ESTRO 2021 Abstract Book

S1666

ESTRO 2021

Conclusion In our study, we only matched in 2 out of 3 cases to the dose constraints used in the Fast-Forward trial despite the very low mean doses obtained to the lung and heart (mean MHD=1.78Gy and mean MLD= 5.58Gy) which are the main consensual constraints used in conventional fractionation radiotherapy, but not adopted in the Fast-Forward trial. An international consensus adopting the most relevant dose constraints in hypofractionated radiotherapy is waited.

PO-1956 Effect of heart dose reduction by IMRT for postmastectomy radiation therapy K. Ono 1 1 Asahi University Hospital, division of radiation service, Gifu, Japan

Purpose or Objective This study aims to reduce the heart dose for patients undergoing postmastectomy radiation therapy (PMRT) for left-sided breast cancer, which is critical for heart disorder. Moreover, this study compared postoperative radiotherapy for left-sided breast cancer intensity-modulated radiation therapy (IMRT) methods of various degrees and examined the details of different heart doses. This resulted in a higher dose reduction effect of organ at risk (OAR) for two-field IMRT (2-F IMRT), which has a narrower irradiation range. However, a detailed evaluation has not been made regarding postmastectomy chest wall irradiation. Consequently, this study compared PMRT methods of various degrees for left-sided breast cancer and examined the details of different heart doses. Materials and Methods Fifteen patients underwent PMRT for left-sided breast cancer from April 2015 to February 2020 at our institute. Moreover, this study conducted four treatment plans: 2-F IMRT, four-field IMRT (4-F IMRT), 40 o dual partial arc volumetric modulated arc therapy (VMAT, 40d VMAT), and 80° dual partial arc VMAT (80d VMAT). In all treatment plans, the prescription dose was 50 Gy/25 fractions for the planning target volume (PTV). Dose calculation was optimized by inverse treatment planning using the Monte Carlo algorithm. Each of the four treatment plans was analyzed by the dose-volume histogram (DVH), and a multiple comparison test was performed. The evaluation items were compared to the heart's V 10 (coverage of 10% of the volume of the heart), V 45 , and D mean (dose mean); left anterior descending (LAD) artery’s V 10 , V 45 , and D mean ; 95% PTV coverage for the chest wall ( D 95 ); homogeneity index (HI); left lung’s V 20 and D mean ; right lung’s D mean ; and contralateral breast V 5 and V 10 . Results D 95 showed good coverage of around 48 Gy in the four plans. The mean doses for the heart and left lung OAR were comparable in the four plans. However, 4-F IMRT showed highest V 20 for the left lung. In the contralateral breast, 80d VMAT resulted in the highest dose among the four plans. The narrower irradiation range of 2-F IMRT reduced V 10 of the heart and LAD, while V 45 tended to be higher due to the deterioration of HI. Moreover, 80d VMAT with the widest irradiation range had the highest reduction effect of V 45 on heart and LAD. Furthermore, V 10 was also able to reduce the dose slightly.

Conclusion 4-F IMRT, 40d VMAT, and 80d VMAT, which have a wide irradiation range in thin targets (e.g., PMRT), were the most effective methods for V 45 dose reduction for OAR of the heart and LAD. The dose reduction effect of V 10 was higher for 2-F IMRT with a narrower irradiation range. Moreover, IMRT is easily affected by the position and body shape of the patient’s organs. Therefore, taking into consideration the patient's body type, enhancing the dose reduction effect of OAR is important to determine the number of fields and irradiation range.

PO-1957 Monte Carlo-based independent verification of online adaptive SBRT plans for prostate S. Moragues-Femenía 1 , C. Laosa-Bello 1 , J. Calvo-Ortega 1 , J. Casals-Farran 1 1 Hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain