ESTRO 2023 - Abstract Book

S1041

Digital Posters

ESTRO 2023

on both FB and DIBH scans, then RT plans were generated. RT was delivered as indicated to primary site with or without regional nodal irradiation using conventional tangential fields or VMAT techniques. Ipsilateral mean lung dose, percent of lung volume receiving >20 Grays (V20), mean heart dose, and percent of heart volume receiving >5 Grays (V5) were calculated in both techniques. Univariate and Multivariate analyses were performed to test the impact of radiation technique, nodal irradiation, patients’ age, smoking status, body mass index (BMI) and type of surgery on heart and lung dose reduction using the mean reduction values as cut off for comparison. Results Results A total of 90 patients were identified. Mean age was 44 years (range 23-69), mean BMI was 29.5 (range 22-40), 9 (10%) patients were planned using VMAT technique, 66 (73%) patients were non-smokers, 58 (64%) patients received radiation to chest wall, while 32 (36%) patients received radiation to breast and 73 (82%) patients received regional nodal irradiation. DIBH mean lung dose reduction ranged from 5%-47% (mean 18%), while heart dose reduction ranged 7%-70% (mean 39%) compared to FB. On univariate analysis, inclusion of regional nodal irradiation resulted in lower effectiveness in mean lung dose reduction (lung dose reduction less than 18% (p= 0.001)) the rest of variable did not significantly impact mean lung dose reduction. In regards to heart mean dose reduction; only high BMI significantly resulted in lower effectiveness in mean heart dose reduction (lower than 39%), which persisted on multivariate analysis (OR 0.843, [95% CI 0.76-0.934] (p= 0.0012)). Conclusion The use of DIBH reduces heart and ipsilateral mean lung dose for left sided adjuvant breast cancer RT. Patients’ BMI was inversely correlated with the dosimetric advantage of DIBH technique. Further studies are warranted to validate these results and dictate other factors that may influence DIBH value. Purpose or Objective Breast cancer treatment typically involves a combined modality approach, with surgical treatment playing the central role. However, due to various reasons, some patients are not able to undergo surgery. In the absence of resection, it is paramount to improve local control of the primary tumor. To that end, we explored the role of ablative radiation dose delivered with SBRT. Materials and Methods Between 2015-2022, 28 patients underwent definitive SBRT for primary breast cancer without planned surgical intervention. SBRT dose was 25-40 Gy in 5 fractions (40 Gy in 5 fractions in 80% of cases). Post-treatment imaging (PET-CT, MRI, MMG and ultrasound) as well as clinical exams were used to evaluate the response. Toxicity rates and response rates were assessed as primary endpoints of the study. Kaplan-Meier curves were used to estimate local control (LC) and overall survival (OS). Results The reasons for breast SBRT included the need for aggressive local palliation, medical comorbidities precluding surgery, unresectable disease, and patient preference. Median age was 69 (36-97) years. Median follow-up was 32 (3.4-70.4) months after RT completion. Initial post-treatment imaging was available in 96% of the cases and was performed at a median 2.2 (0.6-8.1) months after SBRT. Radiographically, complete response (CR) was seen in 28% and partial response (PR) in 64% of the cases. Among those with PR, median decrease in diameter was 44% (16.1-68.8%). Median SUV reduction was 65.2% (27.9-69.7%). Acute toxicity occurred in 93% of the cases, including grade 1 (n=16), grade 2 (n=8), and grade 4 (necrosis; n=2). Late toxicity included grade 2 edema (n=2) and grade 4 skin toxicity (necrosis; n=2, including 1 case of sequential late effect). Acute and late grade 4 toxicities were successfully managed with debridement or steroids. There have been 3 (11%) cases of local progression in the index lesion. Estimated LC after SBRT was 100% at 6 months and 89% at 1, 2 and 3 years. Median OS was not reached. The estimated median OS was 80% at 1 year, and 60% at 2 and 3 years. Conclusion Definitive SBRT to primary tumor in the breast appears to be well tolerated with high rates of radiographic and clinical response as well as high local control rates. To our knowledge, the current study represents the largest series of definitive SBRT in non-surgical breast cancer patients. Larger prospective studies and longer follow up are needed to assess late toxicity and durability of response. PO-1299 SBRT for treatment of primary breast cancer in patients not undergoing surgery: a pilot study A. Stessin 1 , E. Zabrocka 1 , S. Ryu 1 1 Stony Brook University Medical Center, Radiation Oncology, Stony Brook, USA

PO-1300 Tattoo-less Chest Wall Irradiation (VMAT) using Surface Imaging

B. Mueller 1 , Y. Song 2 , W. Chia-Ko 3 , H. Hsu 4 , X. Zhai 3 , P. Tamas 5 , S. Powell 5 , B. McCormick 5 , A. Khan 5 , L. Hong 3 , L. Cervino Arriba 3 , B. Zhao 3 , L. Braunstein 5 1 MSKCC, Radiation Oncology, New York, USA; 2 MSKCC, Medical Physics, New York, USA; 3 MSKCC, Medical Physics, NYC, USA; 4 Columbia University, Medical Physics, NYC, USA; 5 MSKCC, Radiation Oncology, NYC, USA

Purpose or Objective