Abstract PS01-08: Management of ipsilateral breast tumor recurrence following breast conservation surgery for ductal carcinoma in situ – a data-free zone

Abstract Background: Breast-conserving surgery (BCS) is well established for the management of ductal carcinoma in situ (DCIS). Although a growing body of data support re-conservation therapy (RBCS) for ipsilateral breast tumor recurrence (IBTR) following BCS in invasive cancer, neither randomized trials nor guidelines address management of IBTR following BCS for DCIS. Here we aim to compare the outcomes of mastectomy vs RBCS for a large series of DCIS patients with IBTR. Methods: We identified women treated with BCS for DCIS at MSKCC who developed IBTR as a first event. Between those treated with mastectomy vs RBCS, we compared the clinicopathologic characteristics for the initial and recurrent tumors, the use of adjuvant RT (both upfront (“primary RT”) and post IBTR (“secondary RT”)) and/or tamoxifen, the rate of third events (local, regional, distant), breast cancer specific (BCSS) and overall survival (OS). Results: From our service databases among 3001 women treated with BCS for DCIS (1978-2010), we found 383 who developed an IBTR as a first event and were treated at our institution between 1983-2023, 186 (49%) with RBCS and 197 (51%) with mastectomy. RBCS was more frequent over time and comprised 56% of patients treated between 2014 and 2023. Among those treated with mastectomy, the initial tumors were significantly more likely to have necrosis (74% vs 59%), high grade (47% vs 28%), comedo histology (38% vs 20%), and to have received primary RT at the time of BCS (61% vs 21%). Between those who underwent mastectomy vs RBCS, there were no significant differences in disease-free interval, in the pathologic characteristics of their IBTR, or in the proportion of invasive vs in situ disease. For patients treated with RBCS, 11 (5.9%) received both primary and secondary RT and 77 (41.3%) received neither (Table 1a). For patients treated with a mastectomy only 8.5% had secondary RT (Table 1b). At a median follow-up of 5.1 years post-IBTR, third local events in total were more frequent for RBCS vs mastectomy (16.1% vs 3.0%, 0.001), but there were no differences in BCSS or OS. Among RBCS patients, third local events (breast re-recurrence) were least frequent among those who received primary and secondary RT, and comparable between with who did vs did not receive secondary RT (16% (13/82) vs 16% (117/104), Table 2). Conclusions: Our data show that for women with isolated IBTR following BCS for DCIS and treated by mastectomy vs RBCS, 1) treatment with mastectomy was associated with less favorable initial pathology and more frequent use of primary RT, 2) re-recurrence was more frequent with RBCS, and 3) BCSS and OS were comparable. In an era of increasing surgical de-escalation, our data suggest a wider role for RBCS and – as for patients having RBCS for IBTR following invasive cancer – further study of the relationship between secondary RT and the rate of third breast events. Citation Format: Brian Diskin, Varadan Sevilimedu, Kimberly Van Zee, Monica Morrow, Hiram Cody. Management of ipsilateral breast tumor recurrence following breast conservation surgery for ductal carcinoma in situ – a data-free zone abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS01-08.