Auterinen, Iiro

Bjugg, Hanna

Kotiluoto, Petri

Savolainen, Sauli

Seppälä, Tiina

Standard dosimetric phantoms are used in radiotherapy to compare irradiations under standard conditions. They provide volumes of tissue substitute for the measurement of absorbed dose and are large enough to ensure that full contribution to the absorbed dose from scattered radiation is received at the point of measurement. Aim of this study was to find out a recommendation for the boundary values of size of a reference phantom. These reference conditions for the reference measurement methods are created for "A code of practise for dosimetry of BNCT in Europe" project. The major objective of the project is to prepare detailed guidelines for the dosimetry of epithermal neutron beams to be used for treatment of cancer patients by Boron Neutron Capture Therapy (BNCT) at European research reactors and accelerators. For this objective Monte Carlo simulations have been carried out with MCNP code in three different cubic phantoms for studying effect of different phantom sizes in important radiation components. These three phantoms are the proposed reference (measurement) phantom (40*40*20 cm), a phantom that was assumed to model an infinite phantom, and a smaller (15*15*15 cm) cubic phantom which exists in Petten BNCT facility in Netherlands. Function of the smallest phantom was to study acceptable lower limit to the phantom size to still reach the reference conditions. All the simulated phantoms were cubic water phantoms with one 0.5 cm thick (beam side) wall and three 1 cm thick walls of PMMA (polymethyl-methacrylate). The comparisons were done with calculations of the thermal, epithermal and fast neutron fluence rates in analogous points. The source specification of the MCNP runs were accordance of 250 kW FiR 1 research reactor neutron beam with 14 cm beam aperture. In order to minimise the statistical error of the Monte Carlo calculations, over 60 million source particles were simulated for infinite and reference phantom cases. Calculation results were in good agreement with each other. According to these results, the proposed reference phantom can be considered as a good model of an infinite phantom, and all the larger phantoms with same wall thickness and materials are usable as standard dosimetric phantom for BNCT dosimetry. The lower limit for the standard dosimetric phantom size will be presented in addition, when the simulation results are acquired for the indicated smaller phantom

Calculational evaluations of the proposal for a reference dosimetric phantom for BNCT

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