Radiotherapy Exchange Week Report by HC120704021652

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									Mark Penrice                                                                 Radiotherapy Exchange Report


Mark Penrice
Radiotherapy Exchange Week Report
Monday 8th to Friday 12th August 2005
Compared to RRPPS, this week had a slower, more uncertain start, a feeling that pervaded all the way
through. This owing mostly to a general lack of organisation of the exchange as a whole and therefore
not really knowing what I should be doing or witnessing at any one time. To be fair, as one of the
regular staff commented, “only RRPPS have the time to draw up detailed timetables”. Though the pace
seemed quite laid back and relaxed all the way through, this was not an accurate view of the usual
workload on the department, as I visited at the start of the typical holiday season - so, just as in Nuclear
Medicine, there were less staff crowding the place, and many less patients for them to deal with.
Conceivably the previous weeks were a lot more frantic and had less free time! Certainly, the
complexity and time-consuming nature of some treatment planning and preparation stages - and the well
drilled fast turnaround on the simulator and actual therapy machines - suggest a far busier atmosphere
through the rest of the year.

Monday
Even simply making introductions and getting started on the first morning felt the effect of this lack of
planning, and it was not helped by some apparently controversial changes taking place to how the
department is run - starting that day! The previous entrance to radiotherapy and its reception desk being
closed, and all enquiries (and access) being directed through the main Cancer Centre & Chemotherapy
foyer and reception. From the signage, this looked to have been put together in a similarly ad-hoc
manner (reasonable to assume it wasn’t top priority!), and not all of the Chemotherapy staff seemed
aware of this change. Certainly it was hard to find anyone aware of the running of Radiotherapy or even
who could be considered “in charge” of the department, so I could find them and explain the situation.
Eventually, I gave up on being “official” about it and flagged down a passing Radiotherapy technician.
They quickly revealed why I was having so much trouble - I wasn’t even on the right floor - and guided
me to the first floor Mould Room, where I was to be largely based for the rest of the week.

Met Andy, Phillip, Kate and Jo, the main workers on the practical side of treatment preparation,
including immobilisation mask and shielding production, mostly taking place in and around the mould
room, and worked out a rough idea of how the week was to be divided up. Today, I was to observe the
general order of business around this one room, and all the different tasks they are involved with. First
things first, the production of a plastic mask.
The masks found in radiotherapy, moulded from a patient’s face (or entire head, in some cases), have a
variety of uses. Though it is their main function, they are not merely intended to hold them still during
treatment, and provide reliable and precise maintenance of position between treatments; they also allow
the position of anatomical features to be more accurately highlighted and recorded on X-ray films and
CT images, very precise marking out of therapy beam alignments and shielding during planning without
needing the patient to be present (or exposing them further / tying up X-ray sets), and double checking
the aim of these elements during the actual therapy - as well as being a mounting surface for flexible
shields and wax bolus, without risk of lead poisoning or the mess and wasted time of clearing up wax
post-treatment!
For all their precision and many uses, the impression-taking and production of a mask is deceptively
simple and low-tech, being a fully practical, analogue and non-electronic procedure, an increasingly
rare thing in medical physics! First, any particular areas of interest for treatment or shielding (if they are
obvious at the skin surface at this stage, without needing deeper investigation) are outlined with marker
pen, the patient is informed of what the rest of the impression-taking will involve, and positioned on the
couch comfortably and in a reproducible manner (lining up anatomical features e.g. ears, nose with
laser guide cross-hairs if possible). Any particular positional photographs are taken at this time also.
Next, the shape and detail of their face is captured using a thick layer of alginate, spread on with plastic
spatulas and avoiding the nose/mouth if at all possible to avoid asphyxiation, and this is given greater
strength and definition by a second layer formed from fresh plaster-of-paris bandages applied on top.
This is allowed a few moments to set, and then removed - usually without much difficulty - and taken to
the casting room (the patient may now leave at their leisure).
The alginate and bandages are both items adapted from other fields in medicine for their particular
properties. Alginate has its roots and main use in dentistry, taking impressions of teeth for records,
braces and denture casting, though it is used in much smaller quantities and under greater pressure. It is



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Mark Penrice                                                                Radiotherapy Exchange Report


a thick, “creamy” paste (incidentally with a bright green colour and strong smell of mint!) mixed up
with water from granules, that takes the form of whatever it is laid on or pressed against, setting very
quickly after spreading to a firm, rubbery permanent model of that surface. It cannot be returned to the
more fluid form without some difficulty! One problem with alginate - and one not often found in
dentistry - is that it sticks easily to hair, particularly once set. Out of the several patients I saw having
impressions taken, most of them required some part of their face covered up before application, whether
a young girl having to wear a hair net, or an older man, balding but with a beard that called for greater
creativity (taped-down plastic sheeting).
Obviously, plaster bandages are more usually found in accident & emergency for immobilising broken
limbs or joints. In both cases their application method is the same - starting life dry but relatively
flexible, being “activated” and made soft by dipping in water, and draped over or wrapped around the
area in question. The bandage then dries and hardens by both evaporation, and exothermic reaction of
the plaster with water, to form a strong, stiff support structure, for either the alginate mould or a
patient’s fractured arm.

Once complete and transferred to the casting room, the newly made impression has any holes (e.g.
around the mouth & nose) blocked and covered over, and may be built up slightly around the edge,
using further bandages. Supported on christmas-pudding shaped domes of waste plaster, it is filled with
a large amount of freshly mixed plaster of paris, and left to set, which takes a surprisingly short time!
The same exothermic reaction seen with the bandages takes place in the cast, chemically removing
water and producing a great deal of heat (prepared with cold water, ending up almost too hot to touch),
which also speeds evaporation. If the cast is needed in a great hurry, salt may be added to the plaster
mixture, somehow catalysing the reaction (the plaster definitely becoming too hot to touch!).
A dry, mostly completed replica of the patient’s face (or back of their head, or other area of interest) is
broken out of the mould - quite literally, as it can not easily be tapped out without risking damage, the
mould has to be torn and peeled away in layers, with no possibility of reuse. It then requires some
simple but labour intensive tidying-up - chipping away any excess (at the edges, from any covered-up
hair, and again around the mouth), filing away rough edges or overhangs that may make it stick in the
vacuum former, and building up any missing parts with small extra dabs of plaster.

Each completed cast is taken to the vacuum forming room and placed in the machine. This represents
the most mechanised part of the process, but is still barely automated and requires a lot of manual input.
The cast is located securely in the centre of a movable tray, which lowers deep inside the body of the
machine. A new sheet of quite thick, stiff transparent plastic is removed from it’s protective wrapping
and inserted into the holder so that it covers the hole left by the lowered tray. Hot air is blown over the
sheet from beneath, until it can be seen to have softened, by way of it billowing upwards. The air flow
is stopped and the vacuum pump is turned on, sucking the plastic downwards; at the same time the cast
is raised into it’s original position, pressing up through the transparent sheet (held securely at the edges)
and shaping it into a near-perfect replica of the patient’s own contours. It cools again to a relatively
rigid state within a few seconds, and the forming is complete. The entire operation takes a couple of
minutes, with the insertion/removal of cast and plastic, along with the heating stage, needing the most
time - the movement of the cast tray and plastic holders (kept out of the way at first) is fast and a little
violent!
After being formed, the plastic is trimmed down to size using sturdy tin-snips (each sheet is about 1m
on a side, and so a lot is wasted; being thermo-plastic, however, it can be melted and recycled) and
returned to the mould room for the final stages. It needs to be reinforced, have some basic markers
applied, have the fit checked on the patient, and where necessary, clips fitted to hold the front and back
sections together for two-part moulds (these are used mainly for patients that are to be treated prone,
rather than in the usual supine position - the rear mask being used more for marking off beam aim
points rather than holding the patient still).
The reinforcing process is necessary to create a mask suitable for use in therapy. Though fairly rigid,
there is still some degree of movement and distortion possible, and it lacks any proper surface to allow
it to fit the simulator or therapy machine tables. A suitable, simple stand is made up using more of the
same type of plastic, as are legs to hold the mask and stand together, and strengthening “box” sections
that are all stuck together to form a strong, much more rigid structure. For fitting of stand to mask, it is
essential when fitting the initial struts to again lie the patient on the bed used for making the impression,
so it can be made to the right height and position. To shape the plastic strips, a portable heat gun or
miniature bunsen burner are used to warm and soften them; to stick them, ordinary acetone (a solvent
ketone, as found in nail varnish remover) acts as a type of polystyrene cement - temporarily dissolving



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Mark Penrice                                                                Radiotherapy Exchange Report


the top layer of both surfaces after it is brushed on, so that they bond together and stick tight once dry.
One side of the joint being at a high temperature helps the process, supplying already softened plastic,
speeding the dissolution and then drying stages, with an audible hiss as the acetone evaporates.
After the first locating struts are in place (and front/back securing tags, if needed - these are shaped and
glued in the same fashion and will simply be held together with bulldog clips when the mask is used),
the patient is once more free to leave. Further strengthening stages can be completed in the mould room
proper, adding extra supports and side surfaces to the triangular “boxes” using the same methods.
Before this fitting stage, the mouth & nose hole is cut out of the mask and filed down. If it is too small
or needs altering in some way, this will be completed at the same time, along with any other minor
alterations (e.g. cutting out eyes for some reason). Again, simple tools are needed - a smaller pair of
snips, and a small metal file.

The final part of mask production that is handled in the mould room is adding simple markers to each
one, to aid in therapy planning - nothing more than a long strip of stickytape, with small pieces of lead
attached in a rosary-type fashion (cycles of four small ones, one large) at roughly 5mm intervals. Their
positioning does not need to be precise, so long as the tape is stuck securely enough that they will not
move; they are used to highlight and compensate for the 3D soft tissue contours of the patient’s face on
2 dimensional simulator images, or CT imaging where soft structures such as the nose and chin may not
be very well defined compared to bone.
So-far completed masks are labelled with patient name and registration number, and passed along to
planning for mark-up.

An alternative type of mask fitting was also seen, known as “Orfit”. Much simpler than the full
moulding and vacuum forming process, these are intended for patients needing palliative pain-reducing
treatment, rather than precise therapeutic care. Orfit uses a mask of thermoplastic mesh, much like that
used in nuclear medicine brain scans; it is even prepared the same way, by warming in a water-bath.
The intent is the same - for keeping them reasonably still in an acceptably reproducible position,
without the expense and time overhead of a fully tailored mask.
Unlike nuclear medicine, these masks are not re-used. Once formed to the patient’s contours, they are
reserved for that one person and are kept until the end of their treatment, then disposed of.

