reference dosimetry for megavoltage therapy beams: electrons - … · 2010-03-22 · • tg-51 ion...

Reference Dosimetry for Megavoltage Therapy Beams:

ElectronsDavid Followill Ph.D

Radiological Physics CenterUT M.D.Anderson Cancer Center

Houston TX

• Based on an Absorbed Dose to Water (in Water) Standard (60Co): • Conversion to Absorbed dose at other energies and modalities

(electrons) is by correction factors, kecal, k’R50, and Pgr (electrons).• Based on Bragg Gray cavity theory.• All of the chamber correction factors introduced in TG-21 are

incorporated into Kecal, and k’R50.• Many chamber specific corrections are averaged into “classes” of

chambers, so only a few k’R50 values are needed.

Protocol for Clinical Reference Dosimetry of Protocol for Clinical Reference Dosimetry of HighHigh--Energy Photon and Electron Beams.Energy Photon and Electron Beams.

Med. Phys. 26, p1847 – 1870 (1999)TG-51

• Requires Calibration in Water ( at least annually): Plastics may be used for monthly output checks, but

must be referenced to the water calibration by a correction factor.

• Electrons •Energy characterized by depth of 50% of max dose, d50Reference measurement depth, dref, is related to d50.

Protocol for Clinical Reference Dosimetry of Protocol for Clinical Reference Dosimetry of HighHigh--Energy Photon and Electron Beams.Energy Photon and Electron Beams.

Med. Phys. 26, p1847 – 1870 (1999)TG-51

Absorbed Dose Calibration FactorAbsorbed Dose Calibration FactorIdeal:

Where: is the chamber Absorbed Dose Calibration Factor specific for the energy and modality of the beam being measured.

(NK for Kilovolt X-rays)

Pragmatic: Too expensive to be practicalWe understand the behavior of chambers in the megavoltage range.Cobalt 60 is still available, and very reproducible.Cobalt 60 is a near perfect beam to use as the reference energy for

chamber calibration.

• other energy/modality specific corrections.

QwD,N M D •=

QwD,N

CowD,

60

N M D •=

Chamber Calibration FactorChamber Calibration FactorChamber Calibration Factor

• Obtain from ADCL• Chamber waterproofing material:

– 1 mm Acrylic (PMMA) wall– Provided by ADCL– Waterproof chambers

Calibrate P-P ChamberCalibrate PCalibrate P--P ChamberP Chamber• ADCL’s:

– TG-39, in water – 60Co– Evidence of problems with kecal

• User: (Recommended)– TG-39 in water/plastic – Cylindrical Chamber is NIST traceable– TG-51 In water– high energy electron R50 near 7.5 cm– compare with cylindrical chamber

Electron EquationElectron Equation

factor correctionenergy of component gradient-non k

Correction Gradient P

k k P

N k k P M D

'R

Qgr

R'R

Qgr

CowD,ecal

'R50

Qgr

Qw

50

5050

[Gy]60

=

=

=

=

(Eq 6)

kecal is conversion of ND,w from 60Co to electrons

kQ

correction for gradient at the point of calibration (dref)

=QgrP

( )( )refraw

refrefQgr dM

0.5rdM P +=

ecal'RR k kk

5050=

(Eq 21)

(Eq 4)

(Eq 5)

For ElectronsFor Electrons50R

QgrQ k P k =

TG-51 MeasurementsTGTG--51 Measurements51 Measurements

Electrons:• Look up kecal for your chamber.• Search for Imax and I50 (use 0.5 rcav shift)• Determine R50

• Determine dref and k’R50

• Move chamber to physical dref (no shift)• Measure Ppol and Pion

• Move chamber to dref + 0.5 rcav

• Calculate the gradient correction, Pgr

kecal Electronskkecalecal ElectronsElectrons• dummy factor

carries into ( = dose calib factor at reference electron

energy d50 = 7.5cm) • Simplifies the calculations • Allows for a specific electron calibration factor

to be introduced later.• Includes L/ρ and Prepl for R50 = 7.5 cm and

L/ρ, Prepl, Pwall for Cobalt 60.

