# RT02_Phys2_Equip_WEB by xuyuzhu

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```									 IAEA Training Material on Radiation Protection in Radiotherapy

Radiation Protection in
Radiotherapy

Part 2

Radiation Physics
Lecture 2: Dosimetry and Equipment
Rationale
    Radiation dose delivered to the target
and surrounding tissues is one of the
major    predictors     of    radiotherapy
treatment outcome (compare part 3 of
the course). It is generally assumed that
the dose must be accurately delivered
within +/-5% of the prescribed dose to
ensure the treatment aims are met.

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   2
Objectives
    To understand the relevance of radiation
dose and dosimetry for radiotherapy
    To be able to explain the difference between
absolute and relative dosimetry
    To be able to discuss the features of the most
common dosimeters in radiotherapy:
ionization chambers, semiconductors,
thermoluminescence dosimeters (TLD) and
film

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Contents of lecture 2
1. Absolute and relative dosimetry
2. The dosimetric environment: phantoms
3. Dosimetric techniques
 physical background
 practical realization

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1. Absolute and relative dosimetry
    Absolute dosimetry is a technique that yields
information directly on absorbed dose in Gy. This
absolute dosimetric measurement is also referred to
as calibration. All further measurements are then
compared to this known dose under reference
conditions. This means …
    relative dosimetry is performed. In general no
conversion coefficients or correction factors are
required in relative dosimetry since it is only the
comparison of two dosimeter readings, one of them
being in reference conditions.

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Absolute dosimetry
 Required for every radiation quality once
 Determination of absorbed dose (in Gy)
at one reference point in a phantom
 Well defined geometry (example for a
linear accelerator: measurements in
water, at 100cm FSD, 10x10cm2 field
size, depth 10cm
 Follows protocols (compare part 10)

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Absolute dosimetry
 Required for every radiation quality
once
 Determination of absolute dose (in Gy)
at one reference point in a phantom
 Well defined geometry: Eg. water
phantom, 100cm FSD, 10x10cm2 field
size, depth 10cm
 Follows protocols (compare part 10)

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Quick Question

A dose of 1Gy delivers a huge quantity of
energy to the patient - is it true or false?
Answer
FALSE – 1Gy = 1J/kg. Delivering this amount
of energy would raise the temperature of
tissue by less than 0.001oC. Even for a 100kg
person it is much less than the energy
consumed with a bowl of muesli – please note
the amount of energy in food is often listed on
the package.

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Relative dosimetry
 Relates dose under non-reference
conditions to the dose under reference
conditions
 Typically at least two measurements are
required:
 one in conditions where the dose shall be
determined
 one in conditions where the dose is known

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Examples for relative dosimetry
    Characterization of a radiation beam
 percentage depth dose, tissue maximum
ratios or similar
 profiles
    Determination of factors affecting output
 field size factors, applicator factors
 filter factors, wedge factors
 patient specific factors (e.g. electron cut-
out)

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Percentage depth dose
measurement
    Variation of dose
in a medium
(typically water)
with depth
    Includes
attenuation and
inverse square
law components

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Percentage depth dose
Relates dose
at different
depths in water
(or the patient)
to the dose at the
depth of dose
maximum - note
that the y axis is
relative!!!

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TAR, TMR, TPR
    Relative dosimetry for isocentric treatment
set-up (compare part 5)
    All can be converted into percentage depth
dose
    TAR = ratio of dose in phantom with x cm
overlaying tissue to dose at the same point in air
    TMR = ratio of dose with x cm overlaying tissue to
dose at dose maximum (detector position fixed)
    TPR as TMR but as a ratio to dose at a reference
point (e.g. 10cm overlaying tissue)

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TMR, TPR
    Mimics isocentric
conditions
    TMR is a special
case of TPR
where the
reference
phantom depth is
depth of
maximum dose

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Strong ISL
PDD and TMR                                                                         dependence

    Percentage depth dose
(PDD) changes with
distance of the patient
to the source due to
variations in the inverse
square law (ISL), TAR,
TMR and TPR do not.                                                            Weak ISL
dependence

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Output factors
    Compare dose with dose under
reference conditions
 different field sizes
 wedge factor
 tray factor
 applicator factor
 electron cutout factor

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Example: wedge factor
Dose under
reference
conditions

Could also involve different field sizes and/or
different depths of the detector in the phantom

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Quick Question

Is a Half Value Layer measurement for
the determination of X Ray quality
absolute or relative dosimetry?
Answer
 Relative dosimetry:
 we relate the dose with different aluminium
or copper filters in the beam to the dose
without the filters to determine which filter
thickness attenuates the beam to half its
original intensity
 the result is independent of the actual dose
given - we could measure for 10s or 20s or
60s each time, as long as we ensure the
irradiation is identical for all measurements

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2. The dosimetric environment
    Phantoms
    A phantom represents the radiation properties of
the patient and allows the introduction of a
radiation detector into this environment, a task that
would be difficult in a real patient.
    A very important example is the scanning water
phantom.
    Alternatively, the phantom can be made of slabs
of tissue mimicking material or even shaped as a
human body (anthropomorphic).

