ESTRO 2020 Abstract book

S1091 ESTRO 2020

lateral side=Plan 3, medial side=Plan 4, cranial side=Plan 5, caudal side=Plan 6) were made for each patient (A total of 300 plans were created). Regarding these virtual plans, D95 (%), V107 (%), V80(%), Dmean (%), and Dmax (%) of TV were examined in each patient. Results For dose-volume parameters of the TV, changes of ≥10% were uncommon. V107 (%) increased by ≥10% in three plans in two patients (15.3%, 10.6%, 10.9%, respectively), D95 (%) increased by ≥10% in one plan (10.9%), and the other parameters did not change by ≥10%. The mean values of changes in D95 (%), V107 (%), V80(%), Dmean (%), and Dmax (%) were the largest when isocenter displaced to ventral side. Changes in V107 (%) showed comparatively broad distribution when isocenter displaced to ventral and lateral side, and changes in D95 (%) showed comparatively broad distribution when isocenter displaced to ventral or dorsal side. In Bp cases, changes in V107(%) tended to show broader distribution and changes of D95 tended to show narrower distribution compared to Bt cases. Conclusion In PORT for breast cancer using FIFT, setup error of 5 mm rarely brought significant changes of dose distribution in the TV regardless of surgical types. However, displacement of patient body to ventral side tended to cause comparatively large changes of dose-volume parameters. In patient setup of PORT for breast cancer using FIFT, priority should be given to reducing vertical setup errors. PO‐1862 Patient‐specific SRS QA using the PRIMO Monte Carlo software J.F. Calvo Ortega 1 , M. Hermida-López- 2 , S. Moragues- Femenía 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.S, ; 2 Servei de Física i Protecció Radiològica. Hospital Vall d'Hebron. Barcelona Spain, Servei de Física i Protecció Radiològica. Hospital Vall d'Hebron. Barcelona Spain, Barcelona, Spain Purpose or Objective To show a single institution experience using the PRIMO Monte Carlo software to perform pre-treatment independent verification of brain stereotactic radiosurgery (SRS) plans. Material and Methods Twenty seven SRS cases of brain metastases (1-3 fractions) were prospectively included in this study. The number of targets per case ranged from 1 to 5. The total number of lesions was 45, with a mean diameter of 1.9 cm (ranging from 0.8 to 4.6 cm). Each SRS plan consisted of a single- isocenter and multiple non-coplanar IMRT fields of 6 MV photon beams from a Varian 2100 CD linac equipped with a 120 MLC. Inverse optimization and dose calculation were done using DVO and AAA algorithms of the Eclipse TPS (v 13.7), respectively. Calculation grid size of 1.0 mm x 1.0 mm x 1.0 mm was used. Each SRS plan was sent to PRIMO software (v. 0.3.1.1.1681) to be simulated using the DPM algorithm before treatment delivery. Accuracy of PRIMO for dose calculation of static beams was previously benchmarked against the IROC-H (Imaging and Radiation Oncology Core–Houston) reference dosimetry dataset, and a dosimetric accuracy within 2.8% was found for a Clinac 2100 6 MV beam [Radiat Oncol. 2018 Aug 7;13(1):144]. The simulations in the present work were performed on an Intel(R) Xeon(R) CPU E5–2640 v4 @ 2.40 GHz (2 processors) with 64 GB of RAM, with 32 CPU cores available. Simulation conditions: PRIMO default transport parameters for 6 MV from Clinac 2100; 404 million histories; DPM algorithm; particle splitting ×170; CT scanner calibration curve of the CT Optima CT660; default materials assignment; and

were compared using paired sample t-test. The magnitude and statistical significance of the observed dosimetric differences were quantified. The optimization objectives were not modified or individualized between patients to avoid introducing bias. Results Averaged results of planned target volume (PTV) and organs-at-risk (OARs) sparing were summarized in table 1. The results demonstrated that HpVMAT plans are capable to reduce radiation dose to hippocampus, eyes and lenses significantly relative to cVMAT plans, while PTV coverage was not compromised. The HpVMAT technique is also compared favorably to hippocampal sparing reported for RTOG 0933. Beam-on time of HpVMAT had no significant increment compared to cVMAT.

Conclusion HpVMAT improved hippocampal sparing, while simultaneously spare other OARs such as the eyes and lenses. PTV coverage was not compromised with HVMAT. HpVMAT can be considered for a trial for patients with BMs undergoing WBRT with hippocampal sparing. PO‐1861 Impact of setup error in postoperative breast/chest wall radiotherapy with field in field technique M. Kenji 1 , H. Yasushi 2 , N. Kei 2 , K. Teruhito 2 , M. Teruhito 2 1 Saiseikai Imabari Hospital, Department of Radiology, 7- 1-6 Kita-machi- Imabari- Ehime 799-1592, Japan ; 2 Ehime University Graduate School of Medicine, Department of Radiology, 454 Shitsukawa- Toon- Ehime 791-0295, Japan Purpose or Objective Field in field technique (FIFT) has been increasingly used in postoperative radiotherapy (PORT) for breast cancer to improve dose homogeneity in target volumes (TVs). Regarding FIFT in PORT for breast cancer, influence of setup error on dose distribution in TVs was assessed. Material and Methods To evaluate the effect of setup error (SE) on dose distribution in the TVs, data in treatment planning systems (Eclipse, Varian medical systems) of 50 patients who received PORT for breast cancer with FIFT (post breast- conserving surgery (Bp), 25 cases; postmastectomy (Bt), 25 cases) were used. Six virtual plans including a plan in which the isocenter is displaced by 5 mm in the X-axis, Y-axis, and Z-axis directions from the original plan (plan 0) (displaced to ventral side =Plan 1, dorsal side=Plan 2,