Dosimetry Measurement for Beam Commissioning

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