ESTRO 38 Abstract book

S978 ESTRO 38

to the TG-43 formalism. 2) Applicator, shield, source and balloon materials with nominal densities specified, but patient geometry consists of water. 3) Tissue materials assigned to contoured organs in addition to foreign structures, densities overridden with nominal densities. 4) Materials specified as per segmentation 3, with organ densities based on CT densities. In addition, two novel sources were investigated and dosimetrically compared with 192 Ir: 75 Se and 169 Yb. The clinical TG-43 based plan optimized for 192 Ir was used for all simulations. The TG-43 results for 169 Yb and 75 Se were normalized to give the same D 90 as the clinical plan. Results CTV coverage and dose to the OARs, pelvic and femur bone are overestimated for 192 Ir TG-43 based dosimetry, while for 175 Se and 69 Yb dose to CTV and OARs are overestimated, but pelvic and femur bone doses are significantly underestimated. 75 Se delivers similar dose to OARs as 192 Ir but delivers slightly increased bone doses. 169 Yb delivers lower dose to the rectum but significantly higher bone dose. Dosimetric indices and comparisons between segmentation schemes for the CTV, rectum, and pelvis are given in Table 1. Colorwash comparison between segmentation schemes for 192 Ir is given in Figure 1.

delivered dose, specifically for radionuclides with lower average energy than 192 Ir. In addition, our results show that with a future MRI-based treatment planning for HDR-EBT, loss of CT-density data will not significantly affect dosimetry if material composition and nominal mass densities are used. EP-1806 Commissioning an Independent Dose Calculation System for the Unity MR-Linac E. Goodwin 1 , S. Nill 1 , U. Oelfke 1 1 Royal Marsden / Institute of Cancer Research, Joint Department of Physics, London, United Kingdom Purpose or Objective An independent secondary monitor unit check is a quality assurance measure used throughout the radiotherapy community, and is a legal requirement in many European countries. The first patients on the Unity MR-Linac (Elekta AB, Stockholm, Sweden) have recently been treated at The Royal Marsden Hospital. There is no commercial software for performing an independent calculation for a 1.5 T MR-Linac. Therefore a solution using scripting tools for the RayStation (RaySearch, Stockholm) treatment planning system was developed. Material and Methods The primary dose calculation for the MR-Linac is carried out in Monaco 5.4 (Elekta) using a Monte Carlo algorithm that has been benchmarked against beam data. This was used to generate a series of square and rectangular fields on a model of water phantom, with a 2mm voxel size and 0.5% uncertainty per beam. Depth dose curves and dose profiles were extracted and used as the basis for the RayStation model. An FFF beam model was created which matched the dose curves as closely as possible. RayStation requires that MLC-Y machines have a backup jaw, so a ‘dummy’ Y-jaw open beyond the MLCs was placed in each plan. Profiles from the MR-Linac are asymmetric due to the effect of the magnetic field on secondary electrons. This could not be modelled in RayStation. To first approximation, the difference could be accounted for by translating the fields laterally. Each beam isocentre was offset by an empirically derived factor of 0.16 cm (see fig. 1). Beam attenuation through MR components varies with gantry angle, so a monitor unit correction was applied to each beam. Point dose measurements and qualitative comparisons of profiles were made between simple fields (from 1.5 x 1.5 cm 2 to 22 x 58.6 cm 2 ), complex field shapes and IMRT fields calculated in Monaco and RayStation. Ten protocol prostate plans (6 offline, 4 adapted) were created in Monaco.

Results The RayStation profile and depth dose curves were in good qualitative agreement with Monaco for simple fields. In the centre of large fields, the dose difference was comparable to the statistical uncertainty of the Monaco calculation (0.5%). Elsewhere, agreement was worse due to the lack of asymmetry in RayStation. For complex field shapes the agreement was generally good, except near thin segments where RayStation would underestimate in- field dose by up to 3%. The RayStation calculated treatment plans yielded DVHs that were consistent with

Conclusion Ignoring patient geometry and in particular high-Z materials such as the iodine radiographic contrast, bone and tungsten shielding in dose calculations contributes to significant inaccuracies which lead to sub-optimal dose optimization and disagreement between prescribed and