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October 19th 2009 Dosimetry – A. Mack & M. Sassowsky
Dosimetry
PD Dr.Dr. Andreas MackKlinik Hirslanden
Institute for Radio-Oncology – RTAG&
Dr. Manfred Sassowsky (manuscript 2007)Cantonal Hospital Lucerne (KSL), Institute for Radio-Oncology
19.10.2009
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 1
Dosimetry
Dr. Manfred Sassowsky
Cantonal Hospital Lucerne (KSL)
Institute for Radio-Oncology
3.9.2007
• Introduction
• Metrological traceability; the dosimetry chain
• Absolute / relative dosimetry
• Ionisation chambers
• Thermoluminescent Detectors (TLD)
• Film dosimetry
• Portal dosimetry
• Small field dosimetry
KantonsspitalLuzern
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 2
Literature
1. E.B. Podgorsak (Technical Editor): Radiation Oncology Physics: A
Handbook for Teachers and Students, IAEA, Vienna, 2005, ISBN 92–0–
107304–6, http://www.iaea.org/books
2. TRS 398: Absorbed Dose Determination in External Beam Radiotherapy,
IAEA, Vienna, 2000
3. H. Reich (Hrsg.): Dosimetrie ionisierender Strahlung, B.G. Teubner,
Stuttgart, ISBN 3-519-03067-5 (out of print)
4. H. Krieger: Strahlenphysik, Dosimetrie und Strahlenschutz (2 volumes),
B.G. Teubner, 2001, ISBN 3-519-23078-X and ISBN 3-519-43052-5
5. Recommendations of the Swiss Society of Radiobiology and Medical
Physics (http://www.sgsmp.ch/recrep-m.htm#rec)
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 3
Introduction (1)
• Dosimetry = dose measurement
- Dose, here: amount of radiation
- Metrology: science and technique of measurement
(... not to be confused with meteorology ...)
• A dosimeter is a device that measures (directly or indirectly)
- Exposure
- Kerma
- Absorbed dose
- Equivalent dose
- or other related quantities
• Dosimeter system = detector + reader (+ auxiliary equipment)
e.g.: Ionisation chamber + electrometer + check source
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 4
Introduction (2)
• Absorbed dose is the deposited energy per mass:
• SI unit is the gray (Gy):
• Dose rate ist the absorbed dose per unit of time:
• SI unit is the gray per second (Gy/s):
• Water is commonly used as reference material
(Properties similar to tissue, availability, physical properties well defined)
dm
dED =
Gykg
J===
][
][][
m
ED
kg
J1 Gy1 =
dt
dDD =•
s
Gy==
•
][
][][
t
DD
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 5
Introduction (3)
• Ideally a dosimeter system should have the following properties:
- High accuracy and reproducibility
- Linearity of signal with dose
- Adequate spatial resolution
- Large dynamic range
- Small dependence of signal on
o Dose rate
o Beam quality
o Direction
• Not all requirements can be fulfilled by a single dosimeter system
• For a given application, the most suitable system must be chosen
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 6
Introduction (4)
• Accuracy: Proximity of measured values to the “true” value
• Reproducibility: Degree of agreement between repeated measurements
• Our “Target”: measure the “true value”
• Accuracy versus reproducibility:
Reproducibility High High Low Low
Accuracy High Low High Low
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 7
Introduction (5)
• The „true value“ is not known
• Accuracy and reproducibility of a measurement are expressed by its
Measurement uncertainty
• ISO standard „Guide to the expression of uncertainty in measurement“ (GUM)
- Procedure for characterizing the quality of a measurement
- Generally accepted in many fields
- Defines uncertainty as a quantifiable attribute of a measurement
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 8
Introduction (6)
• Ionising radiation can not be measured directly -
- but only by its interaction with matter
• Different types of fundamental interactions
have been treated in the lecture “Basic radiation physics”
• Dosimetry methods presented in this lecture:
Methods... Physical effect ...
- Calorimetry Heating of water
- Ionisation chambers Ionisation of air
- TLDs Excitation of energy levels in crystals
- Film dosimetry Ionisation of AgBr crystals in radiographic film
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 9
Metrological traceability: ... What‘s that ???
• Dose prescription in PTV, e.g.: 30 × 2 Gy = 60 Gy
• How do you know that the delivered dose per fraction is indeed 2 Gy ?
