Thermal Neutron Relative Biological Effectiveness Factors for Boron Neutron Capture Therapy from In Vitro Irradiations.

The experimental determination of the relative biological effectiveness of thermal neutron factors is fundamental in Boron Neutron Capture Therapy. The present values have been obtained while using mixed beams that consist of both neutrons and photons of various energies. A common weighting factor has been used for both thermal and fast neutron doses, although such an approach has been questioned. At the nuclear reactor of the Institut Laue-Langevin a pure low-energy neutron beam has been used to determine thermal neutron relative biological effectiveness factors. Different cancer cell lines, which correspond to glioblastoma, melanoma, and head and neck squamous cell carcinoma, and non-tumor cell lines (lung fibroblast and embryonic kidney), have been irradiated while using an experimental arrangement designed to minimize neutron-induced secondary gamma radiation. Additionally, the cells were irradiated with photons at a medical linear accelerator, providing reference data for comparison with that from neutron irradiation. The survival and proliferation were studied after irradiation, yielding the Relative Biological Effectiveness that corresponds to the damage of thermal neutrons for the different tissue types.

Radiobiology data of melanoma cells after low-energy neutron irradiation and boron compound administration.

The cold neutron beam at the PF1b line at the Institut Laue-Langevin (ILL), without fast neutrons and a low contribution of gamma rays, is a very suitable facility to measure cell damage following low-energy neutron irradiation. The biological damage associated with the thermal and the boron doses can be obtained in order to evaluate the relative biological effectiveness (RBE) for Boron Neutron Capture Therapy. Three different experiments were carried out on the A375 melanoma cell line: the first one in a hospital LINAC, to obtain the reference radiation data, and the other two at the ILL, in which the damage to cells with and without boron compounds added was measured.

A simple approximation for the evaluation of the photon iso-effective dose in Boron Neutron Capture Therapy based on dose-independent weighting factors.

The current methodology for determining the biological effect of Boron Neutron Capture Therapy (BNCT) has recently been questioned, and a more accurate framework based in the photon iso-effective dose has been proposed. In this work we derive a first order approximation to this quantity. The new approach removes the main drawbacks of the current method, being based on new weighting factors which are true constants (dose independent) but which can be evaluated from published data on the existing (dose-dependent) weighting factors. In addition to this, we apply the formalism to allow the comparison to a fractionated conventional radiotherapy treatment, for which there is a lot of knowledge from clinical practice. As an application, the photon iso-effective dose of a BNCT treatment for a brain tumor is estimated. An excel sheet used for these calculations is also provided as supplementary material and can be used also with user-provided input data for the estimation of the photon iso-effective dose for comparison with conventional radiotherapy, both to single and fractionated treatments.