Dosimetrical verification of calculated total and gamma absorbed dose distributions DT, respectively DG, for fast neutron therapy.

Abstract

The physical part of treatment planning for neutron therapy is highly important but quite more complicated than for photons and electrons due to the necessity of separation of total and gamma absorbed dose distributions (DT and DG). Therefore, dosimetrical verifications of dose distributions of complex treatment plans are very rare, and the experimental difficulties are enormous. A method using TLD-300 (CaF2: Tm) detectors is described with the ability to provide from each detector's readings both DT and DG using the different LET dependence of the main glow-curve peaks. The principle of an on-line computer program is given for an automatic mathematical glow-curve analysis which is necessary to achieve the accuracy of about +/- 5% (DT) and +/- 10% (DG) in single fixed fields. Dosimetrically relevant features of the TLD-300 method are discussed. Examples of dosimetrical verifications of calculated dose distributions (DT and DG) for a treatment with one single fixed field, with two wedge filter fields and with a rotational field with d(14)+Be neutrons are shown. Deviations exceeding significantly the uncertainties above are found only in the case of the wedge filter treatment for the absolute (quite less for the relative) values of the DG dose distribution. Probable reasons are mostly the reduced contribution of scattered radiation for phantom sizes, which laterally scarcely exceed the useful beam, and slight neutron energy spectrum changes by the wedge filter.