Kilovoltage X
X-R
R Dosimetry
Ray
for Radiatio
on Therapy
C M Chharlie
C.-M.
h li Ma
M
Dept. of Radiation Oncologgy, FCCC, Philadelphia, PA
Clinical Dosimetryy Meaasurements in Radiotherapy
py
2009 AAPM Summer School
Coloradoo College, CO
June 21-25, 2009
Outlline

Kil
Kilovoltage
lt
x-ray do
dosimetryi t a review
i

TG-61 formalism forr kilovoltage x-ray dosimetry

Clinical implementat
p
tion of the TG-61 protocol
p

Summary of TG-61 recommendations
r

Uncertainty analysiss
Therapax HF150 Superfficial
ficial Unit By Pantak, Inc
A X-ray
X ray unit from Gulmay
Gulmay Medical Ltd
The physics of kV
V x-ray dosimetry





Very short electron rangges (< 0.5 mm water)
L
Large
scatter
tt contributio
t ib tions andd SSD,
SSD field
fi ld size,
i
beam quality dependentt
Kerma = dose (also Kcoll = K as negl. brem., <0.1%)
Bragg Gray cavity cond
Bragg-Gray
ditions very difficult to
fulfill - even for air-filleed ionization chambers
Ionization chambers callibrated as “exposure
meters” and used as “ph
photon detectors”
Kil
Kilovoltage
lt
x-ray dosimetryd i t
a review
i

Kilovoltage x-ray dosimetry is the main theme in the first
70 years following the discovvery of x-rays
x rays

The introduction of the roenttgen
g in 1928 at Stockholm
Congress of Radiology markked the beginning of precise
physical measurement of raddiation exposure (dose)

The universally adopted instrrument to measure exposure
i the
is
h free
f air
i chamber
h b (±1%
% agreement between
b
national
i l
labs in 1932)
Exposure, X
dQ
dQ
X
( kg/c)
d
dm
where dQ is the absolute valu
ue of the total charge of the
ions of one sign produced in (dry) air when all the
electrons liberated by photon
ns in air of mass dm are
completely stopped in air
Measurement off Exposure
X
Free Air Chhamber
D
Q

AD L
“Modern” Dosimetry for
f Kilovoltage X-Ray

ICRU Report
p 23 ((1973)) significant
sg
changes
g made
40-150 kV in-air method
d, >150 kV in-phantom

NCRP Report 69 (1981) only protocol for N. Ame.
10 kV and above, in
in-air
air method, no BSF given

IAEA Report 277 (1987) significant changes made
10-100 kV in-air method
d, >100 kV in-phantom
“Modern” Dosimetry for
f Kilovoltage X-Ray

IPEMB Code of Practice (1996) with three ranges
Very llow- (<
V
( 1mmAl)
1
Al) iin-p
phantom,
h
lowl
(1 8
(1-8mmAl)
Al)
in-air, medium-energy (>0
0.5mmCu) in-phantom

NCS Code of Practice (19997) two energy ranges
50 - 100 kV in-air method
d, 100 - 300 kV in-phantom

IAEA Code
d off Practice
i (20
( 000)) - new recommendations
d i
Absorbed dose based, con
nsistent with other beams
Kilovoltage x-ray dosimetry

For low
low-energy
energy (40 - 150 kV
kV, 8mm Al HVL)
x-rays - the backscaatter method

F medium-energy
For
di
y (100 - 300 kV
kV, 44mm C
Cu
HVL) x-rays
y - the in
n-phantom
p
Method
(except for NC
CRP Report 69)
AAPM TG
G-61 Report
( Med. Phys. 28 (6) 2001 868893 )
Charles Coffey
Chihray Liu
Ravi Nath
Jan Seuntjens
Larry DeWerd
Charlie Ma (chair)
Stephen Seltzer
What’s
W
at s New in AAPM
A
TG-61?
G6 ?

