New approaches to the dosimetry of boron neutron capture therapy at MIT-MGH

Theoretical, computational and experimental advances in the dosimetry of boron neutron capture therapy have been made at MIT-MGH. Monte Carlo calculations of three-dimensional dose-equivalent distributions within a brain model have been carried out using the ANDY code. These calculations indicate that a D2O-moderated 252Cf neutron beam produces essentially the same boron dose distribution as the MIT therapy beam. However, a significant reduction in normal tissue dose is obtained using the 252Cf beam because of its low incident gamma-ray contamination. A ''tailored'' beam of epithermal neutrons, produced by D2O moderation and 10B filtration of a 252Cf source, produces a superior depth/dose distribution in addition to a reduced normal tissue dose. The ANDY code has also been used to calculate the boron dose distribution in a bone tumour located on the femur within an ''adult'' thigh. For the case of 10B concentrations of 50μg/g in normal bone and 200 μg/g in tumour, a ratio of 9 to 1 in total dose between tumour and underlying bone and a ratio of 4:1 between tumour dose and entrance dose is achieved using a reactor neutron beam. Preliminary to the development of an on-line boron dose monitor, a Ge(Li) detector spectrometer was used to measure the 0.477 MeV radiative capture gamma ray produced by the 10B(n,α)7Li reaction within a boron loaded water phantom. The area under the 10B peak is observed to scale linearly with boron concentration over the range of 0.2 to 20.0 μg 10B/g H2O. This technique can also be used to provide a rapid in-vitro assay of 10B in the blood stream before a therapy irradiation. (author)