absolute calibration lecture.pdf · 2015. 2. 5. · absolute calibration ref. aapm tg-51 report...

Absolute CalibrationRef. AAPM TG-51 report

Point of Measurement

point of measurement: the point at which the absorbed dose is measured. For cylindrical ion chambers used for clinical reference dosimetry the point of measurement is on the central axis of the cavity at the center of the active volume of the cavity and for plane-parallel chambers the point of measurement is at the front (upstream side) of the air cavity at the center of the collecting region.

Photons

Photons 1General Formalism: Dw

Q = MND ,wQ (Gy)

ND ,wQ (Gy/c) Absorbed dose to water calibration factor

for beam quality Q

ND ,wQ = KQND ,w

60Co (Gy/c)

DwQ = MKQND ,w

60Co(Gy)

ND ,w60Co (Gy/c) Absorbed dose to water calibration factor

for 60 Co provided by ADCL where ND ,w60Co = Dw

60Co

MADCL

@22ºC and 101.33 kPa

Photons 2DwQ = MKQND ,w

60Co(Gy)

M = Mraw × PTP × Pion × Pelec × Ppol

PTP =273.2 + T295.2

× 101.33P T in ºC and P in kPa

Note: When using Hg barometer P needs to be corrected for latitude and Hg temperature.

PTP : the temperature–pressure correction factor which makes the charge or measured current correspond to the standard environmental conditions for which the calibration factor applies.

Photons 3M = Mraw × PTP × Pion × Pelec × Ppol

Pion: the recombination correction factor takes into account the incomplete collection of charge from an ion chamber.

Pion(VH ) =1− VHVL

MrawH

MrawL − VH

VL

(For pulsed or pulsed swept beam)

If Pion is > 1.05, chamber should not be used!


Pelec : the electrometer correction factor. If the electrometer is calibrated separately from the ion chamber, then Pelec is the electrometer calibration factor which corrects the electrometer reading to true coulombs. Pelec is considered 1.00 if the electrometer and ion chamber are calibrated as a unit.Unit: C/rdg or C/C.


Ppol : the polarity correction factor which takes into account any polarity effect in the response of the ion chamber.

Ppol vary with beam quality and other conditions such as cable position. Therefore, it is necessary to correct for these effects by making measurements each time clinical reference dosimetry is performed.

Ppol =Mraw

+ −Mraw−

2Mraw

Photons 6

Beam quality Q is determined from %dd(10): SSD = 100 cm, field size = 10x10

If cylindrical chamber is used then shift DI curve by 0.6rcavity upstream (no correction needed for plane-parallel chambers) to get %dd curve.

AAPM’s TG–51 Protocol for Reference Dosimetry: Med Phys 26 (1999) 1847 – 1870 page 2

0 5 10 15 20

depth /cm

50

60

70

80

90

100

%d

ep

th!

ion

iza

tio

n

dmax

A

I

II

%dd(10)

0 1 2 3 4 5 6 7 8

depth / cm

0

20

40

60

80

100

I

II

a)

b)

B

dose

I50

Figure 1: Effect of shifting depth-ionization data measured with cylindrical chambers upstream by0.6 rcav for photon beams (panel a) and 0.5 rcav for electron beams (panel b) (with rcav = 1.0 cm).The raw data are shown by curve I (long dashes) in both cases and the shifted data, which are takenas the depth-ionization curve, are shown by curve II (solid line). The value of the % ionization atpoint A (10 cm depth) in the photon beam gives %dd(10) and the depth at point B (solid curve, 50%ionization) in the electron beam gives I50 from which R50 can be determined (see section VIII.C). Forthe photon beams, curve II is effectively the percentage depth-dose curve. For the electron beams,curve II must be further corrected (see section X.D) to obtain the percentage depth-dose curve shown(short dashes - but this is not needed for application of the protocol).

∼dave/tex/tg51/tg51.tex: Edited 18 Aug 1999

Photons 7

For photon energies ≥10 MV, 1mm (±20%) Pb

Pb should be placed either 50±5cm or 30±1cm above the phantom surface.

On %dd curve (with Pb for ≥10Mv or open %dd for <10MV) locate %dd(10)pb or %dd(10).


0 5 10 15 20

depth /cm

50

60

70

80

90

100

%d

ep

th!

ion

iza

tio

n

dmax

A

I

II

%dd(10)

0 1 2 3 4 5 6 7 8

depth / cm

0

20

40

60

80

100

I

II

a)

b)

B

dose

I50



Photons 8

For photon energies < 10MV

%dd(10)x = %dd(10)

For photon energies ≥10MV

foil @ 50±5cm & %dd(10)Pb ≥73%

%dd(10)x = [0.8905+0.0015 %dd(10)Pb]%dd(10)Pb

foil @ 30±1cm & %dd(10)Pb ≥71%

%dd(10)x = [0.8116+0.00264 %dd(10)Pb]%dd(10)Pb

Note: If %dd(10)Pb is less than the thresholds given above, then %dd(10)x = %dd(10)Pb.

