This document discusses various methods and equipment used for radiation dosimetry in radiotherapy. It describes phantoms that are used to simulate patient tissue, including water phantoms, slab phantoms made of tissue-equivalent materials, and anthropomorphic phantoms. It also discusses different radiation detection methods including ionization chambers, thermoluminescent dosimeters (TLD), semiconductor devices, radiographic film, and their uses and limitations in radiation dosimetry. Common dosimeters discussed include Farmer chambers, parallel-plate chambers, TLD crystals, diodes, and radiographic film.

1. Radiation Physics
Dosimetry and Equipment

2. Quick Question
A dose of 1Gy delivers a huge quantity of
energy to the patient - is it true or false?

3. Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 3
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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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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RANDO
phantom
torso
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
Adapted
from Collins
2001
Ion chamber

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Detection of Ionization in Air
Adapted
from
Metcalfe
1998

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

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Ionization Chambers
Thimble chambers
600cc chamber

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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
 small
 designed for
electrons
 Holt chamber
 robust
 embedded in
polystyrene slab

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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

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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

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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

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Importance of thermal
treatment
 Determines the arrangement of
impurities
 sensitivity
 fading
 response to different radiation qualities
 Maintain thermal treatment constant...

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Dose
response of
LiF:Mg,Ti:
wide dosimetric
range
watch
supralinearity

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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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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
Cross section
Often prepacked
for ease of use

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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
From Metcalfe et al. 1996
Mostly used like
a photocell generating
a voltage proportional
to the dose received.

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1. irradiation
2. Charge
carriers trapped
in Si substrate
3. Current
between source
and drain altered

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

49. Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 49
Summary of lecture
Ion chambers Semiconductors TLDs Film
Advantages Well understood,
accurate, variety of
forms available
Small, robust Small, no cables
required
Two dimensional,
ease of use
Disadvantages Large, high voltage
required
Temperature
dependence
Delayed readout,
complex handling
Not tissue
equivalent, not
very reproducible
Common use Reference
dosimetry, beam
scanning
Beam scanning, in
vivo dosimetry
Dose verification,
in vivo dosimetry
QA, assessment of
dose distributions
Comment Most common and
important
dosimetric
technique
New developments
(MOSFETs) may
increase utility
Also used for
dosimetric
intercomparisons
(audits)
New developments
(radiochromic
film) may increase
utility

50. Radiation Protection in Radiotherapy
Any question?
Part 2, lecture 2: Dosimetry and equipment 50