• The Linac displays MU (monitor units), this must be calibrated.
• How and against what ?
=> Against a dosimeter (ionisation chamber) in a phantom
• This dosimeter must also be calibrated
• How and against what ?
• ....
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 10
Metrological traceability: The metrological „Pyramid“
National standards
Primary
Secondary
Local reference
standards
Working standards
Field instruments
e.g. for MV photons:
Water calorimeter
Ionisation
chambers
Dosimeter system
METAS
Radio-oncology
departments
Sent to METAS for verification
at least every 4 years
Traceability: The result of a measurement can be related to a (primary) standard
through an unbroken chain of calibrations all having stated measurement uncertainties.
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 11
Metrological traceability: Intercomparisons
• Primary standards are compared internationally
with other primary standards
• Interlaboratory comparisons with several
participants or bilaterally
• Assure that the primary standards agree within
their measurement uncertainties
• National dosimetry intercomparisons
organised by SSRMP
• TLDs sent to radio oncology
departments for irradiation
• Results evaluated centrally and
published in anonymised form
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 12
Metrological traceability: Primary standards
• Measurement setups with highest metrological quality, lowest uncertainty
• Measurement results deduced from "first principles",
or simple physical relations
• Can eventually be traced back to fundamental constants
• Can obviously not be calibrated
(as they are supposed to serve as the origin in the metrological pyramid)
• Require significant time and effort; not suitable for clinical environment
• Examples:
- Superficial X rays: parallel plate ionisation chamber
- MV photon beams: water calorimeter
- MV electron beams: Fricke dosimetry
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 13
Metrological traceability: Water calorimeter (1)
• Primary standard for MV photon beams
• Measures temperature increase caused by
deposited energy
NTC Glass capillary
Connection wires
Insulation
Epoxy resin
≈ 0.25 mm ≈ 0.5 mm
hdWWW k
cTD−
⋅⋅Δ=1
1
• Vessel with ultra-pure water and 2 miniature
temperature probes
• NTC: temperature dependent resistance
• Measured with bridge circuit
DW = Absorbed dose to water
ΔTW = Measured temperature increase
cW = Specific heat capacity of water
khd = Correction for heat defect
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 14
Metrological traceability: Water calorimeter (2)
Heat exchangerThermal insulation
(styrofoam)Stirrer
Beam
Glass vessel
Thermistors
Water phantom
Monitor chambers
Air
Pt100 temperature probes
BeamGlass vessel
Thermal insulation (styrofoam)
Monitor chambers
Heat exchanger
• Vessel embedded in water phantom
• Temperature stabilised at 4oC
(maximum density of water)
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 15
Metrological traceability: Water calorimeter (3)
-15
-10
-5
0
5
10
15
0 40 80 120 160 200 240 280 320 360
Zeit [s]
Brü
ck
en
sp
an
nu
ng
[ȝV
]
12.06
12.08
12.10
12.12
12.14
12.16
12.18
12.20
220 240 260 280 300 320 340 360
-11.56
-11.54
-11.52
-11.50
-11.48
-11.46
-11.44
-11.42
-11.40
-11.38
-11.36
-11.34
0 20 40 60 80 100 120
ǻU
m2, b2
m1, b1
U1 = m1 t + b1
U2 = m2 t + b2
Bri
dg
e v
olt
ag
e[μ
V]
Time [s]
• One calorimeter run
• Note:
ΔU ≈ 24 μV
ΔTW ≈ 1.2 mK
= 0.0012oC
• Typical measurement
series needs about 100
calorimeter runs
• Measurement
uncertainty (60Co):
ΔDW/DW =0.41% (k=1)
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 16
Absolute / relative dosimetry
• Absolute dosimetry
- Measurement of absolute dose
at a reference point on the central ray of a beam
- Accomplished using primary standards,
secondary standards and local reference standards
- Secondary and local reference standards: ionisation chambers
• Relative dosimetry
- Measurement of dose relative to reference point
o Depth dose curve on central ray of the beam
o Transverse dose distributions in different depths
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 17
Ionisation chamber: Principle
• Cavity filled with gas (usually air)
• Two electrodes on HV (U)
• Beam ionises air molecules
• Charge separation in electric field
• Current (I) => Dose rate
• Charge (Q=∫ I dt) => Dose
• Sensitive volume open to
ambient air (p, T vary)
• Variety of different shapes:
Beam
U
I
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 18
Ionisation chamber: Cylindrical (thimble) chamber
Insulator Outer electrode Central collecting electrodeHousing
• Most popular design
• Signal independent of radial beam direction
• Typical sizes:
- Length: 4 ... 25 mm
- Radius: 2 ... 7 mm
- Volume: 0.05 ... 1 cm3
