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

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

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.

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)

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

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

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

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= 0.034 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

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 0.033 to 0.024 (cGy/s) Dγ decreases from 0.029 to 0.021 (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

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

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.

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.

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

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.

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%

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

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

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

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

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.

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

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)

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