Later on Monday, I saw the first stages of the production of custom shaped “lead” shields, moulded
from low temperature melting point alloy - but I will leave the write-up of this to a later day, as I didn’t
get to observe the full procedure until Thursday.

Also, had a read-through of the protocols and guidelines folder. Not quite the local rules, but included
many safety points and instructions for much of the work around the mould room and simulator,
including those covered above.




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Mark Penrice                                                                Radiotherapy Exchange Report


Tuesday
The first of two days in Therapy Planning, down the corridor from the mould room. Much quieter,
really rather boring to a casual observer despite the otherwise interesting and involved nature of the
work - as Nick Sangha was at pains to warn me and apologise for several times, “it’s not a spectator
sport”! In stark contrast to the mould room, the majority of the work is unavoidably computer-based
owing to the heavy, detailed calculations involved that would be almost impossible to safely and
accurately reproduce mentally and manually - with endless repetition, as a lot of trial and error tweaking
is required to get the best possible result.
On both days, most of my time was spent shadowing Tony who, along with several other members of
staff, was involved mainly with planning beam positions and widths and designing shielding - i.e. the
bulk of the drawn-out, demanding and “invisible” tasks! (I never really had a full introduction to the
other members of staff other than brief hellos, as everybody was busy and going to and fro)

This day, I saw how the beam positions, widths, “automatic” shielding wedges, and custom shadow
masks are designed and planned. As stated above, this is mostly laid out on computer, as allowance has
to be made for how the different layers of patient tissue affect the passage of the X-ray beam, and even
a powerful, modern system with a large monitor seems to update barely-sharp-enough graphics with
considerable delay (a few seconds per change, nothing compared to systems of old, but still feels like an
age in the current era of near-instant response). Each internal part of the patient has varying thickness
and a different absorption coefficient, to say nothing of scatter, and these may affect the beam
differently depending on its output energy (from a few hundred keV for surface treatments, through
more common 6 and 8 MeV beams, up to extra high 15MeV treatments). Also, effort has to be made to
concentrate the dose in the area of the tumour whilst minimising it everywhere else, and particularly in
sensitive areas... which may well be adjacent to the tumour!
For all this information, the computer relies on CT data showing the absorption density from which to
calculate the dose distribution (besides the consultant using this same data to help locate the tumour),
but the basic cross section shape of the patient is commonly found from either the print of a single CT
slice or even planar X-ray films taken on the simulator, copied in a simplified form using tracing paper,
then entered into the computer using a light box and old-style - but still highly accurate - digitiser puck.
(The last time I saw mention of one of those was in an Usborne “World of Computers” book, circa
1989 ... but, if it works, why change it?)

The core idea in planning, however the beam is to be shaped and controlled, is to deliver at least 95%
of the “prescribed” dose (actual figures may well go as high as 105%) to the target area, whilst
minimising it to all other tissues, as intentionally deadly radiation is used, to kill off tumour cells.
Ideally the 95% boundary would follow the contour of the target, with the 90%, 75% etc lines all
following it quite tightly. Unfortunately this is rarely possible to achieve simply due to the limitations of
the machine, technology, and the great amount of time that both such a treatment (requiring 5, 10 or
more “beams” - aim points for the large, heavy linear accelerator) and it’s planning would take.
In practice, up to 3 or 4 individually planned beams (with their own angles, widths, masks and shields)
are used, and they are kept as narrow and masked-off as is practical; however, too much masking would
spoil the exposure and actually reduce the target-to-other tissue dose ratio. For some common tumours
that are found centrally or near the centre of the body, beams fixed at right angles are used, simplifying
the procedure, as the angles do not need to be tweaked, and beams which face each other are
automatically mirrored.

Though it is preferable, it is not essential for the 95% line to strictly follow the target, so long as it
covers most of the “PTV” (patient target volume) without a “significant” amount (above about 2.5%)
falling to 90%. A feature of the target marking-out that can introduce some fuzziness and uncertainty
also allows for small variation and inaccuracy in the planning; the PTV, in fact, does not mark the edge
of the tumour (which is the “CTV” - cancer target volume), but covers a certain extra amount of
surrounding tissue around it. This is necessary to encompass the entire tumour with allowance for
patient bodily movement, plus movement, settling, and filling/emptying of surrounding organs and
vessels, as well as any minor local spread of malignancy that might not be detectable by CT or any
other means. This, combined with a refractive effect particular to X-rays (where they “curve” slightly
around the edge of the shielding), makes the edge of the target zone quite fuzzy and forgiving of an
imperfect end result.




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Mark Penrice                                                                Radiotherapy Exchange Report