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Scanning water
phantom

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Slab phantoms

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Tissue equivalent materials
    Many specifically manufactured materials
such as solid water (previous slide), white
water, plastic water, …
    Polystyrene (good for megavoltage beams,
not ideal for low energy photons)
    Perspex (other names: PMMA, Plexiglas) -
tissue equivalent composition, but with higher
physical density - correction is necessary.

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Anthropomorphic phantom

Whole body
phantom: ART

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Allows placement of radiation detectors in
the phantom (shown here are TLDs)

Includes
inhomogeneities

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torso
RANDO
phantom

CT slice
through lung

Head with
TLD holes
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Pediatric phantom

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Some remarks on phantoms
    It is essential that they are tested prior to use
    physical measurements - weight, dimensions
    radiation measurements - CT scan, attenuation
checks
    Cheaper alternatives can also be used
    wax for shaping of humanoid phantoms
    cork as lung equivalent
    As long as their properties and limitations are
known - they are useful

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3. Radiation effects and dosimetry
Radiation effect                                              Dosimetric method
Ionization in gases                                           Ionization chamber
Ionization in liquids                                         Liquid filled ionization chamber
Ionization in solids                                          Semiconductors
Diamond detectors
Luminescence                                                  Thermoluminescence dosimetry
Fluorescence                                                  Scintillators
Chemical transitions                                          Radiographic film
Chemical dosimetry
NMR dosimetry
Heat                                                          Calorimetry
Biological effects                                            Erythema
Chromosome damage

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Principles of radiation detection

 Ionization chamber
 Geiger Mueller Counter
 Thermoluminescence dosimetry
 Film
 Semiconductors

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Detection of Ionization in Air
Ion chamber                                                                    Adapted
from Collins
2001

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Detection of Ionization in Air
Adapted
from
Metcalfe
1998

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Ionometric measurements
Ionization Chamber                                             Geiger Counter
 200-400V                                                      >700V
 Measures exposure                                             Every ionization
which can be                                                   event is counted
converted to dose                                             Counter of events
 not very sensitive                                             not a dosimeter
 very sensitive

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Ionization Chambers
600cc chamber

Thimble chambers

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Cross section through a Farmer type
chamber (from Metcalfe 1996)

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Ionization Chambers
     Farmer 0.6 cc
chamber and
electrometer
     Most important
chamber in
radiotherapy
dosimetry

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Electrometer

From the chamber

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Ionization chambers
 Relatively large volume for small signal
(1Gy produces approximately 36nC in
1cc of air)
 To improve spatial resolution at least in
one dimension parallel plate type
chambers are used.

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Parallel plate chambers

From Metcalfe et al 1996

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Parallel Plate Ionization Chambers
    Used for
 low energy X Rays (< 60 KV)
 Electrons of any energy but rated as the
preferred method for energies < 10 MeV
and essential for energies < 5 MeV
 Many types available in different
materials and sizes
 Often sold in combination with a suitable
slab phantom
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Parallel Plate Ionization
Chambers - examples
    Markus chamber                                                 Holt chamber
    small                                                          robust
    designed for                                                   embedded in
electrons                                                       polystyrene slab

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment       42
Well type ionization chamber
    For calibration of
brachytherapy
sources

Brachytherapy
source

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Ionization chamber type survey
meters
    not as sensitive as G-M devices but not affected by
pulsed beams such as occur with accelerators
    because of the above,
this is the preferred
device around high
energy radiotherapy
accelerators

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Geiger-Mueller Counter
 Not a dosimeter - just a
counter of radiation events
 Very sensitive
 Light weight and convenient
to use
 Suitable for miniaturization

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   45
Geiger-Mueller (G-M) Devices
    Useful for
 area monitoring
 room monitoring
 personnel
monitoring

    Care required in regions of high dose
rate or pulsed beams as reading may
be inaccurate
Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   46
Thermoluminescence
dosimetry (TLD)
 Small crystals
 Many different materials
 Passive dosimeter - no cables required
 Wide dosimetric range (Gy to 100s of
Gy)
 Many different applications

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Various TLD types

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Simplified scheme of the TLD
process

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TLD glow curves

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Glow curves
 Allow research
 Are powerful QA tools - does the glow
curve look OK?
 Can be used for further evaluation
 May improve the accuracy through glow
curve deconvolution

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The role of different dopants

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   52
Importance of thermal
treatment
    Determines the arrangement of
impurities
 sensitivity
 fading
 response to different radiation qualities

    Maintain thermal treatment constant...