NCowD,

ecalQ

wD,N

ecalQ

wD,N

Electrons:Reference Conditions

Electrons:Electrons:Reference ConditionsReference Conditions

• Field Size:assure full lateral scatter for R50

10 cm x 10 cm for R50 < 8.5 cm20 cm x 20 cm for R50 > 8.5 cm

• Reference Depth, dref

dref = 0.6R50 - 0.1 [cm]

• Nominal SSD

(Eq 18)

ElectronsClinical % dd

ElectronsElectronsClinical % ddClinical % dd

• Measure % ionization• Shift to effective point of measurement• Convert ionization to dose -- using TG-25

(revised L/ρ)• Use %dd to shift from dref to dmax, not %dIon

Parallel Plate and diode need no shift

Effective point of measurementEffective point of measurement

deff = dmeas - f ( rcav)

f = 0.5 rcav for electrons

f = 0 for parallel plateinner surface of front window

rcavCenter of chamber

Effective pt of measurement

Electron source

x

Effective Point of MeasurementEffective Point of Measurement

Water surface

Parallel plateCylindrical

X X

Physical depthEffective depth

Beam Quality Specification for Electron Beams

Beam Quality Specification for Beam Quality Specification for Electron BeamsElectron BeamsSpecified by R50

R50 = depth (cm) at which dose falls to 50% of max for a 10 cm x 10 cm field.

(20cm x 20cm for R50 > 8.5cm)

R50 = 1.029 I50 - 0.06 (cm) [I50 < 10cm]

R50 = 1.059 I50 - 0.37 (cm) [I50 >10cm]

{I50 = depth of 50% ionization}

(Eq 16)

(Eq 17)

Reference depth: Reference depth: ddrefref

dref = 0.6R50 - 0.1 [cm]

• Energy Dependent Factor - chamber specific(Includes ratio of L/ρ • Prep • Pcel for arbitraryelectron energy to that for R50 = 7.5 cm)

• well behaved (observed )

(cyl) = .9905 + 0.071e-(R50/3.67)

(cyl cham for 2cm < R50 < 9cm), 0.2% error for Farmer chambers.

Fig 5 & 7 (cyl) Fig 6 & 8 (pp)

'R50

k (Eq 19)

'R50

k

'R50

k

For ElectronsFor ElectronsFor Electrons

60 Cobalt at evaluatedcelwallrepl

water

air

7.5 Renergy at evaluatedcelrepl

water

airecal

|

|

PPPL

PPL

k50

•••⎟⎟⎠

⎞ρ

••⎟⎟⎠

⎞ρ

==

7.550Renergy at evaluatedcelrepl

water

air

50Renergy the at evaluatedcelrepl

water

air50R

|

|

PPL

PPL

k'

=••⎟⎟⎠

⎞ρ

••⎟⎟⎠

⎞ρ

=

PhantomsPhantomsPhantoms

• Water only– Annual calibration

• Plastics:– Weekly/Monthly– Compare with water at annual calibration.

Chamber ProtectionChamber ProtectionChamber Protection

• Waterproof Chambers–no protective sleeve needed

• Other Chambers–1 mm thick Acrylic protective sleeve

Chambers:parallel plate vs cylindrical

Chambers:Chambers:parallel plate vs cylindricalparallel plate vs cylindrical

• Electrons:– Parallel Plate Chambers: RECOMMENDED– Cylindrical Chambers: ACCEPTABLE

–P-P Chamber required for R50 ≤ 2.6 cm

Chamber PositionCylindrical Chamber (e-)

Chamber PositionChamber PositionCylindrical Chamber (e-)

• Clinical depth dose data: –effective pt of measurement

• Beam quality specification: –effective pt of measurement

• Calibration: –Physical center of chamber–gradient correction included in Pgr

M = PionPTPPelecPpolMraw [C or rdg] (Eq 8)

Pion = Collection efficiency correction

PTP = Temp-Press correction

Pelec = Electrometer factor

Ppol = Polarity Correction

Mraw = uncorrected charge reading

PPionion Pulsed BeamPulsed Beam

(Eq 11)

VH = normal operating potential of chamber

VL = reduced potential on chamber -

= raw reading with full potential

= raw reading with reduced potential

HrawM

LrawM

/2VV HL<

( )⎟⎟⎠

⎞⎜⎜⎝

⎛−

⎟⎟⎠

⎞⎜⎜⎝

⎛−

=

L

HLraw

Hraw

L

H

Hion

VV

MM

VV

VP00.1

0.975

0.980

0.985

0.990

0.995

1.000

1.005

120 130 140 150 160 170 180 190 200

Sequence #

Res

pons

e (N

orm

to 1

st rd

g @

full

bias

)

