ESTRO 2021 Abstract Book

S559

ESTRO 2021

0.001) and 18.8 months (95% CI, 8.2–54.6 months) for patients with PTV >38 cc (Figure 1B). There was no additional significant prognostic factor identified including age, liver metastasis status, other organ metastasis and number of liver metastases for OS in univariate analysis. No difference in PFS in relation to GTV and PTV were seen (Figure 1C-D) and no significant prognostic factors for PFS was observed in univariate analysis. The LC rates for patients with GTV ≤6 cc and PTV ≤38 cc were 100%. Only 2 patients experienced rib fracture 4 and 6 months after completion of treatment, and one patient had duodenal ulcer.

Conclusion Liver SBRT is an effective and safe treatment option in oligometastatic breast cancer patients with excellent LC, promising survival and limited toxicity. Patients with smaller tumors display better OS and local control compared to counterparts, validating the use of curative approach in this group. PD-0729 Underutilization of adjuvant radiation after nipple-sparing mastectomy W. Talcott 1 , G.N. Marta 2 , M.S. Moran 1 1 Yale School of Medicine, Department of Radiation Oncology, New Haven, USA; 2 Hospital Sírio-Libanês, Department of Radiation Oncology, São Paulo, Brazil Purpose or Objective Nipple areola complex-sparing surgeries are increasingly utilized for women with early-stage breast cancer. In the post-operative setting, 2 major indications for radiation (PORT) to the breast/chest wall and PORT with regional nodal irradiation (RNI) are positive margins (pMarg+) and pathologically involved lymph nodes (pN+), respectively. We investigated the frequency of these 2 adverse pathologic features following nipple-sparing mastectomy (NSM) versus nipple-sparing lumpectomy (NSL) to assess trends in PORT delivery using these 2 indications to determine whether PORT is being appropriately recommended in the NSM setting. Materials and Methods Using the National Cancer Database (NCDB), women diagnosed with cT1-3N0M0 invasive ductal/lobular carcinoma between 2004-2017 who received NSM were compared to those who received NSL. The frequencies of pMarg+ and pN+ by surgical subtype and the utilization of breast/chest wall PORT +/- RNI were assessed by cohort to determine if type of surgery was significantly associated with the likelihood of PORT receipt. Overall survival (OS) between the 2 cohorts was also compared. We calculated descriptive statistics, univariable (UVA)/multivariable (MVA) logistic & Cox regression w/ odds ratios (OR) and 95% confidence intervals (CI) to control for confounders. The MVA model included all UVA parameters that were significant to p<0.10.