Commissioning and evaluation of an extended SSD photon model for PINNACLE3: an application to total body irradiation

Total body irradiations (TBIs) are unusual radiation therapy techniques used to treat specific hematological diseases. Most TBI techniques use extended source to patient distances [source-to-skin distance (SSD)] to provide lateral or anteroposterior irradiations. Those techniques differ from one institution to the other since they need to be customized to accommodate for local material constraints. However, with those unusual techniques come additional challenges for dose calculation. The purpose of this study was to obtain an accurate (better than 4%) dose calculation model for extended source-to-skin distance (eSSD) treatment techniques, which will be used for TBI planning. The studied dynamic TBI technique has special aspects (eSSD, beam spoiler, large field, and out of field dose contribution) that need to be considered in dose calculation. The first part of this study presents an eSSD beam model commissioning in PINNACLE3 and its validation. The second part looks at the comparison between two dose calculation algorithms, the 3D pencil beam and the superposition-convolution algorithms implemented in THERAPLAN PLUS and PINNACLE3, respectively. A regular linac beam was commissioned in each treatment planning system and an additional dedicated TBI beam model was implemented in PINNACLE3. The comparison results indicate that the quality of the TBI treatment greatly depends on the treatment planning system and its beam commissioning. The superposition-convolution algorithm (PINNACLE3) provides a better dose calculation tool for TBI than the 3D pencil beam algorithm (THERAPLAN PLUS) with a maximum mean error of 2.2% on a dynamic treatment. The TBI specific beam model of PINNACLE3 (ESSP-P3) also improves the dose calculation. The maximum difference between calculations and measurements (depth doses and beam profiles) was 2% except for extreme cases (build-up region and depth of 20 cm) where the error was higher. Output factor determination and the dose contribution outside the primary beam weaknesses were found in PINNACLE3. Methods are proposed to overcome these limitations. With the correction method applied, the TBI specific beam model allows a maximum mean error of -0.68% on a dynamic treatment. Accurate TBI dose computation necessitates a good dose calculation algorithm combined with a realistic beam model. Inappropriate dose calculation could lead to an important over- or underdose estimation. No perfect algorithm and beam model were found, but methods are proposed to overcome some of the limitations. Those methods are simple and can be used for other eSSD treatment types.