Photons 9KQ (energy correction factor)

KQ =

Lρ⎞

⎠⎟air

water

• Prepl • Pwall • Pcel⎡

⎣⎢⎢

⎤

⎦⎥⎥Q

Lρ⎞

⎠⎟air

water

• Prepl • Pwall • Pcel⎡

⎣⎢⎢

⎤

⎦⎥⎥Co60

Pcel: corrects for the influence of the Al center electrode

Photons 10Now, select KQ (energy correction factor) for your beam’s quality and

chamber from Table 1 or Fig. 4AAPM’s TG–51 Protocol for Reference Dosimetry: Med Phys 26 (1999) 1847 – 1870 page 5

55 60 65 70 75 80 85 90

%dd(10)X

0.94

0.95

0.96

0.97

0.98

0.99

1.00

kQ

kQ_photon

A12,A1,PR05,PR05P,IC!5,IC!10

NE2561PR06C

NE2581

PTW N30002

PTW N30001,31003

NE2571, 25772505/3A, 30004

Figure 4: Values of kQ at 10 cm depth in accelerator photon beams as a function of %dd(10)x forcylindrical ion chambers commonly used for clinical reference dosimetry. When values were the samewithin 0.1%, only one curve is shown. Explicit values are given in Table I, as is a list of equivalentchambers. For 60Co beams, kQ = 1.000.


Photons 11AAPM’s TG–51 Protocol for Reference Dosimetry: Med Phys 26 (1999) 1847 – 1870 page 4

SAD Setup

10 cm

10x10

10 cm

SSD

10x10

SSD Setup

SAD

Figure 3: Schematic of the SSD or SAD setups which may be used for photon beam referencedosimetry. In both cases the ion chamber is at a water equivalent depth of 10 cm in the waterphantom. The actual value of SSD or SAD is that most useful in the clinic (expected to be about100 cm).


Schematic of the SSD or SAD setups which maybe used for photon beam reference dosimetry. In both cases the ion chamber is at a water equivalent depth of 10cm in the water phantom. The actual value of SSD or SAD is that most useful in the clinic(expected to be about 100cm).

Photons 121. Search for maximum ionization Imax (with/without Pb as appropriate)

2. Place chamber at 10cm+0.6rcav to determine %dd at 10 cm.

3. Determine %dd(10)x from ionization measurements using equations (13), (14), or (15) in the protocol.

4. Determine KQ from %dd(10)x using Table1 or Fig.4 for your chamber.

5. Move chamber to calibration depth (center of the chamber @ 10.0 cm depth).

6. Make measurements to determine Mraw, PTP, Ppol, Pion.

7. Calculate Absorbed Dose @ 10 cm depth from equation (3) in the protocol.

8. Calculate dose at ref. depth (i.e. dmax) using clinical %dd or TMR depending on the setup (i.e. SSD or SAD)

Electrons

Electrons 1

DwQ = MKQND ,w

60Co(Gy)

M and ND ,w60Co are the same as the ones in Photon Protocol.

However, KQ for electrons:

KQ = PgrQKR50

Electrons 2

KQ = PgrQKR50

KR50 is a chamber-specific factor which depends on the quality for which the absorbed-dose calibration factor was obtained and the user’s beam quality, Q, as specified by R50. KR50 is the gradient independent component of KQ.

Electrons 3

KQ = PgrQKR50

:the gradient correction factor is the component of KQ in an electron beam that is dependent on the ionization gradient at the point of measurement. For cylindrical chambers is a function of the radius of the cavity, rcav and the local gradient. is unity for plane-parallel chambers. The equivalent factor in photon beams is accounted for within KQ since it is the same for all beams of a given photon beam quality.

PgrQ

PgrQ

PgrQ

Electrons 4KR50 = ′KR50Kecal

KR50 is the gradient independent component of KQ.

Kecal : photon-electron conversion factor, is fixed for a given ion chamber. It is KR50 of an reference electron beam of quality Qecal (R50=7.5cm). Therefore:

ND ,wQecal = KecalND ,w

60Co

We can find Kecal for our ion chamber in Table III (cyl.) and Table II (pp).

Electrons 5KR50 = ′KR50Kecal

K’R50 :electron quality factor, is a function of electron beam quality given by R50.

′KR50 converts ND ,w

Qecal to ND ,wQ for user's beam of quality Q

➠ DwQ = MPgr

Q ′KR50KecalND ,w60Co

Electrons 7

For cylindrical chambers %depth ionization curve is shifted (to correct for gradient effect) by 0.5rcav (no shift for parallel-plate chambers) prior to finding depth of I50.

2≤ I50 ≤10cmI50 >10cm

R50=1.029 I50 - 0.06 cmR50=1.059 I50 - 0.37 cm


0 5 10 15 20

depth /cm

50

60

70

80

90

100

%d

ep

th!

ion

iza

tio

n

dmax

A

I

II

%dd(10)

0 1 2 3 4 5 6 7 8

depth / cm

0

20

40

60

80

100

I

II

a)

b)

B

dose

I50



Electrons 8Now that we R50 we can look up K’R50 in figures 5&7 for cyl. and 6&8 for pp. chambers for beam quality specified by R50.


