RBE of the M.I.T. Medical Therapy Neutron Beam in Monolayer Cultures of Hela Cells

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Abstract
Due to the possibility of initiating another clinical trial of boron-neutron capture therapy of malignant brain tumors, it is important to obtain more information on the physical characteristics and biological effectiveness of the neutron beam to be used for the therapeutic irradiations. This paper describes the first of a series of experiments designed to determine both the physical properties and biological effects of this beam. The latter were studied in HeLa cells with no boron present; a situation simulating that of normal brain tissue which may be exposed during a therapeutic irradiation of neoplastic tissue containing log. The reactor beam was found to be composed of three major factions; gamma rays, fast neutrons, and thermal neutrons. Dosimetry using LiF thermoluminescence and a tissue equivalent ionization chamber, a silicon diode semiconductor, and gold foil activation respectively, showed that 50% of the dose arose from the gamma component and 50% from the neutron component. The RBE for this beam compared to 250 kVp X-rays (RBE = 1.0) was 1.51, the value being obtained by comparing the D° values of each dose-survival curve. Since the RBE of the gamma component compared to 250 kVp X-rays is 0.8 and the reactor beam RBE was 1.51, the RBE of the neutron component was approximately 2.2 for reproductive failure in HeLa cells.

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