Radiation Protection in Radiotherapy

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Radiation Protection inRadiotherapy
IAEA Training Material on Radiation Protection in
Radiotherapy

• Part 2
• Radiation Physics
• Lecture 2 Dosimetry and Equipment

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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.

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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)

Of tremendous importance If the absolute
dosimetry is incorrect EVERYTHING will be wrong
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Quick Question

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

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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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PDD and TMR
Strong ISL 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?

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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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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
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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
• 200-400V
• Measures exposure which can be converted to dose
• not very sensitive

• Geiger Counter
• gt700V
• Every ionization event is counted
• Counter of events 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 (lt 60 KV)
• Electrons of any energy but rated as the
  preferred method for energies lt 10 MeV and
  essential for energies lt 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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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

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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 LiFMg,Tiwide dosimetric
range watch supralinearity
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Variation of TLD response with radiation quality
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Materials oh what a choice...

• LiFMg,Ti (the gold standard)
• CaF2 (all natural, or with Mn, Dy or Tm)
• CaSO4
• BeO
• Al2O3 C (record sensitivity ? 1uGy)
• LiFMg,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 ---gt problems with
  reproducibility
• Two dimensional dosimeter
• High spatial resolution
• High atomic number ---gt 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 SemiconductorField 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 after irradiation gate bias required to
maintain constant current
Gate bias during irradiation determines
sensitivity
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Diodes and other Solid State Devices

• Advantages
• direct reading
• sensitive
• small size
• waterproofing possible

• Disadvantages
• temperature sensitive
• sensitivity may change --gt re-calibration
  necessary
• regular QA procedures need to be followed

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Summary of lecture 2
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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

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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.

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Any questions?
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Question

• Which radiation detectors could be useful for in
  vivo dosimetry and why?

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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.