Though the computers are used for a great deal of the work, particularly with the beam settings, not all
of the work is electronic. Any areas particularly requiring extra shielding are marked up by the patient’s
consultant on the simulator’s planar X-rays used for planning, and from these guidelines, custom lead-
alloy blocks can be produced to further tailor the beams and better protect the patient. The guides are
again copied quite simply using tracing paper, a hard draughting pencil and ruler. The trace is not a
direct copy of the original shape, however; due to the refractive effect mentioned above, the beam tends
to infiltrate slightly into the protected area, and so the shields need to be extended an extra 1cm into the
otherwise unprotected area - possibly even a little into the target zone. This extension is added to the
lines on the film itself, and only the new line is traced. In addition, positional information is added to
the trace, using the standard grid on the film and the edges of the simulated beam, so the shield may be
accurately located in the tray attached to the accelerator - as are details on the scale factor of the trace
(entered into the computer that produces the moulds for the alloy, and automatically calculated by the
one used to plan beams), so that the produced shield will come out at the right size. Typically the final
shield will be less than half the dimensions of the traced outline (usually 40%, to be precise), thanks to
the different locations of shield and film in comparison to the beam origin. As the X-rays are produced
from a point source, in this case the exit of a linear accelerator, but a rectangular beam is produced from
this output, they diverge quite markedly with distance. This divergence is a linear phenomenon, so the
factor relates purely to the two distances (something in the order of 48cm from the source for the shield
and 120cm for the film). Various extra tools, largely forgotten in the computer age, are used for
marking out the 1cm border, e.g. protractors and odd draughtsman’s implements like parallel rules.

After seeing a couple of plans that progressed this way quite easily - if slowly - we moved on to some
other notable exercises. Firstly, a breast cancer treatment plan making use of wax bolus, a material with
similar properties to body tissue that produces extra attenuation to the beam, without shielding it
completely. Rather than reducing the dose, this is aimed at altering the position of maximum dose, as
the pattern of energy deposition with MeV X-rays is an odd one... they shed energy gradually and cause
minor ionisation at first, but they reach a critical point (minimum energy level?) where most of the
remaining energy is all “dumped” in a small area, before depositing the final scraps more diffusely in
the zone beyond this maximum. This non-linear dosing pattern is how radiotherapy can be effectively
used to treat tumours in the centre of the body, such as in the liver or lungs, without savagely burning
all tissue between there and the surface. However, when it comes to peripheral areas such as the breast
(or nose, skin etc), the layer of tissue is usually often too thin to easily locate the “isodose” point close
enough to the surface to be effective and safe - even with the usual long-way-round “glancing” beams
used for these treatments. Instead, bolus is used, layered up above the region of interest to simulate it
being deeper inside the body, and artificially raising the isodose point closer to (or even all the way to)
the skin surface. Like the various shields, the bolus is entered into the computer so that it may be
properly planned; it is added at the simulation stage and marked off on the X-rays and CT, so it can be
traced and digitised the same as the other major features.
At least, that’s how I understand it from a combination of basic college lectures and the gaps Tony tried
to fill in - as a student himself, it seems he was still learning on this particular part of the subject area
and was almost as lost as I was.

Secondly, a complication to show that it doesn’t always go as smoothly as the first three plans (the
central ones being liver and brain, if I remember rightly), which despite the time consumed came out
quite satisfactorily. This time, the patient was having therapy for prostate cancer, but their steel hip
replacement was causing more shielding than the actual adjustable lead wedges, leading to a rod-shaped
area of greatly reduced dose all the way into one side of the PTV. Obviously, this is a piece of shield
that cannot be removed or altered, and must be worked around. The alterations to the plan - mostly
consisting of making the wedge on the non-mirrored beam very thick on the opposite side to the
artificial hip, as it was a fixed position three beam treatment and the mirrored beams could not be
significantly tweaked - attempted to minimise this. It didn’t appear to work terribly well, but it reduced
the severity of the problem at least, and perhaps further down the line from this initial plan (as they go
through several iterations and checking stages) the method may be altered to use different angles.




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Mark Penrice                                                                Radiotherapy Exchange Report


Wednesday
Staying with planning, again overall quite dull to watch, but many more things seen in truth. First of all,
a short session with Nick, who demonstrated the setup for planning treatment of a small lung tumour.
More complicated than other plans seen so far, due to particular extra issues compared to them. Most
critically, taking care to control the dose to the spinal cord. No more than 72% of the prescribed dose
allowed, with no exceptions (cord is allowed 40Gy maximum dose in any treatment - centrally located
tumour was prescribed 55Gy), even if it means potentially compromising the target dose - or, more
likely, adding extra complexity to the treatment. Also, taking notice that the alveolar space in the lungs
effectively counts as “outside”, an air gap that causes very little attenuation compared to body tissue,
reducing the dose received by the other (healthy) lung as much as possible, as well as minimising the
dose to the rest of the body. Keeping limits on the spine and healthy lung doses is especially tricky as
they are so close to the diseased lung, and the tumour itself (making an extra nuisance, as it was located
centrally and to the rear of the lung). All of the planning, at least that I saw, was computerised, with
much reference being made to multi-slice chest CT, and regions of interest drawn directly onto it.
In the end, a three beam treatment was planned, but unlike those from Tuesday, they were all freely
(and very carefully) positioned, ending up within a narrow arc no more than 100 degrees rotated from
each other. The separation angles were not even, and the shielding wedge thicknesses varied widely,
between almost flat (producing a fairly homogenous beam) to almost 45 degrees (producing a beam
with a lot more power at one side than the other). This arrangement was arrived at after a great deal of
beam/wedge shuffling and tweaking, and movement of the isofocus - confusingly, today it meant not the
centre of maximum dose, but the theoretical centre, and the point around which the angled beams are
positioned... altering it’s position feeds down through the process to ultimately change the precise
height and lateral positioning of the patient couch inside the sweep of the accelerator head. All this
work was ultimately worth it, however, as it produced a nicely crafted area of 95% closely following the
outline of the PTV, a satisfactorily low dose to the spinal cord (not very far inside the limit, but at least
not exceeding it) and low doses to the healthy lung and rest of the body. All this despite a lack of any
decent fine control over the beam or shield shaping!