Use bboth
U
th th
the in-air
i i andd in-phantom
i h t
methods
th d for
f tube
t b
potentials 100 - 300 kV
V

More complete data (foor water, tissue & bone)

Recommendations for relative
r
measurements

Recommendations for QA
Q and consistency check
Detectors for kV
k x-ray beams

Air filled ion chamberrs are recommended for
Air-filled
absolute dose measureements

Diode, film,
Diode
film diamond detectors for relative
measurements
Beam quality
y specification

Use a “narrow
narrow beam (good beam) geometry
geometry”
●

Half-Value Layer exprressed in mm Ai or Cu
for 40
40-150
150 kV x-rays
x rayss: use mmAl
for 100 - 300 kV x-raays: use mmCu
Tub
be poten
ntial (k
kV)
300
200
100
1
2
3
HVL (m
mmCu)
4
Beam quality
y specification

Use both tube potentiaal and HVL to specify
beam quality for cham
mber calibration

Use HVL to specify beam quality for
determination of cham
mber correction and
conversion factors
Table 1:
UW ADCL and NIST beams coompared.
NIST BEAM QUALITIES
UW ADCL BEAM QUALITIES
BEAM
Code
L30
L40
L50
L80
L1001
HVL
(mm Al)
0.22
0.49
0.75
1.83
2.8
HC
M20
M30
M40
M50
M60
0.152
0.36
0.73
1.02
1.68
M100
5.0
60
57
58
58
59
BEAM
Code
UW30-L
UW40-L
UW50-L
UW80-L
UW100-L
HVL
(mm Al)
0.22
0.49
0.75
1.83
2.80
56
60
61
58
58
79
64
66
66
68
UW20-M
UW30-M
UW40-M
UW50-M
UW60-M
72
UW80-M
UW80
M2
UW100-M
0.153
0.354
0.73
1.02
1.68
2.96
4.98
6.96
10.2
14 9
14.9
18.5
79
63
64
64
66
68
72
78
87
94
98
1.86
2 82
2.82
63
76
M150
M200
M250
10.2
14 9
14.9
18.5
87
95
98
UW120-M2
UW150-M
UW200 M
UW200-M
UW250-M
S75
S60
1.86
28
2.8
63
75
UW75-S
UW60 S
UW60-S
HC
All beams are matched as closely as possible too available NIST beam qualities.
I i ti Cham
Ionization
Ch mber
b C
Calibration
lib ti
 Free-in-air Kair calibraation
N K  K air
a /M
 Free-in-air X calibration
n
NX  X / M
Chamber stem ef
NK  N X (W / e)air (1- g)
-1
Kair  X (W / e)air (1- g)
-1
in air
Farm
1.6
RK
1.5
1.4
1.3
NACP
1.2
1.1
1.0
Cap
Capin
0.9
0.8
diode
0.7
0.6
0.5
N23342
0.4
0.3
0.2
50
100
150
200
energy / kVp
250
300
Markus
Farmer
1.6
RK
1.4
NACP
1.2
Diode
1.0
Capintec
0.8
0.6
N23342
0.4
Markus
0.2
50
100
150
200
250
300
Spokas
Formalisms
o a s s for
o KV
KV X-ray
ay Dosimetry
os et y

For 40
F
40-300
300 kV bbeam
ms, recommendd th
the backb k
scatter method if point of interest is on the
surface

For 100-300 kV beam
ms, recommend the inphantom
h
method
h d if point
p i off interest
i
is
i at a depth
d h
RK
110
NACP
diode
Farmer
S k
Spokas
100
TLD
N23342
Markus
90
0
1
depth / g/cm2
2
Capintec
RK
NACP
100
diode
90
Farmer
80
Spokas
70
TLD
N23342
60
Markus
50
2
3
4
5
6
7
depth / g/cm2
8
9
10
Capintec
Th Backscatter
The
B k tt r (in-air)
(i i ) M
Method
th d
 For surface dose deterrmination
Dw  MNK (en
e /  ) Pstem,air Bw
w
air
●
D i ti for
Derivation
f the
th Back
B k
kscatter
tt (I
(In-air)
i ) Metho
M th

Determine the air kerma at a point in air in absence of the
chamber
inair
Kair
 MN
M K Pstem,airi
i

Convert air kerma to water kerma
k
by

in-air
in
i
w
Kwini airi  Kair
(

/

)
en
air
r
Derive water kerma on the su
urface using a backscatter factor
Kw  KwinairBw

Derive absorbed dose to watter from water kerma assuming
charged particle equilibrium
Dw  K w CPE exists
Th IIn-Phan
The
Ph ntom
t
Method
M th d
 For dose determinationn at a depth
Dw  MNK (en / ) P
P
w
air sheath Q,cham
●
D i ti for
Derivation
f the
th In-Phantom
I Ph t
M
Method
th d

Determine the air kerma at a point in water in absence of the
chamber
inwater
Kair
 MN
M K PQ,
i
Q cham
h Psheath
h h