6 e@ dm

18 x@ d10

--- MonitorΔ NEL, NEL'X PTW, PTW'

Cap-G, Cap-C

Pion using the equations in TG-51For VH/VL = 2

1

1.01

1.02

1.03

1.04

1.05

1.06

1 1.01 1.02 1.03 1.04 1.05 1.06MH/ML

Pion

continuous radiation

pulsed radiation

Polarity CorrectionPolarity Correction

M+(M-) is the charge collected with positive (negative) polarity on the collector

Mraw = charge collected with normal polarity

raw

rawrawpol 2M

M - M P−+

= (Eq 9)

For ElectronsFor ElectronsFor Electrons

= gradient correction

( )( )refraw

cavrefQgr dM

r5.0dM P += (Eq 21)

QgrP

QgrP =

M (dref + 0.5rcav)

M raw(dref)

Whole equation with %Whole equation with %dddd

[Gy] (1/pdd) N k k P P P P P M D CowD,ecal

'R50

QgrpolionelecTPrawQ

w60

=

ImplementationImplementationImplementation

Current Implementation StatusCurrent Implementation Status

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Nov-99

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May-09

TOTAL 1494 of 1623 ACTIVE INSTITUTIONS (92%)

Only 8% to go

WhatWhat’’s the Holdup?s the Holdup?• New Air Kerma standard

–TG-51 ≈ TG-21 depending on the chamber

• Time and effort required (everyone is very busy)

• New equipment requirements (chambers and phantoms)

Equipment NeedsEquipment Needs

• Properly sized “liquid” water phantom (30x30x30 cm3)– Don’t use the scanning tank– Adequate scatter conditions– Easy reproducible setup

Chamber Holder and PositionerChamber Holder and Positioner• Holder

– Versatile to hold different chambers

– Rigid (sensitive volume perpendicular to water surface)

– No lateral displacement with depth

– Accurate sub-millimeter placement at any depth

– Verify accuracy prior to initial use– Remote electronic control is nice

Ion ChambersIon Chambers• TG-51 ion chambers vs NEW ion

chambers– Most are similar in design but now

waterproof1. Wall material2. Radius of air cavity3. Presence of Al electrode4. Wall thickness

– AAPM working group to determine the kR50,

kecal for new chambers

Ion Chambers Ion Chambers -- ElectronsElectrons• Parallel-plate or cylindrical chambers okay

• Cylindrical for energies > 6 MeV per protocol (R50 ≥ 2.6 cm)• Cylindrical = parallel plate if care in placement

• Always use a parallel plate chamber for 4 MeV beamsCaution as to where the inside surface of the front window is located

Ion Chambers Ion Chambers -- ElectronsElectrons• All chambers must have an ADCL calibration

coefficient EXCEPT PARALLEL PLATE CHAMBERS– AAPM recommendation is to cross calibrate parallel

plate chamber with cylindrical chamber in a high energy electron beam (worksheet C a la TG-39)

– ADCL ND,w – good TG-51 kecal – bad– Use of (ND,w•kecal) results in an error of 1-2%ONE EXCEPTION – Exradin P11 seems to be okay– AAPM working group determining new kecal values

Measurement TechniquesMeasurement Techniques• Accurate placement of cylindrical ion

chamber at depth– Whether manual or electronic motor driven

there must be a starting reference pointTwo techniques

1. Surface method

Water

Air

Correct Position

Measurement TechniquesMeasurement Techniques2. “Cowboy” method

Cut ruler down to minimize surface area

Cut ruler by the chamber radius and wall thickness

weights

U-shape plastic attached flush with end of ruler

Ion chamber

Water surface

• Accuracy depends on cutting ruler

• Used for reference starting point

• Periodic check of depth

Measurement TechniquesMeasurement Techniques• Parallel plate ion chambers

1. Flat surface makes it easy to measure depth2. Accurate ruler needed3. Must know where the inside surface of the

front window is located

Beam Quality Conversion Beam Quality Conversion FactorsFactors

• Electrons – kR50– Only small figures, no tables– Good figures at:

http://www.physics.carleton.ca/~drogers/pubs/papers/tg51_figures.pdf

Beam Quality Conversion Beam Quality Conversion FactorsFactors

• Electrons – 4 MeV beams (R50 < 2.0 cm)– Only use parallel plate chamber– Need to extrapolate curve

