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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

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   3
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.


Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   5
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)

    Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   6
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)

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   7
          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.

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   9
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)

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   11
Percentage depth dose
measurement
    Variation of dose
     in a medium
     (typically water)
     with depth
    Includes
     attenuation and
     inverse square
     law components

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   12
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

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   15
                                                                                    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




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment          16
Output factors
    Compare dose with dose under
     reference conditions
        different field sizes
        wedge factor
        tray factor
        applicator factor
        electron cutout factor



Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   17
Example: wedge factor
                                                                                    Dose under
                                                                                     reference
                                                                                    conditions




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

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment            18
        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

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   20
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).

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   21
Scanning water
phantom




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   22
Slab phantoms




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   23
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.


Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   24
Anthropomorphic phantom




                                                                               Whole body
                                                                               phantom: ART




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment             25
Allows placement of radiation detectors in
the phantom (shown here are TLDs)




    Includes
inhomogeneities

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   26
                                                                                torso
RANDO
phantom




CT slice
through lung

                                                                                        Head with
                                                                                        TLD holes
 Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment                  27
Pediatric phantom




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   28
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


Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   29
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




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment                30
Principles of radiation detection

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




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   31
Detection of Ionization in Air
       Ion chamber                                                                    Adapted
                                                                                    from Collins
                                                                                       2001




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment           32
Detection of Ionization in Air
                                                                                    Adapted
                                                                                     from
                                                                                    Metcalfe
                                                                                     1998




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment     33
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




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   34
Ionization Chambers
                                                                                    600cc chamber




Thimble chambers

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment                   35
Cross section through a Farmer type
chamber (from Metcalfe 1996)




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   36
Ionization Chambers
                                                                    Farmer 0.6 cc
                                                                     chamber and
                                                                     electrometer
                                                                    Most important
                                                                     chamber in
                                                                     radiotherapy
                                                                     dosimetry




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment     37
Electrometer



From the chamber




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   38
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.



Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   39
Parallel plate chambers




                                                                     From Metcalfe et al 1996


Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment               40
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
Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   41
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



Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   43
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




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   44
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



Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   47
                           Various TLD types




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   48
Simplified scheme of the TLD
process




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   49
                            TLD glow curves




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   50
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


Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   51
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




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   55
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?)


Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   56
TLD reader
   photomultiplier based
   planchet and hot N2 gas heating
    available




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   57
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...




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   58
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

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   59
Radiographic film
                       Often prepacked
                       for ease of use




Cross section




 Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   60
Film: dose response
    Evaluation of film via
     optical density
    OD = log (I0 / I)
    Densitometers are
     commercially
     available




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   61
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

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   62
Radiochromic film
 New development
 No developing
 Not (very) light
  sensitive
 Better tissue
  equivalence
 Expensive


Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment   63
Semiconductor Devices
 Diodes
 MOSFET detectors




                                                                  Diodes for water phantom
                                                                  measurements

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment       64
Diodes
                                                                               Mostly used like
                                                                               a photocell generating
                                                                               a voltage proportional
                                                                               to the dose received.




From Metcalfe et al. 1996




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment                65
Metal Oxide Semiconductor
Field Effect Transistor
                                                                                MOSFETs = extremely
                                                                                small sensitive volume




From Metcalfe et al. 1996




Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment                66
                                                                                        1. irradiation



                                                                                        2. Charge
                                                                                     carriers trapped
                                                                                      in Si substrate



                                                                                       3. Current
                                                                                    between source
                                                                                    and drain altered

Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment                67
                                                                                           Readout
  Gate bias during
                                                                                       after irradiation:
     irradiation:
                                                                                      gate bias required
determines sensitivity
                                                                                          to maintain
                                                                                       constant current




  Radiation Protection in Radiotherapy   Part 2, lecture 2: Dosimetry and equipment                 68
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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