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   53
Dose
response of
LiF:Mg,Ti:

wide dosimetric
range

watch
supralinearity

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   54
Variation of TLD response with
radiation quality

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Materials: oh what a choice...
      LiF:Mg,Ti (the ‘gold‘ standard)
      CaF2 (all natural, or with Mn, Dy or Tm)
      CaSO4
      BeO
      Al2O3 :C (record sensitivity  1uGy)
      LiF:Mg,Cu,P (the new star?)

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TLD reader
   photomultiplier based
   planchet and hot N2 gas heating
available

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What can one expect...
 Reproducibility: single chip  2% (0.1Gy,
1SD)
 Accuracy (4 chips standard, 2 chips
measurement)  3% (0.1Gy, 95%
confidence)
 about 30 minutes per measurement...

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Radiographic film
 Reduction of silver halide to silver
 Requires processing ---> problems with
reproducibility
 Two dimensional dosimeter
 High spatial resolution
 High atomic number ---> variations of
response with radiation quality

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Radiographic film
Often prepacked
for ease of use

Cross section

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Film: dose response
    Evaluation of film via
optical density
    OD = log (I0 / I)
    Densitometers are
commercially
available

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Radiographic film dosimetry in
practice
     Depends on excellent
processor QA
     Commonly used for
demonstration of dose
distributions
     Problems with
accuracy and
variations in response
with X Ray energy

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Radiochromic film
 New development
 No developing
 Not (very) light
sensitive
 Better tissue
equivalence
 Expensive

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Semiconductor Devices
 Diodes
 MOSFET detectors

Diodes for water phantom
measurements

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Diodes
Mostly used like
a photocell generating
a voltage proportional
to the dose received.

From Metcalfe et al. 1996

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Metal Oxide Semiconductor
Field Effect Transistor
MOSFETs = extremely
small sensitive volume

From Metcalfe et al. 1996

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1. irradiation

2. Charge
carriers trapped
in Si substrate

3. Current
between source
and drain altered

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Readout
Gate bias during
after irradiation:
irradiation:
gate bias required
determines sensitivity
to maintain
constant current

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Diodes and other Solid State
Devices
    Advantages                                                     Disadvantages
    direct reading                                                  temperature
    sensitive                                                        sensitive
    small size                                                      sensitivity may
    waterproofing                                                    change --> re-
possible                                                         calibration necessary
    regular QA
procedures need to
be followed

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment             69
Summary of lecture 2
Ion chambers         Semiconductors                       TLDs                 Film
Advantages                  Well understood,       Small, robust                  Small, no cables     Two dimensional,
accurate, variety of                                  required             ease of use
forms available
Disadvantages               Large, high voltage    Temperature                    Delayed readout,     Not tissue
required               dependence                     complex handling     equivalent, not
very reproducible
Common use                  Reference              Beam scanning, in              Dose verification,   QA, assessment of
dosimetry, beam        vivo dosimetry                 in vivo dosimetry    dose distributions
scanning

Comment                     Most common and        New developments               Also used for        New developments
important              (MOSFETs) may                  dosimetric           (radiochromic
dosimetric             increase utility               intercomparisons     film) may increase
technique                                             (audits)             utility

Radiation Protection in Radiotherapy       Part 2, lecture 2: Dosimetry and equipment                           70
General Summary: Physics
   In radiotherapy, photons (X Rays and gamma rays)
and electrons are the most important radiation types
   Accuracy of dose delivery is essential for good
practice in radiotherapy
   Absolute dosimetry determines the absorbed dose in
Gray at a well-defined reference point. Relative
dosimetry relates then the dose in all other points or
the dose under different irradiation conditions to this
absolute measurement.
   There are many different techniques available for
dosimetry - none is perfect and it requires training and
experience to choose the most appropriate technique
for a particular purpose and interpret the results

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   71
Where to Get More
Information
 Medical physicists
 Textbooks:
 Khan F. The physics of radiation therapy. 1994.
 Metcalfe P.; Kron T.; Hoban P. The physics of
radiotherapy X-rays from linear accelerators. 1997.
 Cember H. Introduction to health physics. 1983
 Williams J; Thwaites D. Radiotherapy Physics. 1993.

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   72
Any questions?
Question:

Which radiation detectors could be useful
for in vivo dosimetry and why?
In radiotherapy the dose delivered to the patient is typically
too large for radiographic film which in addition to this is
light sensitive. Ionisation chambers are often fragile and
require high voltage, both not ideal when working with
patients. Therefore, TLDs are often used as detectors for in
vivo dosimetry. They are small, do not require cables for the
measurement and there are materials which are virtually
tissue equivalent. TLDs can be complemented by diodes if
an immediate reading (= “active dosimetry”) is required.
As TLDs, diodes are solid state dosimeters and therefore
sensitive and small. Other detectors of interest in this group
would be MOSFETs.
A different class of in vivo dosimeters are exit dose
detectors in the form of electronic portal imaging (compare
part 5). They may prove very useful for on-line verification.

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   75

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