Dose distributions of a proton beam for eye tumor therapy: hybrid pencil-beam ray-tracing calculations

For the case of eye tumor therapy with protons, improvements are introduced compared to the standard dose calculation which implies straight-line optics and the constant-density assumption for the eye and its surrounding. The progress consists of (i) taking account of the lateral scattering of the protons in tissue by folding the entrance fluence distribution with the pencil beam distribution widening with growing depth in the tissue, (ii) rescaling the spread-out Bragg peak dose distribution in water with the radiological path length calculated voxel by voxel on ray traces through a realistic density matrix for the treatment geometry, yielding a trajectory dependence of the geometrical range. Distributions calculated for some specific situations are compared to measurements and/or standard calculations, and differences to the latter are discussed with respect to the requirements of therapy planning. The most pronounced changes appear for wedges placed in front of the eye, causing additional widening of the lateral falloff. The more accurate prediction of the dose dependence at the field borders is of interest with respect to side effects in the risk organs of the eye.

Range modulation in proton therapy--an optimization technique for clinical and experimental applications

A fast optimization algorithm for range modulation in clinical and experimental applications of proton therapy is described. The method is versatile towards the number of parameters provided for range modulation, i.e. the trade-off between accuracy and simplicity of the latter can be chosen freely. The approach is, therefore, adaptable to most operating proton therapy facilities. It requires only a few basic measurements as input data and results in a depth dose uniformity of better than 2%. A typical calculation takes less than 90 s on a DEC VAXstation 3100, FORTRAN. The method has been extensively tested at the TRIUMF Proton Therapy Facility in Vancouver, BC.

A fast algorithm for the dose calculation of irregularly shaped fields in stereotactic convergent beam irradiation

A fast, irregular field dose calculation algorithm is described for use in stereotactic convergent beam irradiation (CBI) with high-energy photons. The dose distribution of an irregular field is approximated by a convolution of narrow pencil beams. Decomposition of pencil beams and a 'pre-selection' algorithm is used to avoid redundant calculations. A typical dose calculation for CBI with nine couch angles and nine different irregularly shaped fields takes less than 90 s on a DEC VAXstation 3100. Fortran. The difference between the measured and the calculated dose values increases with increasing distance from the isocentre. The error in 10% isodose line is less than 3% of the isocentre dose.