dosimetry measurement for beam commissioning
DESCRIPTION
Dosimetry Measurement for Beam Commissioning. Outline. The radiotherapy dosimetry chain The radiation quantities and detectors The cavity theory for dose conversion The dosimetry protocols Dose measurement without CPE present Dose measurement for IMRT verification. The Dosimetry Chain. - PowerPoint PPT PresentationTRANSCRIPT
Dosimetry Measurement for Dosimetry Measurement for
Beam Commissioning Beam Commissioning
OutlineOutline
The radiotherapy dosimetry chainThe radiotherapy dosimetry chain
The radiation quantities and detectorsThe radiation quantities and detectors
The cavity theory for dose conversionThe cavity theory for dose conversion
The dosimetry protocolsThe dosimetry protocols
Dose measurement without CPE presentDose measurement without CPE present
Dose measurement for IMRT verificationDose measurement for IMRT verification
The Dosimetry Chain Standards Labs - calibrate dosimeters
Primary/secondary dosimetry standards
Reference dosimetry - calibrate treatment unitsnational and international protocols on reference dosimetry
Relative dosimetry - obtain data for RT planningnational and international protocols, textbooks
Daily, monthly, annual QA - keep data accuracynational and international protocols, regulations
Plan/patient dose QA - validate real patient dosenational and international protocols, regulations
Radiation Quantities
Exposure
Kerma
Collision Kerma Absorbed Dose
Kerma Kerma – – kinetic energy transferred kinetic energy transferred
by indirect ionizing radiation per unit massby indirect ionizing radiation per unit mass
dm
EK trd
trtrK E
Energy Transferred, Etr
h
h
e
Q )(R - )(R E nonruoutuintr
T 0 hv - hv 21
T
Collision KermaCollision Kerma – – kinetic energy transferred from kinetic energy transferred from
indirect ionizing radiation to charged particles per unit massindirect ionizing radiation to charged particles per unit mass
enen
c EK
dm
d
ntr
c
EK
Net Energy Transferred, Etrn
T'
h
h
h
e
h
Q R- )(R - )(R ru
nonruoutuintr nE
0 ) (- - 4321 hhhh
T
Absorbed Dose -Absorbed Dose - the expectation value of the energy the expectation value of the energy
imparted by ionizing radiation to matter per unit mass at a pointimparted by ionizing radiation to matter per unit mass at a point
dm
dED
To what materials??? To what materials???
Energy Imparted, ET'
h
h
h
e
h
Q )(R-)(R )(R - )(R coutcinuoutuin E
T
0 ) (- - 4321 Thhhh
Exposure, XExposure, X
where dQ is the absolute value of the total charge of the ions of one sign produced in (dry) air when all the electrons liberated by photons (indirect ionizing radiation) in air of mass dm are completely stopped in air
kg/c)(dm
dQ X
(W/e)air is the mean energy expended in air per
ion pair formed. It gives the number of joules of
energy deposited in the air per coulomb of
charge released
CJair
/97.33e
W
Exposure and Air KermaExposure and Air Kerma
airairc e
WXK
)(
airair
en
e
WX
Measurement of ExposureMeasurement of Exposure
Free Air Chamber
Cavity ChamberCavity ChamberThe basis of an air-wall chamber is that the air surrounding the active volume can be "condensed" into a "solid air" wall
The definition of an air-wall chamber is a chamber whose walls interact with radiation in the same manner as air interacts
For a typical ion chamber, the real charge :
rawdispwallelectpolelecTPion MPPPPPPPM
Correction factors Raw reading
Ionization Chamber Dosimetry
Farmer chamber: the thimble wall is made of graphite and the central electrode is made of aluminum. The collecting volume of the chamber is nominally 0.6 cm3.
Energy response of a Farmer chamber
Detectors for Radiotherapy DosimetryDetectors for Radiotherapy Dosimetry
Air-filled ion chambers Air-filled ion chambers are recommended for are recommended for absolute dose absolute dose measurements (and Fricke measurements (and Fricke dosimeters, TLDs)dosimeters, TLDs)
Diode, TLDs, film, and Diode, TLDs, film, and other solid/liquid detectors other solid/liquid detectors for relative measurementsfor relative measurements
EDR2
det,det med
enmed
D
D
Cavity Theory Cavity Theory – –
converts dose from one medium to another mediumconverts dose from one medium to another medium
Large cavity theory (for "photon detectors"): If the detector size is much greater than the mean electron range in a phantom irradiated by a photon beam (and CPE exists), then
Dose to detector
Dose to medium
Conversion factor
det,det
medmed sD
D
Cavity Theory (cont.)Cavity Theory (cont.)
