Abstract P4-06-02: Complete response of ductal carcinoma in situ to neoadjuvant systemic therapy in HER2-positive invasive breast cancer patients: a nationwide analysis
Authors: Ploumen Roxanne , Claassens Eva , Kooreman Loes , Keymeulen Kristien , van Kats Maartje , Gommers Suzanne , Siesling Sabine , van Nijnatten Thiemo , Smidt Marjolein 
Published: March 2023

Abstract
Introduction Neoadjuvant systemic therapy (NST) is increasingly applied in breast cancer to increase breast-conserving surgery (BCS) rates and to improve oncological outcomes. Ductal carcinoma in situ (DCIS) can be present adjacent to invasive breast cancer (IBC), especially in HER2-positive IBC. DCIS was previously considered to be insensitive to NST. Consequently, mastectomy rates are higher in IBC with adjacent DCIS. Recent studies have shown that DCIS can be sensitive to NST, however, only small populations were investigated. Therefore, the aim of this study was to determine the rate of complete response of adjacent DCIS in HER2-positive IBC and to assess the potential influence of clinicopathological variables in a nationwide cohort. Methods All women diagnosed with HER2-positive IBC, treated with NST and surgery between January 2010 and December 2019, were selected from the Netherlands Cancer Registry (NCR). Of these patients, all pre-NST biopsy and postoperative specimen pathology reports were obtained from PALGA, the Dutch Pathology Registry, and assessed for presence of DCIS. Response of DCIS was defined as absence of DCIS in postoperative pathology when a DCIS component was present in the pre-NST biopsy. Clinicopathological factors associated with DCIS response were assessed using logistic regression analyses. Results In total, 5834 patients were included, of whom 1443 (24.7%) had a DCIS component in the pre-NST biopsy. Mastectomy rates were higher in IBC with adjacent DCIS compared to IBC without adjacent DCIS in the pre-NST biopsy (53.6% versus 41.0%, p&lt; 0.001). Of these 1443 patients, 743 (51.5%) showed complete response of the DCIS component. Complete response of DCIS occurred more frequently in patients who also had a complete response of IBC (63.4% versus 33.8%, p&lt; 0.001). Multivariable logistic regression analysis showed ER negative IBC (OR 1.79; 95% CI 1.33-2.42) and treatment with HER2-targeted therapy (OR 5.97; 95% CI 1.82-19.55) to be independently associated with complete response of DCIS. Conclusion More than half of HER2-positive IBC patients with adjacent DCIS in the pre-NST biopsy showed a complete response of the DCIS component to NST. Complete response of DCIS should be considered, especially in ER-negative HER2-positive IBC and in case of complete response of IBC. Future studies should investigate the evaluation of DCIS response by imaging and the possibility of increasing breast-conserving surgery rates.
Citation Format: Roxanne Ploumen, Eva Claassens, Loes Kooreman, Kristien Keymeulen, Maartje van Kats, Suzanne Gommers, Sabine Siesling, Thiemo van Nijnatten, Marjolein Smidt. Complete response of ductal carcinoma in situ to neoadjuvant systemic therapy in HER2-positive invasive breast cancer patients: a nationwide analysis [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-06-02.