Fast neutron dosimetry : a study of basic dosimetric properties of fast-neutrons for external beam radiotherapy and problems associated with corrections of measured charged particle cross-sections

Sammanfattning: Basic dosimetric properties of fast-neutron beams with energies ≤80 MeV were explored using Monte Carlo techniques. Elementary pencil-beam dose distributions taking into account transport of all relevant types of released charged particles (protons, deuterons, tritons, 3He and a particles) were calculated and used to derive several absorbed dose distributions. Broad-beam depth doses in phantoms of different materials were compared and scaling factors calculated to convert absorbed dose in one material to absorbed dose in another. The scaling factors were in good agreement with available published data and show that water is a good substitute for soft tissue even at neutron energies as high as 80 Me V. The inherent penumbra and fraction of absorbed dose due to photons were also studied, and found to be consistent with published values.Treatment planning in fast-neutron therapy is commonly performed using dose calculation algorithms designed for photon beam therapy. These algorithms have limitations in the physical models when applied to neutron beams. Monte Carlo derived neutron pencil-beam kernels were parameterized and implemented into the photon dose calculation algorithms of the TMS (MDS Nordion) treatment planning system. It was shown that these algorithms yield good results in homogeneous water media. However, the heterogeneity correction method of the photon dose calculation algorithm failed to calculate correct results in heterogeneous media for neutron beams.Fundamental cross-section data are needed when calculating absorbed doses. To achieve results with adequate accuracy, neutron cross-sections are still not sufficiently well known. At the The Svedberg Laboratory in Uppsala, Sweden, an experimental facility has been designed to measure neutron-induced charged-particle production cross-sections for (n,xp), (n,xd), (n,xt), (n,x3He) and (n,xα) reactions at neutron energies up to 100 MeV. In order to derive the energy distributions of charged particles generated inside the production target, the measured data have to be corrected for the energy lost by the particles in the target. In this work a code (CRAWL) was developed for the reconstruction of the true spectrum. It uses a stripping method. With the limitation of reduced energy resolution, results using CRAWL compare well with those of other methods.