All the same, some compromise was made, even if the effect of the patient’s breathing is taken into
account - this therapy was to treat a tumour that was of a considerable size in all three dimensions,
particularly in the direction crossing through the CT slices, and with a convoluted shape causing the
core area to shift on each slice. The machine chosen for the treatment was, however, only capable of
producing an effectively 2-dimensional beam profile, for a roughly cylindrical treatment zone. To work
around this in a simple manner that still produces a suitably effective result, all the PTVs from each
slice (individually drawn around the tumour in every one) were added together to form one large one,
and the tissue absorbency samples were averaged (this also makes the computations a lot simpler and
quicker). The planning proper was performed on this strange composite, possibly leading to some
minor artifacting and mis-aiming, but is the best possible for this particular treatment using current
technology and machines. Machines that allow full three-dimensional planning are available, but they
are only capable of treating a relatively thin slice (smaller than required for this treatment), require the
best part of a day to plan, and are only really required for quite intricate treatments where many delicate
tissues might otherwise be damaged.

Nick briefly talked me through a couple other simpler plans, similar to those seen on Tuesday, and gave
a brief explanation of why certain energies are used for particular therapies besides their penetrating
power - high energy beams might otherwise be suitable for a lot more treatments than they are actually
used for, but for the problem of secondary electron production (beta particles - bremsstrahlung
radiation? he didn’t have time to say..) which alters the dose pattern in a less predictable and
controllable fashion. This means most therapies are limited to 6 or at most 8MeV but no higher; the
department has machines that are capable of 10 and 15MeV output, but this ability is not often used
except in specialist cases. After this, he had to leave the department on some unrelated business (and
was busy on return), so I returned to shadowing Tony, who had begun work on something different and
interesting-looking - one of the aforementioned three dimensional plans.

This was a very complicated and impressive looking planning setup for treatment of an extensive
prostate cancer that was touching the bladder, seminal vesicles and other structures, requiring careful
use of the full 3D capability. This is achieved by use of MLCs (multi-leaf collimators), an array of
automatically positioned movable lead strips (thick in the direction of the beam, but individually thin
across the beam face) fitted in front of the beam window. Quicker, easier and often more precise than
using moulded lead shields, certainly more efficient if many separate beams need to be used and
capable of some clever tricks such as changing halfway through an exposure. But, they are of a limited


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Mark Penrice                                                               Radiotherapy Exchange Report


size, only fitted to two of the department’s many machines (at least seven), and restricted in the shape of
shield that can be produced as they protrude into the centre of the beam from the edges and must leave
a continuous gap. They also, once again, require the plan to be completely computerised (not
necessarily a bad thing) and take a long time to accurately set up.
Like the lung plan, the MLCs had to be very carefully positioned and shaped to fit the full 3D outline of
the PTV, with a great deal of trial and error, repetitive slight alterations with non-intuitive outcomes,
and multiple checks around the entire perimeter of the target to make sure the 95% outline neither
ended up crossing inside of a significant amount of target, nor extended wildly beyond it. Even a small
alteration at the end to correct a tiny area of shortfall took several minutes, from having to work out
what to change, and checking that it hadn’t compromised anything else.
Again, problems were caused by the positioning of the beams - not in deciding on where they should
go, but because they were fixed in the standard three positions and only the isodose point could be
changed. The interactions between altering their MLCs and shield wedges may have seemed to make
little sense, but at least the MLC positions could be different on the left and right laterals rather than
having to be mirrored. This did, however mean an extra 50% work in setting them up - they may be
controlled by simple click-and-drag, but with a good ten or more pairs of lead strips covering but the
small prostate, and most of them needing to be changed on any alteration, there was a lot of clicking
and dragging (a DMSA or MAG3 analysis pales in comparison).
The final printouts looked decidedly basic, somewhere between a classic chart plotter or seismograph
and the print from a school-room BBC micro, but obviously this is all that must be required and
anything more complex or flashy would detract from the information contained within. Several pages
resulted, including a couple of life-size (on A3 paper) physical planning charts at different levels of
detail, from just body outline, PTV and 95% dose, through to one highlighting several major dose
contours and all organs/tissues of interest. Another, smaller page showed a (particularly BBC-like)
graph displaying relative proportions of dose (in Grays, not %) received by each of the bounded-off
areas of the body shown on the detailed planning chart - e.g. PTV, entire prostate, bladder, rectum,
seminal vesicles, femora, general body dose, and so on.

The afternoon held more novel sights. I couldn’t hold my curiosity any longer and had to interrupt Nick
to ask him exactly what the noisy machine was, seemingly made out of scrap scaffolding, that he had
been playing with masks on for the past day and a half. Turns out my judgement was half right in this
case. Right in that it had been produced from rather basic metal piping and was used for part of the
planning process involving the plastic masks. Wrong in that it was far from junk or a toy - actually it
was a bespoke planning machine, carefully custom built for the hospital to very exacting tolerances...
about thirty years previous, explaining the noise! It could not even be oiled, or properly serviced, for
fear of throwing the barely-changed alignments out. Apparently it is still in use as it was incredibly
expensive at the time, is possibly the only one in the entire country or even the world (it was literally
made to a one-time-only blueprint by a specialist machine shop - not a mass produced item), and the
budget hardly exists to create another one when the money spent on making the planning experience a
little quieter and better looking could easily go on more worthy causes. It is also unlikely to be scrapped
until it seizes completely, as it is actually an incredibly useful device, allowing the required patient and
beam positioning measurements to be taken very precisely, and marked on the immobilisation masks,
without the patient having to attend and take up time on the simulator, not to mention being tended to
by at least two staff. This frees up an incalculable amount of machine and staff time, and reduces the
demand on the patient also (reducing waiting times, costs, and increasing staff morale and patient
satisfaction), as the machine only requires the mask and one operator. Even that one person saves time,
as each mask can be marked up far more quickly if a similar operation was tried on the simulator.

