ESTRO 2020 Abstract book

S1092 ESTRO 2020

calibration factor of 0.0174 Gy/MU g/eV. Calculation bin size of 1.9 mm x 1.9 mm x 1.0 mm was used. Comparison of the Eclipse SRS plans vs. the corresponding PRIMO plans was done using the 3D gamma analysis tool available in PRIMO. Gamma passing rates (GPRs) were computed for each target. Criteria was 3% and 5% global dose difference, distance-to-agreement of 2 mm, and 10% of the maximum dose given by PRIMO as low-dose threshold. As the inherent accuracy of PRIMO is about 3%, a 5%/2mm criteria seems adequate to evaluate this Monte Carlo-based verification. Results The mean GPRs were 94.4% (SD: 8.8%) and 97.1% (SD: 6.2%) for the brain targets with 3% and 5% dose difference criteria, respectively. Simulation time was 51 min (SD: 13 min), and the statistical uncertainty (k= 2) of the dose calculated by PRIMO was 2.4% (SD: 0.4%). Conclusion PRIMO Monte Carlo software is a promising tool to perform pre-treatment verifications of cranial SRS plans designed with the Eclipse TPS, with simulation times compatible with the routine clinical workflow. PO‐1863 Independent dosimetric evaluation of the Oncentra Brachytherapy TPS S. Moragues Femenia 1 , J. Calvo-Ortega 1 , C. Laosa-Bello 1 , J. Casals-Farran 1 1 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.Servicio de Oncología Radioterápica- Hospital Universitari Dexeus- Barcelona- Spain, 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.Ser, Purpose or Objective Dosimetric commissioning of a brachytherapy TPS using a set of Monte Carlo-based reference plans. Material and Methods Accuracy of the TG-43 formalism of the Oncentra Brachy (OCB) v. 4.6.0 (Nucletron B.V., Veenendaal, The Netherlands) was evaluated in an independent way using the reference plans provided by the AAPM task group TG- 186 (Med Phys. 2012 Oct;39(10):6208-36). These reference plans were computed using the Monte Carlo MCNP6 code for a generic HDR brachytherapy Ir-192 source that was added in the OCB. The DICOM dataset of these plans were downloaded from the IROC Houston file server (http://rpc.mdanderson.org/RPC/home.htm) and imported into the OCB. Three different scenarios (test cases) were studied: a) Case I: the source located at the center (X= 0 cm, Y= 0 cm, Z= 0 cm) of a waterphantom cube (511 mm side); b) Case II: the source located at the center (X= 0 cm, Y= 0 cm, Z= 0 cm) of a waterphantom cube (211 mm side) embedded in an air-filled cube (511 mm side); c) Case III: like Case II but the source is 7 cm displaced (X= 7 cm, Y= 0 cm, Z= 0 cm) from the center; see Figure 1. The source tip was pointing toward positive Y in all cases. Each reference plan was re-calculated in OCB using the TG-43 formalism and compared with the corresponding Monte Carlo plan. Global Dose differences were computed in discrete points such that a set for each test case, see Table 1. The reference point (r ref ) was located 1 cm away from the source center along the -X direction, i.e. r ref = (X= -1 cm, Y= 0 cm, Z= 0 cm) for the cases I and II, and r ref = (X= 6 cm, Y= 0 cm, Z= 0 cm) for the case III.

Results Table 1 shows the global dose differences found for the three cases. Accuracy of the TG-43 based dose calculations of OCB was within 3% for the analyzed scenarios.

Conclusion The reference Monte-Carlo plans given by the IROC Houston are a valuable and helpful tool to be used during the commissioning of the Oncentra Brachy system. These data supposes an alternative to perform dose measurements when dedicated phantoms and equipment are not available for HDR brachytherapy Ir-192 sources. PO‐1864 Can underdosage due to breast swelling be mitigated with robust optimization for breast radiotherapy K. Crama 1 , E. Brondijk 1 , J. Visser 1 , A. Bel 1 1 Amsterdam UMC Location AMC, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Swelling of the breast during radiotherapy sometimes occurs in breast cancer patients. With the use of more