Small (Bragg-Gray) cavity theory (for "electron detectors"): if the detector size is much smaller than the mean electron range in a phantom irradiated by a photon/electron beam (no need for CPE), then
Dose to detector
Dose to medium
Conversion factor
det,
det,det
)1( med
med
enmed sD
D
Cavity Theory Cavity Theory (Cont.)(Cont.)
Burlin cavity theory: for intermediate sized detectors
Dose to detector
Dose to mediumConversion factor
Bragg-Gary Cavity Theory Bragg-Gary Cavity Theory
1st condition: cavity does not perturb electron
fluence
2nd condition: dose deposited by
electrons crossing it
W
Dw
g
Dg
W
Dw
w
g
w
g
w
g
S
S
S
D
D
Bragg-Gary Cavity Theory Bragg-Gary Cavity Theory
Unrestricted stopping power
for primary electronsCPE exists
for knock-on electrons
Primary only
m
g
E
g
g
m
m
E
g
m L
dL
dL
D
D
max
max
Spencer-Attix Cavity Theory Spencer-Attix Cavity Theory
Restricted stopping power Track-end effect
Spencer-Attix theory explicitly takes into account all knock-on electrons above some energy threshold (traditionally called )
Primary & secondary
Spencer-Attix vs. Bragg-GraySpencer-Attix vs. Bragg-Gray(S-A is more accurate than B-G)(S-A is more accurate than B-G)
Charged Particle EquilibriumCharged Particle Equilibrium
Charged Particle Equilibrium (CPE) exists for a volume v if each charged particle of a given type and energy leaving v is replaced by an identical particle entering.
(Rin)c = (Rout) c
i.e., energy carried in and out by charged particles is equal
Break Down of CPEBreak Down of CPE ((CPE does not exit in many situations)CPE does not exit in many situations)
For high-energy photon beams: the attenuation of the
photon beam is significant for a full electron buildup, it
is impossible for CPE to occur.
For example, a 10 MeV photon beam is attenuated 7%
in the maximum range of its secondary electrons.
Transient Charged Particle EquilibriumTransient Charged Particle Equilibrium (TCPE) (TCPE)
((D is proportional to Kc)D is proportional to Kc)
xc
TCPE eKD '
xKcTCPE '1
cKD
Kc
Kilovoltage x-ray dosimetry- a reviewKilovoltage x-ray dosimetry- a review
ICRU Report 23 (1973) significant changes madeICRU Report 23 (1973) significant changes made40-150 kV in-air method, >150 kV in-phantom40-150 kV in-air method, >150 kV in-phantom
NCRP Report 69 (1981) only protocol for N. Ame.NCRP Report 69 (1981) only protocol for N. Ame.10 kV and above, in-air method, no BSF given10 kV and above, in-air method, no BSF given
IAEA Report 277 (1987) significant changes madeIAEA Report 277 (1987) significant changes made10-100 kV in-air method, >100 kV in-phantom10-100 kV in-air method, >100 kV in-phantom
Kilovoltage x-ray dosimetry- a reviewKilovoltage x-ray dosimetry- a review
IPEMB Code of Practice (1996) with three rangesIPEMB Code of Practice (1996) with three rangesVery low- (< 1mmAl) in-phantom, low- (1-8mmAl) Very low- (< 1mmAl) in-phantom, low- (1-8mmAl)
in-air, medium-energy (>0.5mmCu) in-phantomin-air, medium-energy (>0.5mmCu) in-phantom
NCS Code of Practice (1997) two energy rangesNCS Code of Practice (1997) two energy ranges50 - 100 kV in-air method, 100 - 300 kV in-phantom50 - 100 kV in-air method, 100 - 300 kV in-phantom
IAEA Report 398 (2000) - new recommendationsIAEA Report 398 (2000) - new recommendationsAbsorbed dose based, consistent with other beamsAbsorbed dose based, consistent with other beams
Kilovoltage X-Ray Beam Calibration (The AAPM TG-61 Protocol, C Ma et al)
Use of both in-air and in-phantom methods for Use of both in-air and in-phantom methods for tube potentials 100 - 300 kVtube potentials 100 - 300 kV
More complete data (for water, tissue & bone)More complete data (for water, tissue & bone)
Recommendations for relative measurementsRecommendations for relative measurements
Recommendations for QA and consistency checkRecommendations for QA and consistency check
Formalism for kV x-ray dosimetry
The backscatter method
/ cMKN cK
wairstemenKw BPMND ,wair )/(
Dose to detector Conversion factor
Formalism for kV x-ray dosimetry
The in-phantom method
/ cMKN cK
chamQ,sheathwair )/( PPMND enKw
Dose to detector Conversion factor
50 100 150 200 250 300
energy / kVp
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
in airFarm
RK
NACP
Capin
diode
N23342
Markus
50 100 150 200 250 3000.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Farmer
RK
NACP
Diode
Capintec
N23342
Markus
Spokas
Megavoltage Photon & Electron Calibration (The AAPM TG-51 Protocol)
TG-51 applies to clinical reference dosimetry for external beam radiation therapy using ion chambers.