2 3 4 5 6 7 8 9

R50

/cm

1.00

1.01

1.02

1.03

k’ R

50 a

t d

ref

kR50.prime

N23331

N30001

A12

NE2505.3A NE2571

NE2581PR06C/G

N30002IC10/5

N31003

PR06C/G

0.9905+0.071e(!R50/3.67)

NE2577 N30004

Exradin A1,PR05,PR05P

NE2561

Figure 5: Calculated values of k′R50

at dref as a function of R50 for several common cylindrical ionchambers. These values can be used with Eq.(6), (with a measured value of PQ

gr and a kecal valuefrom Table III) to determine the absorbed dose to water at the reference depth of dref = 0.6R50− 0.1cm.



2 3 4 5 6 7 8 9

R50

/cm

1.00

1.01

1.02

1.03

k’ R

50 a

t d

ref

kR50.prime

N23331

N30001

A12

NE2505.3A NE2571

NE2581PR06C/G

N30002IC10/5

N31003

PR06C/G

0.9905+0.071e(!R50/3.67)

NE2577 N30004

Exradin A1,PR05,PR05P

NE2561

Figure 5: Calculated values of k′R50

at dref as a function of R50 for several common cylindrical ionchambers. These values can be used with Eq.(6), (with a measured value of PQ

gr and a kecal valuefrom Table III) to determine the absorbed dose to water at the reference depth of dref = 0.6R50− 0.1cm.


fig. 5 fig. 6

Electrons 9

Or we can use R50 in the equations provided in the protocol to find K’R50.

2≤ R50 ≤9cm K’R50 (cyl) = 0.9905 + 0.0710 e(-R50/3.67)

K’R50 (pp) =1.2239 - 0.145 (R50)0.2142≤ R50 ≤20cm

Electrons 10From R50 we can determine our dref.

M = Mraw × PTP × Pion × Pelec × Ppol

Mraw, Pion, and Ppol are measured at dref.


0 2 4 6 8 10 12

depth /cm

0

10

20

30

40

50

60

70

80

90

100

110

% d

ep

th!

do

se

dd_R50_dref

dmax

R50

dref

= 0.6 R50

! 0.1 cm

Figure 2: R50 is defined as the depth, in cm, at which the absorbed dose falls to 50% of its maximumvalue in a ≥ 10 × 10 cm2 (≥ 20 × 20 cm2 for R50 > 8.5 cm) electron beam at an SSD of 100 cm.The depth for clinical reference dosimetry is dref = 0.6R50 − 0.1 cm, in the same sized beam at anSSD between 90 and 110 cm. Note that for low-energy beams, dref is usually at dmax.


dref ≈ dmax for e- < 10MeV but deeper for higher energies.

dref = 0.6 R50 - 0.1 cm➠

Electrons 11

➠

DwQ = MPgr

Q ′KR50KecalND ,w60Co

is not needed for (pp) chambers and is close to 1 for e-<10MeV; when dref ≈ dmax and is <1 when dref > (dmax + 0.5 rcav)

PgrQ

PgrQ =

Mraw (dref +0.5rcav )

Mraw dref

electrons 121. Look up Kecal for your ion chamber in Table II or III.

2. Measure the maximum ionization, Imax, and search for I50 (use 0.5rcav).

3. Determine R50 from I50 using equations provided [equations (16) & (17)].

4. Determine dref from R50 using equation (18).

5. Determine K’R50 from R50, using equations (19) & (20), or figures 5-8.

6. Move chamber center to dref (no shift).

7. Make measurements to determine Mraw, PTP, Ppol, and Pion.

8. Move chamber center to dref + 0.5rcav and measure ionization.

9. Calculate the gradient correction Pgr.

10. Calculate Absorbed Dose @ dref depth from equation (3), (4) & (5).

11. Calculate dose @ dmax using clinical %dd.

Cross calibration

cross calibration 1

1. Determine the beam quality for high energy e- beam.

2. Determine ref. depth.

3. Measure Pgr for the cyl. chamber.

4. Make ionization measurement with cyl. chamber (point of measurement @ dref).

5. Make ionization measurement with parallel-plate chamber (point of measurement @ dref).

cross calibration 2

Since: Dw( )PP = (M ′KR50KecalND ,w60Co )PP

and; Dw( )Cyl = (MPgrQ ′KR50KecalND ,w60Co )Cyl .

in order to cross calibrate the two chambers should read the same. Therefore: Dw( )Cyl = Dw( )PP

➠ Cross Calib. factor = (KecalND ,w60Co )PP =

(MPgrQ ′KR50

KecalND ,w60Co )Cyl .

(M ′KR50)PP

we don’t know this!}

cross calibration 3

from this point forward we can use this parallel-plate chamber in electron beams to determine the dose @ dref.

DwQ = M ′KR50 (KecalND ,w

60Co )PP}cross calibration

factor

Setup: Parallel Plate chamber

absolute calibration lecture.pdf · 2015. 2. 5. · absolute calibration ref. aapm tg-51 report...

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