A new TEPC for BNCT: microdosimetric measurements at the thermal column of the TAPIRO reactor

Boron Neutron Capture Therapy (BNCT) is a rather innovative cancer radiotherapy; at present it is object of increasing interest especially for treatment of those tumors where conventional therapy gives not satisfactory results (Glioblastoma and Malignant Melanoma). The therapy takes advantage of both chemical and physical aspects B: the possibility of selectively accumulating in tumor tissue the isotope B through tumor-seeking boron compounds and the high cross section of the nuclear reaction of thermal neutrons with B. The short range in tissue (∼ 10 μm) of the emitted alpha and Li particles allows localized energy release in tumors, with consequent damage to the cancerous cells, while preserving the surrounding healthy tissue. Thermal and epithermal neutron beams are commonly used in BNCT. Both types of beams include contributions by fast, epithermal, and thermal neutrons, as well as gamma rays from the neutron source and from the capture and scattering of neutrons in the beam line structures. Further radiation components are produced within the body in the form of boron disintegration products, protons from nitrogen capture reactions and gamma rays from hydrogen capture reactions. The gamma, neutron and boron neutron capture dose components; all contribute to the total physical dose. Hence the dosimetry of BNCT is a complex task. requiring the discrimination of the different components of the radiation field. Experimental microdosimetry, through the analysis of lineal energy spectra, allows relatively easy evaluation of different dose components at a given point, while requiring fewer and faster measurements than conventional dosimetry techniques for photon and neutron dose discrimination. In the framework of SPES project [1] the construction at LNL of a BNCT center for treatment of skin melanoma is foreseen. In order to optimize the neutron moderator, an experimental BNCT facility has been installed at the 7 MV CN Van de Graaff accelerator of the LNL. Microdosimetric measurements of this irradiation field have already been performed [2]. On the other hand, in order to perform radiobiological measurements aimed to optimize the treatment procedure, much more intense fields are needed. For this purpose an intense thermal neutron field is available at the ENEA-TAPIRO reactor. We have investigated microdosimetric features of such a radiation field with the counter used to study the LNL moderated neutron field.