Nuclear Science and Technology - Volume 9, Number 4, December 2019

e 
ionization chamber (Razor chamber). The 
output correction factor [ 
 ]
is 
a 
DO DUC CHI et al. 
51 
obtained from the tabulated output correction 
factors with respect to the machine specific 
reference field as below: 
[ 
 ]
[ 
 ]
[ 
 ]
 (4) 
III. RESULTS and DISCUSSION 
A. Output factors of collimation systems 
using TRS398 CoP 
Using the conventional formula from 
TRS398 CoP, we got the result as Table I. 
Table I. Output factors of collimation systems using TRS398 CoP and Razor chamber. 
Cone (mm) 4 5 7.5 10 12.5 15 17.5 100 (square) 
6X 0.420 0.523 0.662 0.746 0.797 0.834 0.859 1 
6XFFF 0.473 0.574 0.702 0.772 0.816 0.846 0.866 1 
MLC FS (mm) 0.5 1 2 3 4 5 7 10 
6X 0.529 0.754 0.861 0.895 0.921 0.940 0.968 1 
6XFFF 0.560 0.782 0.876 0.909 0.932 0.948 0.975 1 
Jaw FS (mm) 0.5 1 2 3 4 5 7 10 
6X 0.345 0.704 0.850 0.888 0.914 0.936 0.967 1 
6XFFF 0.377 0.736 0.867 0.906 0.929 0.947 0.974 1 
B. Output factor of collimation systems 
using TRS483 CoP: 
Intermediate field ( ) of 4 × 4 
cm
2
 was selected for calculation of jaw-
shaped fields and MLC-shaped fields. For 
cone, intermediate field was 17.5 mm 
conical field because we need to 
normalize these data to that of 10 × 10 
cm
2
 field size. The results were obtained 
as Table II. 
Table II. Output factor of collimation systems using TRS483 CoP (Razor chamber and Razor diode) 
Cone (mm) 4 5 7.5 10 12.5 15 17.5 100 (square) 
Square Equi. Field 
size (cm) 0.708 0.885 1.327 1.77 2.212 2.655 3.097 10 
6X 0.522 0.599 0.713 0.773 0.814 0.841 0.864 1 
6XFFF 0.578 0.648 0.745 0.797 0.830 0.856 0.871 1 
MLC Field size(mm) 0.5 1 2 3 4 5 7 10 
6X 0.608 0.774 0.871 0.905 0.927 0.945 0.971 1 
6XFFF 0.643 0.792 0.881 0.918 0.938 0.954 0.978 1 
Jaw Field size(mm) 0.5 1 2 3 4 5 7 10 
6X 0.619 0.756 0.84 0.876 0.901 0.924 0.961 1 
6XFFF 0.652 0.771 0.846 0.884 0.907 0.928 0.963 1 
C. Comparison of results between TRS483 
and TRS398 CoP: 
Based on these results, the difference 
between ROF curves is significant between the 
two different methods (CoPs) for MLC-shaped 
field size and for jaw-shaped field size less than 
3 × 3 cm for both the 6X and 6XFFF beams. 
The smallest difference was observed 
with MLC-shaped fields while the biggest 
difference was observed with cone-shaped 
RELATIVE OUTPUT FACTORS OF DIFFERENT COLLIMATION SYSTEMS IN TRUEBEAM 
52 
fields as seen in Fig.3 and Fig.4. At 0.5 × 0.5 
cm
2
 squared field and 4 mm conical field, the 
output factor difference of 6X and 6XFFF 
beams were -44.2%/-42.2%, -13.0%/-13.0%, -
19.6%/-18.2% for jaw-shaped, MLC-shaped 
and cone-shaped fields, respectively. TRS398 
CoP gave underestimation of a relative output 
factor in comparison with TRS483 CoP. The 
large difference was always seen at field sizes 
smaller than 4 × 4 cm
2
. 
Fig. 3. Difference of TRS398 and TRS483 CoP in relative output factor of MLC and Jaws collimations. 
Fig. 4. Difference of TRS398 and TRS483 CoP in relative output factor of cone collimations. 
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
R
O
F 
Jaw Field Size (cm x cm) 
6X Jaw Output Factors 
TRS483
TRS398
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
R
O
F 
Jaw Field Size (cm x cm) 
6XFFF Jaw Output Factors 
TRS483
TRS398
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
R
O
F 
MLC Field Size (cm x cm) 
6X MLC Output Factors 
Razor Chamber -
TRS398
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
R
O
F 
MLC Field Size (cm x cm) 
6XFFF MLC Output Factors 
Razor Chamber -
TRS398
TRS483
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90 100
R
O
F 
Coned Field Size (mm) 
6X Cone Output Factors 
TRS483
TRS398
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90 100
R
O
F 
Coned Field Size (mm) 
6XFFF Cone Output Factors 
TRS483
TRS398
DO DUC CHI et al. 