Beam quality range: 60Co - 50 MV for photons 4 - 50 MeV for
electrons A water phantom (at least 30cm x 30cm x 30cm) for
clinical reference dosimetry, other phantom materials for routine checks and relative dosimetry measurements.
Megavoltage Photon & Electron Calibration (The AAPM TG-51 Protocol)
Simplification compared to TG-21 (less tabulated data). TG-21: TG-51: for photons: for electrons:
gasreplwall
w
aircapionw NPPLCPMD /
CowDQ
Qw NMkD
60
,
CowDQpolelecTPionraw
Qw NkPPPPMD
60
,
CowD
QgrecalRelecionpolTPraw
Qw NPkkPPPPMD
60
50 ,'
Beam SpecificationPhoton beam specification: %dd(10)x
%dd(10) : measured PDD at 10 cm depth in water for a 10cm x 10cm field at 100cm SSD
%dd(10)x : the photon component of the PDD at 10 cm depth in water for a 10cm x 10cm field at 100cm SSD
%dd(10)pb : the PDD at 10 cm depth in water for a 10cm x 10cm field at 100cm SSD with a 1 mm lead foil at about 50 cm from the phantom surface (or 30cm if 50cm clearance is not available)
Reference ConditionsPhoton beam measurements:
The reference depth: dref = 10 cm depth in water for a 10cm x 10cm field at 100cm SSD or SAD.
SSD SAD
10 x 10
Reference ConditionsElectron beam measurements:
The reference depth: dref = 0.6 R50 – 0.1 cm depth in water
The field size is 10x10 for E 20 MeV or 20x20 for E > 20
MeV
SSD = 90-110cm are allowed dref
dmax
R50
Equipment: Ion chamber and electrometer (calibration traceable to national standards Ion chamber and electrometer (calibration traceable to national standards
laboratories).laboratories).
Equipment for two independent checks.Equipment for two independent checks.
Voltage supply (two voltages, both signs)Voltage supply (two voltages, both signs)
Waterproofing for ion chamber (if needed): < 1 mm PMMAWaterproofing for ion chamber (if needed): < 1 mm PMMA
Water phantom: at least 30cm x 30cm x 30cmWater phantom: at least 30cm x 30cm x 30cm
Lead foil for photons 10 MV and above: 1 mm + 20%Lead foil for photons 10 MV and above: 1 mm + 20%
System to measure temperature and pressure System to measure temperature and pressure
Step-By-Step Photon Calibration Procedure
Obtain a traceable for the ion chamber. Obtain a traceable for the ion chamber.
Measure %Measure %dddd(10)(10)pbpb with a lead foil. with a lead foil.
Deduce %Deduce %dddd(10)x from %(10)x from %dddd(10)(10)pbpb for an open beam. for an open beam.
Measure MMeasure Mrawraw at 10 cm water equivalent depth with a 10cm x 10cm field at 10 cm water equivalent depth with a 10cm x 10cm field
defined at 100 SSD or SAD. defined at 100 SSD or SAD.
M = PM = Pion ion PPTP TP PPelecelecPPpolpol M Mrawraw. .
Look up Look up kkQQ (Table I or Figure 4 in TG-51 report) for the chamber. (Table I or Figure 4 in TG-51 report) for the chamber.
Finally, (Gy)Finally, (Gy)
Derive dose at other depths using PDD, TPR or TMR.Derive dose at other depths using PDD, TPR or TMR. CowDQ
Qw NMkD
60
,
CowDN
60
,
Other Dosimetry Protocols
TG-25: clinical dosimetry protocols for electron beams (absolute and relative dosimetry).
TG-40: radiotherapy QA (linacs, TPS, special procedures).
TG-53: commissioning and QA for treatment planning systems
TG-65: inhomogeneity corrections for RT dose
determination