MICRODOSIMETRIC ANALYSIS OF ABSORBED DOSE IN BORON NEUTRON-CAPTURE THERAPY

The biological effectiveness of absorbed dose in BNCT depends on relat ive contributions from the gamma, neutron and the thermal neutron capt ure dose components, and on the resulting spectrum of secondary charge d particles. This spectrum is a variable, likely to change within the patient, with subcellular boron location, and between different beams. In the present work, a dual proportional counter microdosimetric tech nique to measure and analyse the absorbed dose and its single event sp ectrum is described. Single event spectra of the gamma, neutron and B- 10 dose components have been separately calculated from experimental m easurements. Spectra for a 6 mu m diameter volume (simulating a cell n ucleus), with a uniform distribution of B-10 and with B-10 localised e xternal to it have been calculated. For the same macroscopic B-10 dose , the dose to the nucleus with the B-10 localised outside the nucleus is reduced to half of that when it is uniformly distributed. The reduc ed dose is of a lower average lineal energy compared to the uniform do se; its relative effectiveness depends on the radiosensitivity of the irradiated tissue, being larger for sensitive tissues and smaller for radioresistant tissues.