ESTRO 2021 Abstract Book

S906

ESTRO 2021

Conclusion HFRT for OM-BC is a feasible and tolerable approach with significant impact on oncological and quality of life outcomes.

PO-1088 Cardiac conduction system exposure during breast radiotherapy : evaluation of IMRT and protontherapy P. Loap 1 , A. Fourquet 1 , Y. Kirova 1 1 Institut Curie, Department of Radiation Oncology, Paris, France Purpose or Objective Breast IMRT is increasingly used in order to reduce coronary risk but it tends to expose larger volumes to low dose bath, which clinical consequences are still debated. In particular, precise quantification of radiation exposure to the cardiac conduction system has never been studied, while conduction disorders and arrhythmias are described radiation-induced toxicities. With this in mind, we quantified the radiation exposure to conduction nodes during adjuvant breast irradiation with rotational IMRT and estimated the dosimetric gain with pencil beam scanning intensity modulated protontherapy (IMPT). Materials and Methods Based on published delineation guidelines for the atrioventricular (AVN) and sinoatrial (SAN) nodes (Loap et al., Pract Radiat Oncol. 2021), these conduction nodes were retrospectively delineated on the simulation scans of 12 breast cancer patients (7 left-sided and 5 right-sided) treated with breast conserving surgery and adjuvant locoregional VMAT. According to these guidelines, the SAN was delineated by a 2 cm-diameter sphere, tangent to the external wall of the right atrium, centered at the height of the ascending aorta origin and the AVN was delineated by a 2 cm-diameter sphere centered at the junction between the four cardiac chambers, 1 cm above the last slice where the left atrium is visible. We replanned IMPT treatment on the simulation CT scans for left-sided breast cancer patients. Mean and maximum doses to the SAN and the AVN were retrieved from dose-volume histograms. Results Average mean doses to the SAN and to the AVN were 2.8 Gy and 2.3 Gy respectively for left-sided irradiation and 9.6 Gy and 3.6 Gy respectively for right-sided irradiation. Average maximum doses to the SAN and to the AVN were 3.5 Gy and 2.8 Gy respectively for left-sided irradiation and 13.1 Gy and 4.6 Gy respectively for right-sided irradiation. There were no significant differences between mean and maximum dose to the AVN and to the whole heart between right- and left-sided irradiations, while the SAN was significantly more exposed for right-sided irradiation. For left-sided breast cancer patients, IMPT significantly reduced mean dose to the SAN from 2.8 Gy to 0.0 Gy and to the AVN from 2.3 Gy to 0 Gy, and maximum dose to the SAN from 3.5 Gy to 0.2 Gy and to the AVN from 2.8 Gy to 0 Gy (p <0.01). Conclusion During breast irradiation, SAN and AVN can be substantially exposed with VMAT, especially for right-sided irradiation. For patients with underlying rhythmic or conduction disorders, additional cardiac sparing techniques, such as protontherapy, could be beneficial. PO-1089 Long-term follow-up of a single-center neuroendocrine breast cancer cohort P. Loap 1 , F. Laki 2 , P. Beuzeboc 3 , A. Fourquet 1 , Y. Kirova 1 1 Institut Curie, Department of Radiation Oncology, Paris, France; 2 Institut Curie, Department of Surgery, Paris, France; 3 Hôpital Foch, Department of Medical Oncology, Suresnes, France Purpose or Objective Neuroendocrine breast cancer (NEBC) is a rare breast cancer histology, for which. no treatment guideline currently exists; in practice, there are treated the same way as ductal invasive carcinomas. The exact place of radiotherapy for NEBC treatment has been questioned by epidemiological studies that failed to demonstrate any clinical benefit. In this study, we report the long-term survival and toxicity data in NEBC patients treated with radiotherapy at Institut Curie. Materials and Methods All NEBC patients who were evaluated between 1995 and 2005 in the Department of Radiation Oncology of the Institut Curie for local or locoregional breast radiotherapy were identified. A dose of 50 Gy was delivered to the whole breast or the chest wall in 25 fractions, and a 16 Gy boost could be delivered to young patients having undergone breast conserving surgery with risk factors (positive surgical margins or histological considerations). A dose of 46 gray could be delivered to regional lymph nodes (including the internal mammary chain), according to local guidelines. Adjuvant anthracycline-based chemotherapy or hormonotherapy (aromatase inhibitor or tamoxifen) were delivered based on clinical and histological considerations. Patients were followed every 6 months to 5 years and annually thereafter. Results Eighteen NEBC patients benefited from locoregional irradiation, complementing breast surgery. With a median follow-up of 5.7 years, three patients died of their cancers, two patients developed local recurrences and three distant metastases. No regional relapse, contralateral breast cancer or second non-breast cancer were observed. Five-year overall and 5-year metastasis-free survivals were both 88% [73%-100%]; 5-year locoregional control and 5-year local control were both 100%. In univariate analysis only, tumor diameter was statistically associated with overall survival and metastasis-free survival (HR=1.26 [95%CI: 1.00-1.58], p=0.04 and HR=1.13 [95%CI: 1.00-1.28], p=0.04, respectively). No patients had grade ≥2 toxicity; two patients experienced a grade 1 fibrosis, 2 patients had a grade 1 breast pain on the last clinical evaluation and 1 patient developed a persistent grade 1 breast oedema. Conclusion Adjuvant locoregional radiotherapy for NEBC management appears effective and well-tolerated.