ESTRO 2022 - Abstract Book

S1033

Abstract book

ESTRO 2022

Maryland, USA; 4 Advent Health Florida Hospital, Radiation Oncology, Orlando, Florida, USA; 5 Martin Health System, Surgery, Stuart, Florida, USA Purpose or Objective Obesity is a known disparity for breast cancer patients (pts) in the United States. Obese women with breast cancer have more complications from surgery, radiation, and chemotherapy, and are at increased risk for local recurrence. Ipsilateral breast tumor recurrence (IBTR) pt outcomes based on body mass index (BMI) were analyzed in the IRB-approved single arm prospective multi-institution ExBRT trial designed to determine the efficacy and outcome of single fraction 20 Gy intra- operative radiation therapy (IORT) using Xoft electronic brachytherapy at the time of breast conserving surgery for early- stage breast cancer. Materials and Methods Between May 2012–July 2018, 1200 enrolled breast cancer pts were successfully treated per protocol with lumpectomy plus single 20 Gy fraction IORT. Data collection and retrospective chart review included demographics, BMI (not recorded for 1 pt), histopathology, medical therapy, IBTR, and survival. The Exact Chi-square, 2 sided test was used for statistical analysis. Results 1200 pts (mean age 66 years, mean tumor size 11.7 mm, mean BMI: 29.4) completed IORT treatment per protocol. At median 4.0-year follow-up, there were 42 (3.5%) IBTR (original mean tumor size: 13.7 mm, mean BMI: 31.0). Pt BMI and outcomes are shown in Table 1. Although IBTR was highest in the BMI 46-50 group, this did not reach statistical significance (p=0.0832).There was one breast cancer-related death. Table1. IORT treatment outcomes by BMI BMI 16-20 21-25 26-30 31-35 36-40 41-45 46-50 50+ Pt number 59 308 391 250 110 51 23 7 Mean BMI 19.2 23.4 28.0 32.7 37.9 42.4 47.6 53.5 IBTR 0 13 (4.2%) 7 (1.8%) 14 (5.6%) 3 (2.7%) 2 (3.9%) 3 (13.0%) 0 IBTR Mean BMI N/A 23.6 33.1 33 38 42 48 N/A Conclusion At median 4.0-year follow-up, 1200 breast cancer pts successfully treated with lumpectomy plus single 20 Gy fraction IORT per protocol were found to have an IBTR rate of 3.5% which was independent of BMI. Pts with higher BMIs were successfully treated with breast conserving surgery followed by IORT. Pts with BMIs of 46-50 had a high but not statistically significant IBTR incidence. Purpose or Objective The role of post-mastectomy radiation therapy (PMRT) in breast cancer patients who have achieved ypN0 after neoadjuvant chemotherapy (NAC) has not yet been established. This study aimed to evaluate the benefit of PMRT according to pathologic node status and molecular subtype in breast cancer patients who were treated NAC. Materials and Methods We retrospectively reviewed 511 patients with clinical stage II-III breast cancer who underwent NAC and mastectomy at our institution from 2013 to 2017. After mastectomy, ypN+ and ypN0 were identified in 289 (56.6%) and 222 (43.4%), respectively. In this study, molecular subtypes were classified according to hormone receptor (HR) and HER2 receptors as follows: luminal (HR+/HER2-), luminal-HER2 (HR+/HER2+), HER2 enriched (HR-/HER2+), triple-negative (HR -/HER2-). All HER2-positive patients were treated with trastuzumab-containing NAC. Of the total patients, 448 (87.7%) received PMRT, and 63 (12.3%) did not. In ypN+ patients, PMRT was performed in 265 (91.7%) and 24 (8.3%) did not. In ypN0 patients, PMRT was performed in 183 (82.4%) and 39 (17.6%) did not The effect of PMRT on locoregional recurrence-free survival (LRRFS), disease-free survival (DFS), and overall survival (OS) was evaluated based on pathologic nodal status and molecular subtypes. Results The median follow-up duration was 58 months. The cumulative incidence of locoregional recurrence and distant metastasis were 11.8% and 31.5% in ypN+ patients, and 6.8% and 13.5% in ypN0 patients, respectively. In ypN+ patients, PMRT showed significant benefit for LRRFS, DFS, and OS in multivariate analysis (all P < 0.001). In ypN0 patients, triple-negative subtype and lymphovascular invasion were significant prognostic factors for LRRFS (all P < 0.001) and DFS (P = 0.002 and P < 0.001, respectively) by multivariate analysis. However, in ypN0 patients, PMRT was not significantly associated with LRRFS, DFS, and OS. In addition, PMRT showed no significant difference in LRRFS in all molecular subtypes of ypN0 patients. Conclusion PMRT should be performed in patients with residual lymph node following NAC and mastectomy. Although a limited number of patients, this study suggests that the role of PMRT in ypN0 patients regardless of subtype remains unclear. PO-1222 Effect of postmastectomy radiotherapy in breast cancer after neoadjuvant chemotherapy D. Kim 1 , J.H. Kim 1 , J.H. Chang 1 , K.H. Shin 1 1 Seoul National University College of Medicine, Radiation Oncology, Seoul, Korea Republic of