A pencil beam algorithm for proton dose calculations

Abstract

The sharp lateral penumbra and the rapid fall-off of dose at the end of range of a proton beam are among the major advantages of proton radiation therapy. These beam characteristics depend on the position and characteristics of upstream beam-modifying devices such as apertures and compensating boluses. The extent of separation, if any, between these beam-modifying devices and the patient is particularly critical in this respect. We have developed a pencil beam algorithm for proton dose calculations which takes accurate account of the effects of materials upstream of the patient and of the air gap between them and the patient. The model includes a new approach to picking the locations of the pencil beams so as to more accurately model the penumbra and to more effectively account for the multiple-scattering effects of the media around the point of interest. We also present a faster broad-beam version of the algorithm which gives a reasonably accurate penumbra. Predictions of the algorithm and results from experiments performed in a large-field proton beam are presented. In general the algorithm agrees well with the measurements.

Commissioning and initial experience with the first clinical gantry‐mounted proton therapy system

“…The beam model was commissioned in an Eclipse V11.0.30 (Varian Medical Systems, Palo Alto, CA), which employs a pencil‐beam algorithm (8) . As all TPS commercially available use similar algorithms with slight differences in the modeling of Bragg peak and the calculation of spread‐out Bragg peak (SOBP), the results and discussion in this study apply generically.…”

Application of dose kernel calculation using a simplified Monte Carlo method to treatment plan for scanned proton beams

“…It starts calculation at the entrance of patient body with initial beam parameters provided by the effective source model (7) . The model parameters are calculated and prepared in advance for different combinations of the binary RS plates.…”

“…Since we are using the water‐equivalent model, we calculate the rms scattering angle in the voxel as if the material is equivalent to water with the water‐equivalent thickness (13) . The dose deposition in a voxel is obtained from a measured depth‐dose distribution in water using the water‐equivalent model (7) . Shorter calculation time compared with FMC methods comes from simplification in which the dose deposition in materials is obtained by using the measured depth‐dose distribution in water, and ignoring nuclear reactions.…”

“…To assess the usefulness of SMC calculations in the clinical environment, we compared the dose calculation results obtained by using the SMC method with those using the PBA method if the PBS would be used as a beam delivery method for the target. The software for dose calculation using the PBA method has been developed in‐house for the study based on the algorithm shown in Hong (7) . All of these plans are SFUD (single‐field uniform dose) plans.…”

“…A number of facilities begin to apply the PBS technique to cancer treatment in moving organs like lung and liver by using a repainting technique and a gating technique 5 , 6 . Currently, a treatment planning system (TPS) for the PBS uses an analytical dose calculation method using a pencil beam algorithm (PBA) for proton therapy 7 , 8 . Although this algorithm allows short computation time and is suitable for dose calculation in homogeneous or moderately inhomogeneous media, the accuracy limitation appears in certain clinical sites with large density heterogeneity.…”