• Thin walls: ~0.1 g/cm2
• Used for photon and electron beams
PTW Farmer chamber
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 19
Ionisation chamber: Parallel plate chamber
1 – Polarising electrode
2 – Collecting electrode
3 – Guard ring
Cut A-B
• Recommended for dosimetry of electron beams
• Useful for depth dose measurements
• Useful for measurements in build-up
region of MV photon beams
PTW Roos chamber
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 20
Ionisation chamber: Well type chamber
• Used for brachytherapy sources
(=> Lecture "Brachytherapy")
• High sensitivity
• Large volumes
• Can be designed to accommodate
various source sizes
PTW well type chamber
To electrometer
Source holder
Outer electrode
Collecting electrode
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 21
Ionisation chamber: Segmented chamber
• Many individual chambers arranged in an array
• Used to measure 2D dose distributions in a plane
• Application: IMRT quality assurance
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 22
Ionisation chamber: Electrometer
• Currents / charges to be measured are very low
(Currents in the range of 0.1 ... 100 nA; 1 nA= 10-9 A)
• Device to measure such low currents / charges:
electrometer
• Operational Amplifier with high input impedance
• Feedback with resistor for current measurement
• Feedback with capacitor for charge measurement
PTW Unidos E electrometer
R
Uin
Iin
Uout
R
UI out
in −=
+
-
Uin
Iin
Uout
C
-
+
CUtI outin ⋅−=⋅
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 23
Ionisation chamber: Dose determination
= Absorbed dose to water at beam quality Q
= Calibration factor at calibration beam quality Qc
= Correction factor for (eventual) difference between Q and Qc
= Corrected instrument reading at beam quality Q
QQQQWQW MkNDCC⋅⋅= ,,,
CQWN ,
CQQk ,
QM
QWD ,
C
Gy=][ , CQWN
C=][ QM
Gy=][ ,QWD
Favourable situation in CH:
• Qc very close to Q• => very close to 1
CQQk ,
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 24
Ionisation chamber: Correction factors
= Corrected instrument reading at beam quality Q
= Air density correction factor
= Ion recombination correction factor
= Uncorrected instrument reading at beam quality Q
STpQ kkMM ⋅⋅=
0
0
T
T
p
pkTp ⋅=
Tpk
Sk
QM
M
C=][ QM
C=][M
00 , Tp
Tp ,
= Pressure and temperature at reference conditions
= Pressure and temperature at measurement conditions
K hPa ==== ][][;][][ 00 TTpp
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 25
Ionisation chamber: Reference conditions (1)
MV Photons:
• Temperature T0 = 293.15 K = 20oC
• Pressure p0 = 1013.25 hPa
• Relative humidity 50 %
• Beam quality QC = TPR20,10 (calibration)
• Source chamber distance 100 cm
• Depth in water (d) 5 cm (60Co)
10 cm (MV Photons)
• Field size (50% isodose) 10 x 10 cm2 at depth d
SCD = 100 cm
d
Field size = 10 × 10 cm2
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 26
Ionisation chamber: Reference conditions (2)
Calibration beam qualities QC available in CH for MV Photons
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 27
Ionisation chamber: Reference conditions (3)
Electrons:
• Temperature T0 = 293.15 K = 20oC
• Pressure p0 = 1013.25 hPa
• Relative humidity 50 %
• Beam quality QC= R50 (calibration)
• Source surface distance 100 cm
• Depth in water (d) d = 0.6 R50 – 0.1 gcm-2
• Field size (50% isodose) 15 x 15 cm2 at phantom surface
SSD = 100 cm
d
Field size = 15 × 15 cm2
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 28
Ionisation chamber: Reference conditions (4)
Calibration beam qualities QC available in CH for MV Electrons
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 29
Ionisation chamber: Advantages / Disadvantages
• High accuracy and reproducibility
• Necessary correction factors well understood
• Instant readout
• Finite measurement volume
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 30
Thermoluminescent detectors (TLD) Principle (1)
• Upon absorption of radiation, some materials retain part of the absorbed
energy in meta-stable states
• When this energy is subsequently released in the form of light, this
phenomenon is called luminescence
• Light may be ultraviolet, visible or infrared – depending on the material
• Two types of luminescence, distinguished by time delay between
stimulation and emission of light:
- Fluorescence: time delay 10-10 … 10-8 s
- Phosphorescence: time delay > 10-8 s
• Most commonly used materials in clinical dosimetry:
- LiF:Mg,Ti
- LiF:Mg,Cu,P
- Li2B4O7:Mn
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 31
TLD: Principle (2)
Irradiation
• Crystals contain impurities (type 1)
• They lead to meta-stable energy levels
(„Storage traps“)
• Upon irradiation, electrons are shifted to
conduction band
• They may either recombine directly ...