Convert air kerma to water kerma
k
by
in-water
w
w
Kw  Kair
(en / )air

Derive absorbed dose to watter from water kerma assuming
charged particle equilibrium
D w  K w CPE exists
In-air mass energy-abso
orption coefficient ratio
Table 2 Ratios of mass energy-absorption coeffficients averaged over the primary photon
spectrum for the beams described in Table 1.
HVL
(mm
Al)
0.300
0.381
0.875
1.35
2.65
4.76
9.17
14.5
17.6
19.8
20.8
HVL
(mm
Cu)
0.010
0.012
0.027
0.422
0.090
0.195
0.574
1.71
3.01
4.32
4.92
w/air
tissue/w
1.037
1.033
1.023
1.022
1.025
1.034
1.057
1.088
1.102
1.108
1.109
0.917
0.918
0.922
0.926
0.933
0.942
0.960
0.979
0.986
0.989
0.990
muscle/
w
lung/w
skin/w
bone/w
1.016
1.020
1.030
1.032
1.032
1.029
1.018
1.003
0.996
0.993
0.992
1.031
1.035
1.045
1.047
1.046
1.040
1.026
1.005
0.997
0.993
0.992
0.890
0.893
0.902
0.909
0.920
0.934
0.956
0.979
0.985
0.988
0.989
4.20
4.28
4.51
4.45
4.25
3.82
2.885
1.741
1.276
1.080
1.032
In-water mass energy-ab
bsorption coefficient ratio
Table 3 Ratios of mass energy-absorption coeffficients averaged over the photon
spectrum
p
at 2 cm depth
p in water irradiated by
y thhe beams described in Table 1. The field
2
size is 100 cm defined at 50 cm SSD.
HVL
(mm Al)
0.300
0.381
0.875
1.35
2.65
4.76
9.17
14.5
17 6
17.6
19.8
20.8
HVL
(mm Cu)
0.010
0.012
0.027
0.422
0.090
0.195
0.574
1.71
3 01
3.01
4.32
4.92
w/air
tissuue/w
muscle/w
lung/w
bone/w
1.022
1.019
1.018
1.020
1.025
1.032
1.049
1.077
1 094
1.094
1.103
1.105
0.921
0.922
0.924
0.929
0.935
0.941
0.954
0.972
0 981
0.9
0.986
0.988
1.030
1.033
1.035
1.035
1.033
1.030
1.022
1.008
1 000
1.000
0.996
0.994
1.046
1.049
1.050
1.049
1.047
1.042
1.031
1.013
1 002
1.002
0.996
0.994
4.54
4.61
4.63
4.46
4.23
3.91
3.23
2.145
1 560
1.560
1.255
1.150
Water kerma based
d backscatter factor
Table IVb ((continued).
) Water kerma based backscatter
b
factors for a water pphantom as
a function of field diameter (d), source surrface distance (SSD), and radiation quality
Beam quality
(HVL).
HVL (mm Cu)
SSD
(cm)
10
SSD
20
30
50
100
d
(cm)
1
2
3
5
10
15
20
1
2
3
5
10
15
20
1
2
3
5
10
15
20
1
2
3
5
10
15
20
1
2
3
5
10
15
20
0.1
1.062
1.120
1.159
1.210
1.269
1 287
1.287
1.292
1.061
1.116
1.158
1.214
1.290
1 320
1.320
1.333
1.063
1.120
1.164
1.220
1.297
1 330
1.330
1.348
1.065
1.121
1.163
1.225
1.308
1 345
1.345
1.361
1.062
1.121
1.163
1.224
1.310
1 353
1.353
1.373
0.2
1.057
1.118
1.161
1.224
1.306
1 335
1.335
1.344
1.058
1.118
1.164
1.232
1.331
1 377
1.377
1.397
1.060
1.122
1.168
1.242
1.348
1 401
1.401
1.426
1.059
