Calculations of the size of standard dosimetric phantom for BNCT

A protocol for dosimetry of epithermal neutron beams to be used for treatment of cancer patients by Boron Neutron Capture Therapy (BNCT) has been under construction by "A code of practice for dosimetry of BNCT in Europe" project. The objective of the project is to prepare detailed guidelines for dosimetry of BNCT beams at European research reactors and accelerators. One task of the project was to establish reference geometry for beam calibration and beam geometry measurements. The aim of this study was to find out the phantom size that is large enough to ensure that full contribution to the absorbed dose from scattered radiation is received at the points of measurement. The main objective was to meet the criteria at the reference point of BNCT. Secondary objective was to establish the full back scattering situation at the measurement points down to the isodose of 5 % up to the maximum FiR 1 beam size (20 cm). The standard dosimetric phantom should be independent of the beam size. Water is agreed to be suitable reference phantom material in BNCT [1]. The maximum allowed disturbance from the limited phantom size to the dose and fluence values was set to 3.5 % at the reference point and to 5 % at the 5 % isodose. To study the phantom size which meets the requirements of the standard dosimetric phantom in FiR 1 [2] neutron beam, Monte Carlo simulations were carried out with MCNP 4B and 4C codes [3] in many different sized rectangular open top water phantoms. All the simulated phantoms had 0.5 cm thick beam-side wall and 1 cm thick other walls of PMMA (polymethyl-methacrylate). The simulations were first done in an "infinite" (100x100x50 cm) water phantom to find out the full contribution of the beam profile. The simulations were done altogether in about 200 profile points at the 17 depths, from the inner-surface (0.5 cm distance from the aperture plane) of the phantom until the depth of 23 cm. The total neutron and the total photon spectra and absorbed dose rates (to ICRU brain [4]) for each beam components was calculated in the profile points. According to the preliminary results with FiR 1 neutron beam, the standard dosimetric phantom for BNCT should be at least size of 58 cm 58 cm square cross section and 26 cm deep. The result is valid for every beam size (up to 20 cm) used at FiR 1. [1] T. Seppälä et al. Radiat. Phys. Chem. (1999) 239. [2] I. Auterinen et al. Med. & Biol. Eng. & Comp., vol 37, suppl. 1. (1999) 398 [3] J. F. Briesmeister Los Alamos National Laboratory Report LA-12625-M (1993) [4] ICRU Report 46 (1992)