Indications for Postmastectomy Radiation (PMRT) Following Neoadjuvant Chemotherapy (NAC) in ypN0 and ypN1-3 Axillary Node Positive Women

Abstract

MDPurpose/Objective(s): Preoperative breast radiation therapy is actively under investigation in clinical trials but practical experience with this technique and follow-up is limited. Our two single institutional trials evaluating preoperative partial breast irradiation are the largest US experience to date. In this study, we evaluated routine breast surveillance images following preoperative radiation in order to identify post-treatment mammographic imaging patterns unique to the preoperative approach. Materials/Methods: At one institution, women 55 years or older with clinically node negative, ER+ and/or PR+, HER2-, T1 invasive carcinomas or low-intermediate grade in situ disease <2cm were enrolled (nZ32). Intensity-modulated radiation therapy was used to deliver a single dose of 15, 18, or 21Gy to the tumor plus a 1.5cm margin. Lumpectomy was performed within 10 days of radiation. At the second institution, women with clinically node negative infiltrating ductal carcinomas measuring < 3 cm were enrolled. The pre-operative regimen consisted of 38.5 Gy in 3.85 Gy fractions delivered BID using NSABP B-39 expansions. Surgical resection followed at a median of 31 days (no sooner than 21 days) following completion of RT. For both cohorts, baseline mammography was reviewed and compared to all available posttreatment mammograms by an expert breast imager at each institution (nZ 31 at institution 1, n Z 27 at institution 2). Descriptive statistics were utilized to summarize findings. Results: In patients treated with larger single dose of preoperative radiation, median breast imaging follow-up is currently 26 months. In 17/31 (55%) of patients, exuberant and unusually well-demarcated fat necrosis beyond that expected following post-operative adjuvant radiation therapy was observed. For most patients (13/17; 76%), this change was evident by the first post-treatment mammogram (median 13 months) and persistent with additional follow-up. In contrast, patients treated with a fractionated course of preoperative radiation had a lower rate (2/27; 7%) of fat necrosis exceeding the typical post-BCT findings at a median follow-up of 38 months. However, 55% of patients were felt to have a wider area of scar tissue density at the lumpectomy cavity site. Conclusion: Postoperative MMG findings differ between two preoperative radiation therapy regimens. Patients treated with a large single dose of radiation, in contrast to fractionated radiation, demonstrate frequent and well-demarcated fat necrosis that appears to correlate with treated target volume. These imaging findings may be attributable to radiation-induced tissue ablation in the treated tissues, which differ according to treatment regimens. These findings have implications for surveillance in women treated with preoperative radiation.