1.121
1.169
1.240
1.350
1 408
1.408
1.439
1.059
1.121
1.169
1.239
1.349
1 413
1.413
1.446
0.3
1.056
1.119
1.161
1.226
1.316
1 348
1.348
1.361
1.055
1.119
1.168
1.242
1.352
1 407
1.407
1.434
1.056
1.119
1.169
1.242
1.363
1 417
1.417
1.446
1.054
1.118
1.170
1.247
1.367
1 433
1.433
1.471
1.055
1.117
1.170
1.245
1.370
1 447
1.447
1.490
0.4
1.054
1.113
1.155
1.221
1.313
1 348
1.348
1.362
1.054
1.114
1.161
1.238
1.353
1 412
1.412
1.441
1.054
1.113
1.161
1.239
1.366
1 429
1.429
1.464
1.053
1.114
1.163
1.244
1.372
1 443
1.443
1.486
1.053
1.114
1.165
1.243
1.378
1 456
1.456
1.502
0..5
1.05
52
08
1.10
1.15
50
17
1.21
1.31
11
1 3448
1.34
1.36
62
53
1.05
1.11
10
55
1.15
1.23
33
53
1.35
1 4115
1.41
47
1.44
1.05
52
08
1.10
1.15
55
35
1.23
1.36
67
1 4338
1.43
1.47
78
52
1.05
1.11
11
57
1.15
1.24
40
76
1.37
1 4552
1.45
99
1.49
1.05
52
11
1.11
1.16
60
41
1.24
1.38
83
1 4663
1.46
1.51
13
0.6
1.050
1.106
1.147
1.214
1.310
1 348
1.348
1.363
1.051
1.107
1.152
1.229
1.349
1 411
1.411
1.443
1.050
1.105
1.152
1.231
1.360
1 433
1.433
1.473
1.050
1.108
1.154
1.235
1.371
1 450
1.450
1.498
1.050
1.108
1.156
1.237
1.378
1 461
1.461
1.514
Applicator size
s
0.8
1.046
1.103
1.143
1.209
1.307
1 347
1.347
1.364
1.048
1.102
1.147
1.219
1.339
1 403
1.403
1.436
1.047
1.101
1.146
1.221
1.347
1 422
1.422
1.464
1.047
1.103
1.148
1.226
1.360
1 446
1.446
1.495
1.047
1.104
1.150
1.227
1.369
1 458
1.458
1.516
1.0
1.043
1.097
1.135
1.199
1.294
1 332
1.332
1.349
1.045
1.097
1.140
1.209
1.326
1 389
1.389
1.421
1.044
1.096
1.139
1.211
1.332
1 405
1.405
1.446
1.045
1.097
1.140
1.214
1.344
1 428
1.428
1.478
1.045
1.098
1.142
1.217
1.353
1 441
1.441
1.499
1.5
1.037
1.081
1.116
1.170
1.254
1 289
1.289
1.303
1.038
1.084
1.122
1.184
1.291
1 350
1.350
1.381
1.038
1.084
1.121
1.184
1.292
1 360
1.360
1.399
1.038
1.084
1.121
1.184
1.304
1 379
1.379
1.428
1.038
1.085
1.122
1.188
1.311
1 393
1.393
1.447
2.0
1.033
1.071
1.102
1.151
1.227
1 260
1.260
1.273
1.033
1.074
1.107
1.164
1.260
1 316
1.316
1.345
1.033
1.073
1.107
1.164
1.263
1 327
1.327
1.364
1.034
1.073
1.106
1.163
1.274
1 346
1.346
1.391
1.034
1.074
1.107
1.167
1.278
1 356
1.356
1.406
3.0
1.026
1.057
1.081
1.122
1.186
1 213
1.213
1.225
1.024
1.056
1.082
1.127
1.204
1 251
1.251
1.278
1.024
1.056
1.084
1.130
1.214
1 270
1.270
1.302
1.025
1.056
1.084
1.131
1.222
1 285
1.285
1.325
1.025
1.057
1.085
1.132
1.226
1 291
1.291
1.334
4.0
1.021
1.046
1.067
1.101
1.154
1 178
1.178
1.188
1.020
1.046
1.067
1.104
1.168
1 207
1.207
1.230
1.020
1.046
1.068
1.106
1.177
1 226
1.226
1.254
1.020
