Abstract PO2-23-05: Practice patterns and outcomes in the ongoing neoadjuvant ATNEC trial: node marking, response to NACT and breast conservation rates

Abstract Background In ATNEC trial, cT1-3N1M0 patients receive NACT followed by SNB/TAD. If the sentinel nodes(SNs) have converted to benign(ypN0), patients are randomly assigned to Axillary Treatment vs no Axillary Treatment. The study promotes standardized node marking procedures and includes a radiotherapy quality assurance program. Objectives – This study prospectively evaluates practice patterns, NACT response, adoption of node marking, rates of marked node identification, and breast conservation rates in the ongoing ATNEC trial across 70 centers in the UK. The trial aims to recruit 1900 patients. Materials and Methods Data from 196 randomized patients (median age: 54 range: 28-79 years; median BMI: 26.5 range: 17.6-51.1) from December 2021 to June 28, 2023, across 70 UK centers were analyzed. These patients presented with cT1-3N1M0 breast cancer, received NACT, and exhibited no residual nodal disease (ypN0) on SNB/TAD. Results Among the randomised patients 54%(106) were post-menopausal, 71%(140) underwent breast conserving surgery(BCS) and 29%(56) mastectomy. At presentation, 12% (23) had T3, 63% (124) had T2, and 23% (46) had T1 tumors. Grade 3 tumors accounted for 70% (138) of cases. Among the patients, 60% (117) were HER2 positive, 31% (61) were triple negative, and 9% (18) were HER2 negative (ER or PgR positive). Anthracycline and taxane-based NACT were administered to 57% (106), while 38% (71) received a platinum-containing regimen. During SNB/TAD, a median of 4 nodes (interquartile range, 3-5) were removed. Among 184 patients with node marking data, 74% (136) had the involved node marked. The marked node was successfully removed in 94% (128) of these patients. Clip/coil only, Magseed and black dye were the commonest techniques used for node marking. The marked node was the sentinel node in 83% (106) of cases. Among 185 randomized patients with post-NACT response data, 68% (125) achieved a complete pathological response (pCR) in the breast, while 8% (15) had DCIS only and 22% (41) had invasive cancer. Among 105 patients with complete breast tumor response on imaging, 15% (16) had residual DCIS or invasive cancer. In the subset of patients with partial response or stable breast tumors on imaging (69 patients), 46% (32) had no residual tumor on histology (ypT0). Furthermore, 14% (26) exhibited partial or stable disease in the axilla on imaging but achieved complete pathological response on histology. Conclusions HER2-positive and triple-negative breast cancer cases predominate among patients undergoing NACT in the UK. Although 68% achieved a complete pCR (ypT0) in the breast, rates of BCS remain low. Notably, around 75% of randomized patients underwent node marking, with an intra-operative identification rate of 94%, demonstrating the successful implementation of node marking in the UK through the ATNEC trial. Citation Format: Amit Goyal, Andrea Marshall, Sophie Nicholls, Natalie Hammonds, Beatrix Elsberger, Duncan Wheatley, Janice Rose, Helen-Teresa Edwards, Abeer Shaaban, Roeum Butt, Gareth Jackson, Tara Homer, Luke Vale, Samreen Ahmed, Shama Puri, Sophie Gasson, Julie Bruce, Helen Higgins, Janet Dunn. Practice patterns and outcomes in the ongoing neoadjuvant ATNEC trial: node marking, response to NACT and breast conservation rates 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 PO2-23-05.