DOSE HOMOGENEITY IN BORON NEUTRON-CAPTURE THERAPY USING AN EPITHERMALNEUTRON BEAM

Simulation models based on the neutron and photon Monte Carlo code MCN P were used to study the therapeutic possibilities of the HB11 epither mal neutron beam at the High Flux Reactor in Petten. Irradiations were simulated in two types of phantoms filled with water or tissue-equiva lent material for benchmark treatment planning calculations. In a cubo id phantom the influence of different field sizes on the thermal-neutr on-induced dose distribution was investigated. Various shapes of colli mators were studied to test their efficacy in optimizing the thermal-n eutron distribution over a planning target volume and healthy tissues. Using circular collimators of 8, 12 and 15 cm diameter it was shown t hat with the 15-cm field a relatively larger volume within 85% of the maximum neutron-induced dose was obtained than with the 8- or 12-cm-di ameter field. However, even for this large field the maximum diameter of this volume was 7.5 cm. In an ellipsoid head phantom the neutron-in duced dose was calculated assuming the skull to contain 10 ppm B-10, t he brain 5 ppm B-10 and the tumor 30 ppm B-10. It was found that with a single 15-cm-diameter circular beam a very inhomogeneous dose distri bution in a typical target volume was obtained. Applying two equally w eighted opposing 15-cm-diameter fields, however, a dose homogeneity wi thin +/-10% in this planning target volume was obtained. The dose in t he surrounding healthy brain tissue is 30% at maximum of the dose in t he center of the target volume. Contrary to the situation for the 8-cm field, combining four fields of 15 cm diameter gave no large improvem ent of the dose homogeneity over the target volume or a lower maximum dose in the healthy brain. Dose-volume histograms were evaluated for t he planning target volume as well as for the healthy brain to compare different irradiation techniques, yielding a graphical confirmation of the above conclusions. Therapy with BNCT on brain tumors must be perf ormed either with an 8-cm four-field irradiation or with two opposing 15- or 12-cm fields to obtain an optimal dose distribution. (C) 1995 b y Radiation Research Society