1.047
1.069
1.108
1.184
1 237
1.237
1.272
1.020
1.047
1.070
1.109
1.188
1 244
1.244
1.282
5.0
1.017
1.038
1.054
1.082
1.126
1 146
1.146
1.155
1.018
1.039
1.057
1.088
1.141
1 174
1.174
1.194
1.018
1.038
1.055
1.087
1.147
1 189
1.189
1.213
1.018
1.040
1.057
1.089
1.152
1 195
1.195
1.226
1.018
1.040
1.057
1.090
1.155
1 204
1.204
1.237
Ratio of backscatter factors,
f
bone to water
Table 4. Ratios of backscatter factors, bone to wateer, for photon beams 50 - 300 kV
(
(0.875
- 20.8 mm All HVL)) with
i h different
diff
field
fi ld size
i s defined
d fi d at different
diff
SSD.
SSD
HVL
(cm)
50
(mm Al)
0.875
2.65
9 17
9.17
14.5
17.6
20.8
0.875
2.65
9.17
14.5
17.6
20.8
0 875
0.875
2.65
9.17
14.5
17.6
20.8
30
10
(1 x 1 cm2)
0.943
0.972
1 022
1.022
1.039
1.036
1.022
0.943
0.973
1.023
1.039
1.037
1.023
0 943
0.943
0.973
1.023
1.039
1.037
1.022
(2 x 2 cm2)
0.916
0.938
1 015
1.015
1.065
1.069
1.048
0.916
0.935
1.017
1.067
1.067
1.046
0 916
0.916
0.941
1.017
1.064
1.066
1.044
Bbone
Bw
(4 x 4 cm2)
0.890
0.892
0 984
0.984
1.079
1.100
1.073
0.890
0.888
0.988
1.076
1.101
1.078
0 893
0.893
0.901
0.980
1.071
1.092
1.075
(10 x 10 cm2)
0.865
0.829
0 885
0.885
1.028
1.095
1.094
0.867
0.834
0.891
1.025
1.090
1.091
0 874
0.874
0.849
0.915
1.032
1.086
1.087
(20 x 20 cm2)
0.858
0.798
0 827
0.827
0.958
1.049
1.079
0.861
0.807
0.835
0.967
1.047
1.078
0 873
0.873
0.836
0.878
1.005
1.070
1.078
Overall chamber correction factor
Table VII.
VII Overall chamber correction factorrs PQ,cham for common cylindrical chambers in
medium-energy x-ray beams. The data appliees to 2 cm depth in the phantom, and 100 cm2
field size.
Chamber
Type
HVL
(mmCu)
0.10
0.15
0 20
0.20
0.30
0.40
0.50
0.60
0.80
1.0
1.5
2.0
2.5
3.0
4.0
NE2571
Capintec
PR06C
PT
TW
N30001
Exradin
A12
NE2581
NE2611
or
NE2561
1.008
1.015
1 019
1.019
1.023
1.025
1.025
1.025
1.024
1.023
1.019
1.016
1.012
1.009
1.004
0.992
1.000
1 004
1.004
1.008
1.009
1.010
1.010
1.010
1.010
1.008
1.007
1.006
1.005
1.003
1.0004
1.0013
1 0017
1.0
1.0021
1.0023
1.0023
1.0
023
1.0022
1.0021
1.0018
1.0
015
1.0012
1.0010
1.0006
1.002
1.009
1 013
1.013
1.016
1.017
1.017
1.017
1.017
1.016
1.013
1.011
1.010
1.008
1.005
0.991
1.007
1 017
1.017
1.028
1.033
1.036
1.037
1.037
1.035
1.028
1.022
1.017
1.012
1.004
0.995
1.007
1 012
1.012
1.017
1.019
1.019
1.019
1.018
1.017
1.014
1.011
1.009
1.006
1.003
Chamber Sheath Correction
C
Factor
Table IV: The Monte Carlo calculated correctioon factors ps for polystyrene ( = 1.06
gcm3) sleeves of thickness t.
a the same as Table II
II. The 11-