• ... or become trapped
Conduction band
Valence band
Storage trap
Direct
recombination
Ionising
radiation
Conduction band
Valence band
Storage trap
Heat
Light emission
Recombination center
Readout
• Crystals contain impurities (type 2)
• They facilitate recombination of electrons
with holes („recombination centers“)
• Upon heating, electrons are shifted to
conduction band
• They release light when they combine
with a hole at the recombination center
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 32
TLD: TLD reader
• Heater
• Photo-multiplier tube (PM)
- detects light from TLD
- converts it to an
amplified electrical signal
• Electrometer records PM signal
• Display of signal vs. temperature:
“Glow curve”
• Dose determined from area below peak
- Calibration
- Energy correction
- Non-linearity correction
- Fading
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 33
TLD: Applications
• In-vivo dosimetry
• Monitoring for radiation protection
• Dose distributions
• TLD intercomparisons organised by SSRMP
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 34
TLD: Advantages / disadvantages
• TLDs are available in various geometric shapes
• Can be made small in size => point dose measurements
• Many TLDs may be used in a single exposure
• Cheap
• No instant readout
• Readout time consuming
• Accurate results require careful calibration and handling,
as well as significant time and effort
• Signal erased during readout
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 35
Film dosimetry: Principle
Radiographic film:
• Base layer covered with a sensitive
emulsion of AgBr crystals in gelatine
• Irradiation: AgBr is ionised:
Ag+ ions are reduced to elementary Ag:
Ag+ + e- → Ag
• Ag is black and forms a „latent“ image
• Development: other Ag+ ions in one crystal
are reduced, if elementary Ag is present
• Fixation: rest of AgBr (in undeveloped
grains) is washed away
• => Permanent image of dose distribution
Coating
Base (typ. 200 μm)
Emulsion (10 … 20 μm)
Electron micrograph of AgBr grains in gelatine
Typical size 0.1 … 3 μm
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 36
Film dosimetry: Optical density
• Light transmission through the film is a function of the film opacity
• Can be measured in terms of Optical density (OD) with a densitometer
• Optical density is defined as:
⎟⎠⎞
⎜⎝⎛=
I
IOD 0
10log 0I
I= Initial light intensity
= Intensity transmitted through the film
• Relationship between dose and OD:
- „Not strictly“ linear
- Depends on film and processing
- Described by sensitometric curve
- Must be established before use of film for dosimetry
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 37
Film dosimetry: Sensitometric curve
Regions / parameters:
• Fog: OD of unexposed film
• Speed: exposure required to
produce an OD>1 over the fog
• Toe: transition to linear part
• Gamma: slope of the linear part
• Latitude: range of exposures
that fall in the linear part
• Shoulder: Saturation of OD for
high exposures
OD
Exposure
7
6
5
4
3
2
1Fog Toe
Linear part
Shoulder
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 38
Film dosimetry: Applications
• Portal imaging
• Qualitative dose measurements
• Quantitative dose measurements: need careful calibration, use and analysis
• Quality control of radiotherapy machines, e.g.:
- Congruence of light and radiation fields
- Dose profile at given depth in a phantom
- …
• Verification of treatment techniques in phantoms
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 39
Film dosimetry: Advantages / Disadvantages
• Film can be archived
• High 2D resolution
• Very thin: does not disturb beam
• Processing facilities (development, fixation) required
• Not trivial to achieve reproducible processing of the film
• Variation between films and production batches
• Quantitative dosimetry needs careful calibration
• Useful dose range of film is limited
• Energy dependence, in particular for lower photon energies
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 40
Film dosimetry: Radiochromic film
• More recent development: Radiochromic film
• Principle: contains dye that is polymerised
and develops a blue color upon exposure to radiation
• Self-developing, requires neither development nor fixation
• Sensitometric curve must be measured with densitometer
• Advantages with respect to radiographic film:
- No film processing => no quality control of film processing
- Grain-less material => higher resolution