Another two breast treatments needed to be planned, and I was allowed a little input on them to see
first-hand how altering the angles, widths and wedges affects the dose pattern. However all did not go
entirely smoothly; the consultant in charge of these two wished to alter them in a simple-looking, but
ultimately complicating way, by adding an extra centimetre of shielding “behind” the treated area
(deeper into the body) to protect the lungs. This meant they had to be almost completely replanned from
their original setups, narrowing the beam and raising the isocentre closer to the skin surface (without,
for some reason, using any bolus). The now very thin slice of tissue that we were aiming for proved to
be almost impossible to effectively plan for, and in the end we simply could not produce an exact match
- a third beam would have been needed, which is a rare thing for such a treatment and probably would
have required a much lower dose from any perpendicular, non-glancing beam. As it was, we came very
close - the 95% dose area could not be made to fill the PTV (standard target point - isodose centre? -




                                                  Page 7
Mark Penrice                                                              Radiotherapy Exchange Report


registering a close 94%), nor could it be made to avoid overlapping into a large area of non-target tissue
without reducing the target dose still further.
All the same, after Tony & me checked with Nick, it was deemed acceptable, as it was the best possible
solution with the available tools, and not incredibly excessive. As in nuclear medicine, great
precautions are taken - so occasionally in radiotherapy they can be slightly overstepped, when
guaranteeing a positive result overrides the inclusion of slight additional risk. This is by no means the
most extreme example of this policy - a more graphic illustration may be the grudging but intentional
induction of cataracts when treating a brain tumour close to the eyes, if even the best possible shielding
is not quite enough. An operation to remove them may then result, but at least this is far more possible
and far, far less risky than one to remove the tumour - not to mention infinitely preferable to either
ignoring it, or giving insufficient dose to destroy it.
Producing the plan proved to be more difficult than it previously looked (these types seemed relatively
simplistic when viewed on Tuesday) - even digitising the body outline and the target area (in this case,
an imaginary line barely inside of the skin between two marked points) was relatively tricky... having to
follow the traced (or in this case, pre-printed, as we were re-planning) line very accurately with a light-
pen type digitiser, including all of the near-imperceptible little curves and corners in it.

There was a brief demonstration of some of the difficult choices that often have to be made in planning,
when the file of a teenage boy came round for planning his first course of treatment, and I was given his
medical notes to see the full story. He had leukaemia, and though chemotherapy was progressing well,
either that or the disease itself was causing inflammation of and/or pressure on his optic nerves, slowly
destroying his vision. The aim of the radiotherapy treatment was not so much to have any healing effect
(leukaemia, as a disease of the white blood cells, is too diffuse to be treated effectively this way), but
more along palliative, gently therapeutic lines, to somehow reduce the inflammation (targeting nearby
blood vessels to close them down) and restore or at least preserve the remainder of his sight. Again, the
trouble is both that the treatment may cause more harm than good if the diagnosis is flawed, and carries
a good risk of inducing cataracts, compromising the original intent. There was also talk of similar
trouble in the groin area, and the treatment for which could affect the testes and leave the patient
permanently sterile - a difficult thing to discuss with a subject who wasn’t quite 15 yet.

Right near the end of the day, I noticed two “new” men sitting up in the corner doing something on a
computer that looked for all the world like they were playing an old wire-frame 3D space game. Turned
out they were checking how capable the radiotherapy “CMS” planning system was of producing Proton
Therapy plans, should the hospital even come into ownership of such a system. Turns out that CMS is
eminently capable of drawing up highly complex and precise PT plans without too much effort; the
difficult thing would be actually finding a use for it. Proton therapy is a highly advanced, somewhat
futuristic (echoes of Star Trek) radiotherapy technique that has yet to really arrive in Britain
(discounting one or two small research centres), despite being massive in Japan and quickly gaining a
foothold in the USA and some parts of Europe. In this way, medically, we are behind a large section of
the developed world - but at least, if the software works, a consultant may be able to draw up a custom
PT plan for a patient in need of it, and send them briefly abroad to a non-British specialist centre with
plan in hand and on CDR.
Part of PT’s cleverness lies in it’s ability to produce a true 3D dose map even with a single beam, no
overly complicated calculation of intersecting beams or compromising with uneven areas of high and
low dose. No MLCs or custom shields really needed, except as safety precautions. Simply an accurate,
highly customisable area of high dose, and very little elsewhere - all achieved by varying the output
energy in different areas. The proton beam is divided into a fairly high resolution array of cells
(achieved by a scanning method?), each of which can have a single, or varying kinetic energy (in the
MeV range). From there on out, they act like MeV X-rays (albeit far less “fuzzy”), or beta particles.
The energy, or range of energies in a cell dictates the protons’ maximum range(s), and the size &
position of the zone of maximum deposition (usually not far removed from the range). Careful
manipulation of these can provide a good, high resolution 3 dimensional dose map inside the patient -
far better than anything achievable even with the best MLCs or wedges.




