CLINICAL DOSIMETRY OF AN EPITHERMAL NEUTRON BEAM FOR NEUTRON-CAPTURE THERAPY - DOSE DISTRIBUTIONS UNDER REFERENCE CONDITIONS

Purpose: The aim of this study was to asses the dose distribution unde r reference conditions for the various dose components of the Petten c linical epithermal neutron beam for boron neutron capture therapy (BNC T), Methods and Materials: Activation foils and a silicon alpha-partic le detector with a Li-6 converter plate have been used for the determi nation of the thermal neutron fluence rate, The gamma-ray dose rate an d the fast neutron dose rate have been determined using paired ionizat ion chambers, Circular beam apertures of 8, 12 and 15 cm diameters hav e been investigated using a 15 x 15 x 15 cm(3) solid polymethyl-methac rylate phantom, a water phantom of the same dimensions and a 30 x 30 x 30 cm(3) water phantom at various phantom to beam-exit distances, Res ults: The effect of phantom to beam-exit distance could be modeled usi ng an inverse square law with a virtual source to beam-exit distance o f 3.0 m. At a reference phantom to beam-exit distance of 30 cm, three- dimensional dose and fluence distributions of the various dose compone nts have been determined in the phantoms, The absolute thermal neutron fluence rate at a reference depth of 2 cm in the 15 cm water phantom increased by 43% when the field size was increased from 8 to 15 cm, Si multaneously the gamma-ray dose rate increased by 46% while the fast n eutron dose rate increased by only 5%. Conclusion: A reference treatme nt position at 30 cm from the beam exit allows convenient patient posi tioning with a relatively small increase in irradiation time compared to positions very close to the beam-exit, A more homogeneous distribut ion of thermal neutrons over a target volume, a higher absolute therma l neutron fluence rate and a lower contribution of the fast neutron do se to the total dose will result in improved treatment plans when usin g a 12 cm or 15 cm field compared to a 8 cm field, The dose distributi ons will be used as benchmark data for treatment planning systems for BNCT, (C) 1997 Elsevier Science Inc.