Performance of the Currently Available Epithermal Neutron Beam at the Massachusetts Institute of Technology Research Reactor (MITR-II)

Abstract

This paper describes the performance of the currently available epithermal neutron beam at MITR-II. This beam is one of the few clinically useful epithermal neutron beams available for BNCT in the world. The MITR-II epithermal neutron beam has a peak thermal neutron flux of greater than 4 × 10⁸ n/cm² s at 2 cm depth in tissue. Assuming a B-10 concentration of 30 µg/g in tumor and an effective 10 to 1 ratio of B10 in tumor to healthy tissue, the most important figures of merit for the beam are as follows. The advantage depth (useful therapeutic penetration) is in excess of 7 cm. The dose rate to tumor at the advantage depth is 9.6 RBE cGy/min. The integral ratio of dose to tumor vs. dose to healthy tissue is greater than 3 when measured along the centerline of an ellipsoidal water phantom. When used in bilateral irradiation of tumors, this beam can treat the entire brain with therapeutic advantage ratio of greater than 2.9. In addition, plans for improving the current epithermal beam through straightforward modifications are presented. This improved beam is expected to have peak thermal neutron flux in excess of 10⁹ n/cm² s with proportionally greater therapeutic dose rate and higher advantage ratio than the current epithermal neutron beam.