                                                  Page 8
Mark Penrice                                                                 Radiotherapy Exchange Report


Thursday
Returned to the mould room to see more of the practical side of things. Particularly, another run through
of the shield production.
The finished, traced outline - or outlines, as several may be needed - from planning is brought through
to the mould room to be kept with its accompanying mask or other items. When ready for production,
the trace is taken to the shield casting room to begin the journey from lines on paper to a finished,
professional looking item.
First of all, it requires re-reading into the dedicated computer system (though it runs windows, it
appears to be off the network, and only carries out the one task), again using a digitiser tablet and puck,
moved in a particular pattern and sequence around the outlines. The scaling factor is checked and re-
entered if necessary. At the end of it all should be an on-screen shape matching that of the trace, with a
couple extra parts.
The careful sequencing and extra lines are essential, as what has been entered into the computer are
cutting instructions, that will be passed to a machine connected to it, which uses a cheese-wire heated to
a very high temperature to cut through polystyrene blocks. It is these blocks which are used to cast the
low-melting-point lead alloy into the correct shape - despite the general fragility of the material, it melts
at a higher point than the alloy, and holds it’s shape perfectly when not stressed by other means.
The cutting wire follows exactly the path digitised into the computer, so to make sure it can properly
enter and exit the block, and all the pieces will come out cleanly, the extra movements are programmed
to allow for this. Its path may also be altered to allow multiple blocks to be cast from the same piece of
polystyrene (e.g. mirrored shields to placing in the paths of mirrored lateral beams), or to separate
weighty shields into more manageable pieces. Going by the regulations, the heaviest allowable lead
weight is 10kg; any larger than this (and, preferably, slightly smaller), and it should be split somehow if
at all possible - it can still be reassembled later into an almost seamless whole, on the tray itself. Quite
what the rules say about the shield’s weight when fully assembled is sketchy - e.g. how large a load the
trays that hold the shields should be expected to tolerate.

Each of the cut polystyrene blocks is not straight, but slightly angled, so that they are a little wider at
the bottom than the top. There are a couple of good reasons for this, the best being because of the
divergence effect. The angle is set so that it is parallel to the diverging rays, minimising the diffraction
effect and only shielding, fully, those rays that strike the top edge “square” to the direction of greatest
thickness. Also, it makes both the waste polystyrene, and the solid alloy blocks a little easier to remove,
particularly the alloy; because of it’s fluid nature, it tends to stick to the polystyrene by infiltrating all
the bubbles and cracks and forming a vacuum seal which has to be defeated before it can be removed.

When cutting is complete, the waste material is pushed out of the template and discarded, and the
template itself is inserted into a holder. This is a simple construction of one plastic backing plate with a
central metal protective layer and bolt threads sticking up out of it, a top one that is pushed down firmly
with wingnuts and is missing it’s centre section. The template goes between the two, and has a small air
gap between it’s top and the upper plastic part maintained by a pair of small spacers, no matter how
tightly the lid is screwed down. Securely fastened, the template is 3/4 filled with hot alloy from a well-
insulated crucible by means of a large spoon (entire scene reminiscent of a soup tureen and ladle). This
is carried a mercifully short distance to the sink, already part filled with water, enough that it comes
about 1/2 way up the side of the template, to begin the setting process. At the same time, extra alloy is
carried over from the crucible in a small holder like a thick mug, and poured in the top to bring the level
up to the brim before any layers (and weak points, or boundaries that might cause refraction) can form.
This is now left at least a half hour, preferably several hours to fully set. Despite it having only a small
volume, and being kept only just over it’s 70-ish celcius melting point such that scraps solidify instantly
on the spoon during pouring, there is considerable mass (which can be unexpectedly felt through the
spoon, the first time you handle it) which retains a lot of heat. Even if it solidifies completely long
before this time is up, it will still be just a little too warm to easily or safely handle.

Once set, the apparatus is removed from the sink to a bench, and the template plus alloy freed from the
holder. In the same way as the plaster cast for mask production, the template is a one-time-only affair,
and must almost always be cut, chiselled, and broken away from the alloy because of the vacuum effect
from all the pores sticking it too tight to slide easily out. Though they don’t need the same level of filing
and correction as the plaster models, any excess alloy or rough edges are broken and smoothed off,
including any inadvertent connections between parts, needing less effort as the material is quite weak
and brittle. There should now be several fairly clean-looking, very thick (about 7.5cm) chunks of lead



                                                   Page 9
Mark Penrice                                                               Radiotherapy Exchange Report


alloy on the bench, ready for fitting to the sliding boards that fit in front of the beam collimator window
on the therapy machines.

The thickness is something I wasn’t fully expecting, but it is essential. The aim of shielding is, of
course, to reduce dose to as low a level as possible, or cut it out completely. In this case, passing around
3% of the original dose (or, attenuating 97%) is deemed quite acceptable. This is achieved from about
five half-thicknesses of the lead alloy at typical treatment energies; it’s HVL then being about 1.5cm.
This is also why the blocks may be up to 10kg a time despite having a quite small cross section - they
are thick! It also provides a lot of material for securing each block to the holding tray, a good idea with
the weights involved..