- Can be used in regions with high dose gradients
- Energy dependence less pronounced
• Disadvantage: less sensitive than radiographic film
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 41
Portal dosimetry
• Dose measurement / imaging in treatment beam
- Verify treatment portals, compare with simulator radiographs
- Verify patient setup
• Traditional method: film dosimetry using dedicated film types
- Drawbacks:
o Image quality poor compared to conventional X ray images
o Requires time and effort
o Offline evaluation
• More recent development
and nowadays a standard:
EPID = Electronic portal imaging device
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 42
Portal dosimetry
EPID system consists of:
• Suitable radiation detector
- Fluoroscopic detector
- Segmented ionisation chamber
- Amorphous silicon detector
• Data acquisition system to transfer detector information to a computer
• Software to process information and convert it to an image
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 43
Portal dosimetry
• Amorphous Silicon Detector
• Array of typ. 200 000 pixels
• Pixel pitch: typ. 0.8 mm MV photon
Electron
a-Si Photodiode
Phosphor
Copper plate
Glass substrate
Light
FET transistors
for readout
• Cut through
one pixel:
Typ. 500 columns
Typ.
400
rows
Beam
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 44
Small field dosimetry
• Smallest “standard” photon fields have transverse field size down to
approximately 4 x 4 cm2
• Certain advanced radiotherapy techniques use the superposition of
multiple small fields, e.g.
- IMRT
- Stereotactic radiosurgery
(see lectures “Treatment planning systems”, “Special techniques”)
• Issues with small fields:
- Penumbra regions overlap
- Multiple steep gradients at individual field edges
- Modelling in treatment planning systems
- Dose measurement (small volumes / steep gradients)
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 45
Small field dosimetry
• Overlapping of penumbra regions
• Transverse dose profile
(see lecture “Beam production”)Drel (%)
D0
100
50
xField size
Penumbra region
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 46
Small field dosimetry
• Overlapping of penumbra regions
Drel (%)
x
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 47
Small field dosimetry
• Overlapping of penumbra regions
Drel (%)
x
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 48
Small field dosimetry
• Overlapping of penumbra regions
Drel (%)
x
Slight reduction of output factor
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 49
Small field dosimetry
• Overlapping of penumbra regions
Drel (%)
x
Significant reduction of output factor
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 50
Small field dosimetry
• Dose measurement (small volumes / steep gradients)
- Sensitive volume of detector should be small compared to field size
- Positioning of detector
- Disturbance of radiation field by detector
- No secondary electron equilibrium in transverse field direction
- …
• No “standard approach” yet; ongoing investigations / “research”
PTW pinpoint ionisation chamber
Vsens = 16 mm³
PTW silicon diode detector
Vsens : disc of 1mm2 area
PTW diamond detector
Vsens = 1 … 6 mm³
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 51
Small field dosimetry
• Dose measurement in fluence modulated fields (IMRT):
- Film
- Segmented ionisation chamber
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 52
Small field dosimetry
• Dose measurement in fluence modulated fields (IMRT): Gamma method
Reference distribution
Measured distribution
Position
Acceptance criteria
ΔDmax (e.g. 3 %)
Δdmax (e.g. 3 mm)
• γ < 1 =>
• γ > 1 =>
ΔDmax Δdmax
2max
2
2max
2
mind
d
D
D
ΔΔ
+ΔΔ
=γDose
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 53
Small field dosimetry
• Dose measurement in fluence modulated fields (IMRT): Gamma method
Position
ΔDmax Δdmax
Dose
Reference distribution
Measured distribution
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 54
Small field dosimetry
• Dose measurement in fluence modulated fields (IMRT): Gamma method
Position
ΔDmax Δdmax
Dose
Reference distribution
Measured distribution
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 55
Small field dosimetry
• Dose measurement in fluence modulated fields (IMRT): Gamma method
Position
ΔDmax Δdmax
Dose
Reference distribution
Measured distribution
Dosimetry / 3.9.2007 / Dr. M. Sassowsky / KSL 56
The end ...
• Thank you for your attention !
• Questions ?