1. Sheng Yang a, Yen-Wan Hsueh Liu b
The Collimator Design of Accelerator-based Epithermal Neutron Beam for Boron Neutron Capture Therapy
Sheng Yang a, Yen-Wan Hsueh Liu b
aDepartment of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan , ROC
bInstitute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu, Taiwan , ROC
June 2014

2. Outline Introduction Material and Method Result and discussion
Neutron source
Result and discussion
Collimator Design
In-air calculation
In-phantom calculation
Conclusion

3. Introduction
Accelerator based BNCT has become more and more attractive due to its being able to be installed in the hospital.
This presentation is focused on the collimator design for a chosen beam shaping assembly (BSA) for 30 MeV/1 mA proton beam bombarded on Be target.
Compared with Li target, Be target has
higher melting point,
higher neutron yield,
lower gamma-ray yield per neutron.

4. Material and Method Monte Carlo code: MCNPX
Cross section library : ENDF/B-7
Be target thickness =12cm × 12cm × 0.55cm
Flux-to-dose conversion factor: based on MIT Caswell data (R. G. Zamenhof et al,1990)

5. Be(p,n) neutron source Neutron yield per proton:0.0299
Material and Method
Be(p,n) neutron source
Neutron yield per proton:0.0299
Forward:70% Backward: 30%
Neutron source produced by 30MeV/1mA proton : 1.9 x 1014 s-1

6. Design Criteria of Epithermal Neutron Beam for BNCT
Material and Method
Design Criteria of Epithermal Neutron Beam for BNCT
IAEA Recommendation
Фepi at collimator exit > 1 ×109 n cm-2 s-1
Df/Фepi < 2×10-11 cGy-cm2/epi
Dγ/Фepi < 2×10-11 cGy-cm2/epi
Фth/Фepi at collimator exit < 0.05
Forwardness J/Фepi > 0.7

7. Result and Discussion- Collimator Design
The chosen BSA composed of iron and Fluental as fast neutron moderator, and Bi for gamma ray attenuation. The total thickness is 80 cm.
Collimator
Bi truncated cone: thickness10 cm, exit diameter =14 cm
Surrounded by PE (for fast neutron shielding) mixed with Li2CO3 (for reducing thermal neutron and gamma ray)
followed by 5 cm of lead (for gamma ray shielding).
Design 1
Entrance diameter = 70 cm
Length = 25 cm
Design 2 (longer)
Length = 40 cm

8. Result and Discussion – beam characteristics at collimator (design 1) exit
Фepi =1.74 ×109 n cm-2 s-1 (+ 30%, compared to BSA exit)
Df= cGy/s
Dγ=0.029 cGy/s
Df/Фepi : 1.93 ×10-11 cGy cm2/n (- 25% compared to BSA exit)
Dγ/Фepi : 1.65 ×10-11 cGy cm2/n
Фth =7.3 x 107 n cm-2 s -1
Фth /Фepi : 0.043
forwardness J/Фepi= 0.66
J/Фepi
Фth
(n/cm2 s)
Фepi Df
(cGy /s) Dγ
Df/Фepi
(cGy cm2/n)
Dγ/Фepi
After BSA -
2.68×107
1.35×109
0.0341
0.0046
2.53×10-11
3.43×10-12
Collimator 1
0.66
7.34×107
(1σ=0.1%)
1.74×109
0.0336
(1σ=1.1%)
0.029
1.93×10-11
1.65×10-11

9. Design 2 : Longer collimator (+15cm)
At collimator exit
Epithermal neutron flux decreases by 24% from 1.74 to 1.32×109 n cm-2 s-1
Df decreases from to (cGy/s)
Dγ decreases from to (cGy/s)
Df/Фepi decreases to 1.84×10-11 cGy cm2/n
Dγ/Фepi decreases to 1.61×10-11 cGy cm2/n
Better forwardness (J/Фepi changes from 0.66 to 0.7 )
Forwardness
Фth/Фepi
Фepi
(n/cm2 sec) Df
(cGy/s) Dγ
Df/Фepi
(cGy cm2/n)
Dγ/Фepi
Collimator 1
(long 25 cm )
(diameter 70 cm )
0.663
0.042
1.74×109
(1σ=0.1%)
0.0336
(1σ=1.1%)
0.029
1.93×10-11
1.65×10-11
Collimator 2
(long 40 cm )
0.709
0.047
1.32×109
0.0243
(1σ=1.26%)
0.021
(1σ=1.3%)
1.84×10-11
1.61×10-11

10. Effect of Collimator Length on Flux Profile at Beam Exit
Collimator length : 25 cm vs 40 cm
Longer collimator gives better flux profile at beam exit.
lower Фf and Фγ at outside region (radius >7 cm)
107

11. Effect of Collimator Length on Dose Profile at Beam Exit
The longer collimator gives better dose profile.
lower Df/Фepi and Dγ/Фepi the outside region (radius >7 cm)
Longer collimator may be a better choice.

12. In-phantom Calculation
The free beam quality of both collimator designs 1 and 2 satisfy the criteria suggested by IAEA
Their performance are further evaluated by the in- phantom calculations.
A simple brain equivalent phantom is used.