Using a reduced photocopy of the traced sheet, a few screws are lined up in suitably positioned, ready-
drilled holes in the holder, followed by the blocks themselves on the other side. With the whole affair
exactly lined up, the (self tapping) screws are gently hit against the bottom side of the blocks to create
guide indents. The blocks are removed, and the dents are turned into holes, first with the aid of a centre-
punch and the same hammer, followed by a heavy duty bench drilled that requires several attempts to
reach full depth, interjected by removal and having a large amount of metal swarf banged out of the
bit’s thread. With all the holes fully repaired, the blocks are once more carefully positioned, and
securely screwed to the holders, position being repeatedly checked after each screw (and altered if
necessary - I saw it to happen) and at the end when it is all finished. The holders are marked up with
their positions (so they are inserted on the right beam angle) and orientation stickers (so they go in the
right way round), and the patient’s name, ID, and treatment type.

Most of the rest of the day was spent making other types of shields, and more masks, for many patients
- but differently from before. These two items intersected for several of them.
The shields in question are smaller, much thinner, flexible and shaped pieces of lead with cutouts in the
middle, intended for patients with e.g. skin cancer, where the energy is quite low and can be attenuated
near to zero with very little lead. For some of them, the cancer showed far away from the head,
requiring only the shield to be made and the patient given some positional tattoos. Others, it was on the
face or scalp, requiring a mask AND a shield. (Interestingly, all the patients today were apparently skin
cancer sufferers. Conceivably, radiotherapy has days dedicated mainly to particular illnesses, just as
nuclear medicine has days where it is expected to book more MPIs, bones, kidney scans, iodine
therapy, breath tests or VQs than on others)
For a simple example, a young man suffering in the first case from AIDS (but seemingly keeping it
mostly at bay so far), and thanks to it, Karposi’s Sarcoma, a characteristic type of melanoma specific to
patients with auto immune deficiency. The cancer showed up as sore, red patches in two places on his
arm (with a typical “milky” or “pearly” border that could be seen under the mould room’s sun-like
Maglite torch kept for just that very reason); they were outlined with marker pen, which tracing paper
was laid over to copy the marks onto. This was then later transferred again to a couple of lead sheets
(large enough to cover the collimated beam, with a little extra to allow for contouring), and a suitable
hole was cut in the middle of both, matching each traced sarcoma. These lead sheets were then wrapped
in gauze-free elastoplast (to prevent lead poisoning), and had their fit tested on the patient to ensure the
shapes and sizes were all correct once the sheet was wrapped to match the shape of his arm. I expect in
this instance the beam would be collimated down to a very small area, and this would be the only
shielding really necessary.

* 2 older gents, one with cancer on nose, other on shoulder – problems and solutions unique to each
(miniature face mask, cheek-to-armpit cast etc)
* elderly lady with scalp cancer, shielding attempted similar way to young man (no mask despite
location), photograph taken of position. Something about using it in a headboard-type mount and lining
up largely by eye (same as the arm shielding). Possibility of treating her sitting upright in chair? How
would this work?? (pen didn’t clean off white hair very well either)
* harder type of plaster (“stone” plaster) used for casts, so thicker/harder lead shields can be hammered
to fit it without damaging. Particularly with gent suffering it in his shoulder (previous mandibular,
spread to lymph nodes) – large L-shaped shield needed to protect spinal cord, heart…
* gent with cancer on nose being taken for immediate treatment downstairs, we took him, got to see a
little bit of treatment area. Beam being used administers very low energy (100-250keV) “ortho-dose” x-
rays which deposit most of their energy between skin surface and about 1cm depth, very effective
against skin and shallow cancers



                                                  Page 10
Mark Penrice                                                               Radiotherapy Exchange Report


* early finish… quiet week. Cut up some bandages to pass time. Left about 4.30 … others not so lucky,
have to stick around til 5 “just in case” (seen happening about 4 when a mask needed a quick alteration
before treatment)

Friday
Shouldn’t take more than 1 side A4 when expanded - quite empty half day. Went home at 1.30 for lack
of anything to do.
Not much going on with mould room, instead saw simulator and therapy machines
Simulator good copy of treatment room (millimetre perfect!) with same alignment lasers etc. otherwise
fairly simple, just a standard x-ray set with movable film holder to check beam path and what it will
pass through, or to take initial assessment shots with mask in place before full planning.
Downstairs machines – serious heavy equipment, with good reason. Very powerful X-ray beams that
stay on for far longer than I realised. Cause noticeable damage even to their own CCTV cameras that
are well out of the beam path. Isolation “maze” and several interlocks, heavy door etc. patient
positioned using mask/laser, triple checked, for all beam angles. Beam only active when ONLY patient
is in room. Communication thru cctv and 2 way audio links. Planning and positioning info checked by 2
people against each other before each and every exposure.
Very rapid patient turnover, totally different pace to upstairs, and this was “quiet”! CD player in corner,
a little pointless but for comforts sake, only get to use it for about 3 minutes… large number of shields,
masks etc kept in room so easy to hand for all that day/weeks patients.
Great tact needed as some needed to strip right off for the treatment, drop pants etc. explains why some
NM patients who have had RT automatically go to pull everything off before lying down..

Nice people all if a little rushed…!
In fact impressed at the welcoming attitude despite the disorganisation and dullness. Never judge a dept
by the work, I guess…




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