Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Health Sciences

Committee Chair

Linda H. Nie

Committee Member 1

Shuang Liu

Committee Member 2

Jean M. Plantenga

Committee Member 3

Keith Stantz

Abstract

Boron neutron capture therapy (BNCT) is an attractive radiotherapy modality that utilizes high-LET particles to deliver the radiation dose. Different from conventional treatments, BNCT has the ability to target tumor cells by injecting patients with a boron10 (B-10) compound that selectively accumulates inside the tumor and irradiating the target area with a neutron beam. The radiation dose produced is very localized due to the short travel range of the resulting particles and limited to the B-10 containing cells. The surrounding healthy tissues receive minimal dose. At present, the BNCT neutron sources are mainly nuclear reactors and large particle accelerators. These types of neutron sources have high capital expenses, are difficult to maintain and manipulate, require high voltage, and cannot be widely installed in clinical settings. A deuterium-deuterium (DD) neutron generator is a competitive alternative for neutron source due to its low cost, compact size, low acceleration voltage, and relatively simple installation. The objective of this dissertation research is to investigate and design a DD neutron generator-based BNCT system.

In the first study, the optimal neutron energy for BNCT of brain tumors at various depths was determined. When the neutron source had an energy in the epithermal range, between 0.5 eV and 10 keV, the dose ratio between the tumor and the brain was maximized. The alpha dose component accounted for approximately 80% of the total tumor dose. As the neutron energy increased to 2.45 MeV, the alpha dose fraction was reduced to 5%. With an epithermal neutron source, 50% of the total brain dose originated from photons while neutrons and alphas contributed to the other 50%. Although higher energy neutrons delivered more dose per source neutron to the tumor, more than 80% of the dose was deposited by neutrons, and the brain received the same amount of dose as the tumor. The benefits of the high-LET particles were reduced because the high-energy neutrons were not thermalized when they reached the tumor site.

The second specific aim focused on designing a beam shaping assembly for a DD neutron generator source to moderate the fast DD neutrons and reduce radiation contaminations in the beam. The final optimized layout included a moderator combination of 45-cm Li7 F and 10-cm MgF2, a 30-cm lead reflector, 10-cm lead collimator, and 0.02- cm cadmium filter. The neutron spectrum in air had 9.4 x 104 nepi/cm2 -s, 0.03 for thermalto-epitherml ratio, 5.9 x 10-13 Gy-cm 2 /nepi, and 2.1 x 10-13 Gy-cm 2 /nepi. For the in-phantom evaluation, the advantage depth (AD) was 12.5 cm, the advantage ratio was 4.4, and the dose rate at AD was 2.9 x 10-3 cGy-Eq/min. The maximum skin dose was 0.6 Gy-Eq. The only deficiency of the system was the inadequate neutron flux that DD neutron generators currently produce.

Finally, the dose distributions of the designed BNCT system in a cadaver-based phantom were examined in MCNP. The brain obtained a maximum dose of 12.5 Gy-Eq, minimum dose of 1.2 Gy-Eq, and average dose of 5.3 Gy-Eq. Results from this dissertation demonstrated the feasibility of a DD neutron generator-based BNCT system for treatment of brain tumors.

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