– Equation good downto 1 cm

1

Charge MeasurementsCharge MeasurementsM = Pion • PTP • Pelec • Ppol • Mraw

• PTP correction factor– Mercury thermometers and barometers most

accurate (but they are no longer kosher)– Hg barometers T&G corrections needed– Quality aneroid or digital can be used

• Check annually against a standard• Digital purchased with a calibration does not mean

accurate but rather what it read at certain pressures or temperatures

Charge MeasurementsCharge Measurements• Pelec correction factor

– ADCL calibration for each scale needed

• Ppol correction factor– Change polarity requires irradiation (600 to 800 cGy) to

re-equilibrate chamber– Use of eq 9 in TG-51 requires that you preserve the sign of

the reading or

– Ppol should be near unity for cylindrical chambers and slightly larger correction for parallel plate chambers

raw

rawrawpol M

MMP

2

−+ +=

Charge MeasurementsCharge Measurements

• Pion correction factor– High dose rate capabilities result in higher Pion

– Change in bias requires irradiation (600-800 cGy) to re-equilibrate chamber.

0.975

0.980

0.985

0.990

0.995

1.000

1.005

1.010

1.015

0 20 40 60 80 100 120 140 160 180

Sequence #

Res

pons

e no

rm to

1st

rdg

(-300

V) fo

r a b

eam

Cl 2100CD (2105) data (75 mu) dated 19-Feb-00Monitor's drift due to Ktp & machine

fluctuation(All other chamber data are norm to monitor)

@ d5

6 x

--- Monitor ◊ A-12' Δ NEL'' X PTW''

A-12

PTW' '

NEL' '

@ d10

6 x @ d10

18 x

@ d5

18 x@ dm20 e@ dm

6 e

Charge MeasurementsCharge Measurements• Pion correction factor

– Use eqs. 11 and 12 to calculate Pion– As a check if using VH/VL = 2 (within 0.1%)

• Pulsed beam : Pion = MH/ML if MH/ML < 1.02

• Continuous beam : Pion = {(MH/ML – 1)/3}+1

– Pion depends on chamber, beam energy, linac and beam modality

• Tends to increase with energy

Charge MeasurementsCharge Measurements• Electron beam gradient (Pgr) correction factor

– Only for cylindrical ion chambers– Ratio of readings at two depths

– The reading at dref+0.5rcav should have the same precision as the reading at dref since:

Dose = M(dref) • (many factors) • M(dref+0.5rcav)M(dref)

( )( )refraw

cavgr dM

rP 5.0dM ref +=

Charge MeasurementsCharge Measurements• Electron beam gradient (Pgr) correction factor

– E < 12 MeV; Pgr >1.000– E ≥ 12 MeV; Pgr ≤ 1.000– Why? Because for low electron energies dref = dmax

and this places the eff. pt. of measurement in the buildup region thus a ratio of readings greater than 1.000.

– At higher electron energies dref is greater than dmaxand as such the eff. Pt. of measurement is on the descending portion of the depth dose curve thus a ratio of readings less than 1.000.

0

20

40

60

80

100

0 2 4 6

Depth (cm)

Perc

ent D

ose


60

80

100

Dos

e

6 MeV

xx

Physical depth

Effective depth

( )( )refraw

cav

dMr5.0dM ref +

0

20

40

60

80

100

120

0 5 10 15Depth (cm)

Perc

ent D

ose


80

100

120 Physical depth

xxdref

dref+0.5rcav

Effective depth

( )( )refraw

cav

dMr5.0dM ref +

Clinical Depth DoseClinical Depth Dose• For electrons – depth dose correction for ≥

16 MeV is significant (~98.5% - 16 MeVand ~95.5% - 20 MeV)– Caution!!! Super big problem if you use %

depth ionization data (3-5% error for high energy electron beams)

SummarySummary• Implementation is straightforward

1. Must read the protocol and follow the prescriptive steps

2. Many suggestions to clarify confusion have been made

3. RPC will assist you and answer questions• Differences between TG-51 and other

protocols such as TG-21 and TRS 398 are minimal.

reference dosimetry for megavoltage therapy beams: electrons - … · 2010-03-22 · • tg-51 ion...

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