t Other conditions are
statistical uncertainties are smaller than 0.001.
Beam quality
B
lit
(mm A1)
t = 0.5 mm
1.04
2.94
4.28
9 20
9.20
13.0
16.6
21 5
21.5
0.990
0.995
0.996
0 999
0.999
1.000
1.000
1 000
1.000
ps for
f
Pl t
Polystyrene
t = 1 mm
t = 2 mm
0.9981
0.9990
0.9993
0 9997
0.9
0.9999
0.9999
1 0000
1.0
0.962
0.981
0.986
0 994
0.994
0.998
0.999
1 000
1.000
t = 3 mm
0.943
0.972
0.979
0 992
0.992
0.997
0.999
1 000
1.000
Consistency bettween the in-air
and in-phant
in phanttom methods

S l a method
Select
h d bbased
d on
o point
i off interest
i

Check consistency onlyy if PDD can be measured
accurately

Experimental
p
studies inndicated consistent results
(about 1%) using both methods
m
at 100 and 300 kV
Ma, Li and Seuntjens (19998) Med Phys 25: 2376-84
Table II Dose ratios calculated based on Eq. 12 for a 100 kV (2.43 mm Al) beam with a
100 cm2 field defined at 80 cm SSD. The chamber readings
r
were corrected for
temperature, pressure, polarity and ion recombinatio
on. The PDD curves were measured
using
i th
the NACP chamber
h b and
d corrected
t d using
i the
th Monte
M t Carlo
C l calculated
l l t d Cz factors.
f t
The
Th
ratios of mass energy-absorption coefficients for waater to air, the backscatter factors and
the chamber correction factors were taken from the ICRU3 (reference depth = 5 cm),
IPEMB25 (reference depth = 2 cm) and the NCS26 (rreference depth = 2 cm) dosimetry
protocols. The IAEA chamber correction factors weere taken from14 while mass-energy
2
absorption
b
i coefficient
ffi i ratios
i were from
f
.For
F this
hi wo
ork,
k the
h correction
i factors
f
were taken
k
8
from ref .
ICRU ((1973)) IAEA ((1987,, 1996))
Mair
IPEMB ((1996))
NCS ((1997))
This work
27.99
1.252
27.99
1.260
27.99
1.277
27.99
1.281
27.99
1.280
13.69
1 000
1.000
13.69
1 030
1.030
25.84
1 023
1.023
25.84
1 005
1.005
25.84
0 990
0.990
  
  
1.000
1.000
1.000
0.996
1.000
PDDz=ref
0.375
0.375
0.707
0.707
0.707
R
0.960
0.938
0.956
0.976
0.990
Bw
Mz=ref
PQ,cham
en
w ,air air

en
w,air z ref

Guidelines for dosimetry
d
in other
phantom
m materials

Determine the surface dose for
f other phantom materials from
Dmed,z 0  C

Dw,z 0
where
h
C
med
w

med
w

Bmed
med

(en /  )w
Bw

Vary significantly
air
i
The backscatter factor ratios are significant for bone to water
but close to 1.0 for soft tissu
ues.
Ratio of Backscatter Factors,
Factors Bone to Water
1.10
10cmx10cm
m
1.05
1cmx1cm
1.00
20cmx220cm
0.95
0.90
0.85
0.80
0
1
2
3
4
Beam Quallity (mm Cu)
5
Relative dosimetrry measurement

L
Large
uncertainty
t i t in
i PDD measurements
t

Large uncertainty in profile measurements

Effect of electron con
ntamination

Choice of detectors

Choice of phantom materials
m
100
90
Diode
80
70
NACP
60
RK
50
Farmer
40
30
film
20
10
0
0
2
49
10
11
al axis / cm
distance from centra
12
13
(300 kkV
V beam)
(300 kV beam)
Summary of TG-611 Recommendations

Water p
phantom for absolu
ute dose determination, 2 cm
depth for > 100 kV, plastiic phantoms for routine
checks

Effective point of measureement: center of air cavity
40-70
40
70 kV: parallel plate chamber
70-300 kV: cylindrical chamber

Use both tube potential an
nd HVL for chamber
calibration

A
Appropriate
i t b
build-up
ild
f parallel
for
ll l plate
l t chambers
h b
Summaryy of TG-611 Recommendations


N
Narrow
beam ggeometry
y for HVL
V determination
What method to use depen
nding on beam quality and point
of interest (POI)
(
)
40-100 kV : only the in
n-air method should be used
100-300 kV : the in-air method if POI on surface
100-300 kV : the in-phan
ntom method if POI at a depth

Inter-compare chamber foor correction/conversion factors

Use HVL as beam quality specifier
s
for conversion and
correction factor (tabular data
d
preferred)

Quality assurance (daily,
(daily monthly,
m
monthly
annually)
Estimated combined standard uncertainty
33.5%
5% o
surfac
4.7%
a dept
C lusions
Conclu
i

Exposure/kerma basedd dosimetry procedures

Backscatter method forr both low- and mediumenergy x-ray beams
b

Complete data set avaiilable for en/,
/ B,
B PQ,cham
Q cham
and Psheath

Consistent results using both formalisms
Questions for kV
V x-ray dosimetry
1.
2.
3
3.
4.
Does a Farmer chambeer have enough buildup for
kV x-ray beams?
Does the Bragg-Gray cavity
c
theory apply to a
Farmer chamber for kV
V x-ray beams?
Is the difference between Kcoll and K significant for
kV x-ray beams?
A air-filled
Are
i fill d ionization
i i ti n chambers
h b usedd as “photon
“ h t
detectors” or “electron detectors” for kV beams?
Answ
wers:
1.
2.
3.
4.
Yes, electron ranges < 0.5mm of water
No, significant eneergy deposition from
electrons generated
d in the air cavity
No, g < 0.1%
“Photon detectors””