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!
Photons 4M = Mraw × PTP × Pion × Pelec × Ppol
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.
Photons 5M = Mraw × PTP × Pion × Pelec × Ppol
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).
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 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.
∼dave/tex/tg51/tg51.tex: Edited 18 Aug 1999
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).
∼dave/tex/tg51/tg51.tex: Edited 18 Aug 1999
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
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
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.
AAPM’s TG–51 Protocol for Reference Dosimetry: Med Phys 26 (1999) 1847 – 1870 page 6
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.
∼dave/tex/tg51/tg51.tex: Edited 18 Aug 1999
AAPM’s TG–51 Protocol for Reference Dosimetry: Med Phys 26 (1999) 1847 – 1870 page 6
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.
∼dave/tex/tg51/tg51.tex: Edited 18 Aug 1999
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.
AAPM’s TG–51 Protocol for Reference Dosimetry: Med Phys 26 (1999) 1847 – 1870 page 3
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.
∼dave/tex/tg51/tg51.tex: Edited 18 Aug 1999
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