13. In-phantom Calculation
Description of phantom
Rectangle 18cm ×18cm ×20cm
Brain tissue with density 1.04g/cm3
Tissue composition is from ICRU-46 report
Phantom location
at beam exit and 10 cm away from exit.
RBE=3.2 for neutron, =1 for gamma ray
CBE=1.3 for normal tissue ,=3.8 for tumor
assuming T/N=3
Tally center region of phantom :1cm × 1cm × 20 cm

14. 1-D Flux distribution in phantom
Collimator 1 (inner diameter:70 cm, length:25 cm)
Phantom at collimator exit
Fast and epithermal neutron flux decrease in the phantom ( mainly due to scattering with hydrogen), and leading to the increase of thermal neutron flux.
The neutron interaction with hydrogen causes the production of photons.

15. In-air and in-phantom flux comparison
When phantom is located at the beam exit
Longer collimator design (design 2) although gives lower epithermal neutron flux at the beam exit (24% lower), but due to the improvement of forwardness, the maximum thermal neutron flux in the phantom is only 20% lower.
The effect of forwardness is even more pronounced If the phantom is placed at 10 cm away from the beam exit (as usually happens in the clinical trial):
Longer collimator design (design 2) gives 19% lower epithermal neutron flux at 10 cm from the beam exit. But the maximum thermal neutron flux in the phantom is only 13% lower.
Phantom location (cm)
Epithermal neutron flux in air (n/cm2/s)
Epithermal neutron flux at 0 cm of phantom (n/cm2/s)
Maximum Thermal neutron flux (n/cm2/s)
Collimator 1
(25 cm long ,
diameter 70 cm )
1.74×109
2.7×109
3.75×109 10
5.94×108
1.09×109
1.75×109
Collimator 2
(40 cm long,
1.32×109
(76%)
2.12×109
(79%)
3.04×109
(81%)
4.82×108
9.26×108
(85% )
1.52×109
(87%)
1σ<1%

16. Parameters to assess the in-phantom beam quality
Advantage depth (AD):the depth in tissue at which the total weighted dose rate of tumor equals the maximum weighted dose rate of the health tissue.
Advantage depth dose rate (ADDR):The dose rate at AD, which is also the maximum normal tissue dose rate.
Advantage ratio (AR):the ratio of the integrated tumor dose to the integrated health tissue dose (from the surface to AD).
Therapeutic ratio (TR): Ratio of tumor dose to maximum normal tissue dose.
ADDR AD

17. Comparison of dose rate distribution in phantom - located at beam exit

18. Comparison of dose rate distribution in phantom - at 10 cm away from the beam exit

19. In-phantom Performance Comparison of Both Collimator Designs
(long 25 cm )
(diameter 70 cm )
Distance
from exit(cm)
B concentration
in blood (ppm) AD
(cm) AR
ADDR
(cGy/
min)
*Treatment
time(min)
*Max tumor
dose (Gy)
Фepi at exit:
1.74×109 (n/cm2/s) 10
8.07
3.77
87.6 18
40.8 25
9.11
5.67
86.4
11.5
58.4
9.29
5.66
41.2 24
57.9
*Normal tissue tolerance:10Gy
Collimator 2
(long 40 cm )
(diameter 70 cm )
Distance
from exit(cm)
B concentration
in blood (ppm) AD
(cm) AR
ADDR
(cGy/
min)
*Treatment
time(min)
*Max tumor
dose (Gy)
Фepi at exit:
1.32×109 (n/cm2/s) 10
8.08
3.83 44
22.7
41.8 25
9.11
5.75
70.5
(81%)
14.1
58.7
9.15
5.71
35.5
(86%)
28.1
58.4

20. In-phantom Performance Comparison of both Collimator Designs
for blood boron =25 ppm, : AD = 9 cm, AR = 5.7,
As the blood boron concentration increases, the AD, AR and tumor dose increases.
If phantom is located at the beam exit,
the ADDR of longer collimator design (design 2) is 19% lower, compared to design 1.
If phantom is located at 10 cm away from the beam exit
the ADDR of longer collimator design (design 2) is 14% lower compared to design 1.
The treatment time of collimator design 2 is only ~ 20% longer.
Both collimator designs give the same beam in-phantom quality in terms of AD, AR
Although collimator design 2 gives lower epithermal neutron flux intensity at the beam exit. it makes the beam more forward. Therefore the difference of in-phantom maximum normal tissue dose are not as large. The difference in treatment time is therefore only ~20%. All within 30 minutes under 10 Gy (W) limitation for the normal tissue.

21. Therapeutic Ratio Boron concentration in blood:25ppm
TR for collimator design 1 and 2 are about the same.
TR for phantom at beam exit and at 10 cm away from beam exit are about the same

22. Conclusion Longer collimator ( length 40 cm)
In-air
Lower epithermal neutron flux, but
Better flux/dose profile at the beam exit
more forward
Beam in-phantom performance
Very close beam quality,
the difference of maximum normal tissue dose are not as large
(for phantom at 10 cm from exit, 25ppm 10B in blood, T/N= 3)
AD ~ 9 cm , AR ~ 5.7
Treatment time < 30 min, for normal tissue <10 Gy (W),
Maximum tumor dose : 58 Gy (W)

23. Contributors
Zhen-Fan You, Institute of Nuclear Engineering and Science, National Tsing Hua University, Taiwan 
Min-Hao Hsieh, Institute of Nuclear Engineering and Science, National Tsing Hua University, Taiwan