53 
Noticingly, the Razor chamber’s 
reading differences between 10 × 10 cm
2
MLC-shaped field and 10 × 10 cm
2
 jaw-
shaped field were just 0.61% and 0.25% for 
6X and 6XFFF, respectively. Therefore, these 
relative output factors could be used for direct 
comparison between jaw-shaped field and 
MLC-shaped field of “the same” nominal 
field size. 
D. Comparison of results between 6X, 
6XFFF (TRS483 CoP): 
For the same collimation system, output 
factor comparisons were also made for 6X and 
6XFFF beams after applying TRS483 CoP 
(Fig. 5). The biggest differences in output 
factor were seen at 0.5 × 0.5 cm
2
 jaw-shaped 
field, 0.5 × 0.5 cm
2
 MLC-shaped field and 
4mm cone-shaped field with values of 5.3%, 
5.8% and 10.5%, respectively. 
Fig. 5. Difference in output factor of 6X and 6XFFF beams in each collimation system. 
E. Comparison of Output Factor curves 
between different collimation systems 
(TRS483 CoP): 
The relative output factor comparisons 
were made between MLC, jaws and Cone 
systems for both 6X and 6XFFF beams. 
Conical collimators are independent 
from MLC and Jaws systems. The conical 
collimation system has smallest relative 
output factor in comparison with that of MLC 
and Jaws systems for both 6X and 6XFFF 
beams as Fig. 6. 
For field sizes bigger than 1 × 1 cm
2
, jaw 
system has lower relative output factor than 
MLC’s but it is inverse for field size less than 1 
× 1 cm
2
. 
In a multi-centre analytical study of 
small field output factor calculations in 
radiotherapy reported by Krzysztof Chełmiński 
and Wojciech Bulski, for 2 × 2 cm
2
 MLC-
shaped fields of Varian linacs, the differences 
between the treatment planning system output 
factors (based on collected beam data) often 
exceeded 5% and were below 10% [11]. In 
our study, these differences were -1.1% (6X) 
and -0.5% (6XFFF) for MLC-shaped fields, 
1.3% (6X) and 2.6% (6XFFF) for jaw-shaped 
fields, -7.0% (6X) and -5.7% (6XFFF) for 
cone-shaped fields. The smaller differences 
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
R
O
F 
Jaw Field Size (cm) 
TRS483 Jaws ROF 
6X
6XFFF
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10
R
O
F 
MLC Field Size (cm) 
TRS483 MLC ROF 
6X
6XFFF
0.5
0.6
0.7
0.8
0.9
1
2.5 5 7.5 10 12.5 15 17.5
R
O
F 
Coned Field Size (mm) 
TRS483 Cone ROF 
6X
6XFFF
RELATIVE OUTPUT FACTORS OF DIFFERENT COLLIMATION SYSTEMS IN TRUEBEAM 
54 
observed in our study for MLC-shaped field 
may came from our small field detector, the 
Razor chamber. 
A multinational audit of small field 
output factors calculated by treatment 
planning systems used in radiotherapy, the 
ROF for small fields calculated by TPSs 
were generally larger than measured 
reference data. On a national level, 30% and 
31% of the calculated ROF of the 2 × 2 cm
2
field exceeded the action limit of 3% for 
nominal beam energies of 6 MV and for 
nominal beam energies higher than 6 MV, 
respectively [12]. 
The discrepancy above may come 
from accuracy of treatment planning 
algorithms on measured output factors, 
especially for small fields. 
Fig. 6. Difference in output factor of difference collimation system for 6X and 6XFFF beams. 
CONCLUSIONS 
An international code of practice for the 
dosimetry of small static fields used in external 
beam radiotherapy (TRS483 CoP) was 
successfully applied to recalculate relative 
output factors for cone system with correction. 
Relative output factors for jaw collimation 
system were extensively obtained for field size 
less than 3 × 3 cm
2
 for Eclipse v.13.6 for 6X 
and 6XFFF beams using TRS483 CoP. 
Relative output factors were also measured for 
MLC collimation system to be compared with 
that of the jaw collimation system. The 
discrepancy of output factor between jaw-
shaped fields and MLC-shaped fields suggests 
that jaw-based beam data itself may not 
suitable for MLC-based treatment planning. 
Additional measurement of small beam 
percentage depth dose and profiles as well as 
specific modelling of photon beam for MLC 
system may be required. 
REFERENCES 
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[2]. International Atomic Energy Agency, TRS483, 
“An International Code of Practice for 
Reference and Relative Dose Determination”, 
November, 2017. 
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
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R
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