Abstract PS4-08-02: Prognosis and Management of DCIS with Microinvasion: Insights from SEER Database and a Chinese Cohort

Abstract Objective Ductal carcinoma in situ with microinvasion (DCISM) is a subtype of DCIS characterized by microscopic invasive foci (≤1 mm). Representing about 1% of breast cancer cases and 5%-10% of DCIS diagnoses, DCISM has both in situ and invasive features, complicating its clinical management. This study compares the prognostic outcomes of DCISM with DCIS and evaluates therapeutic approaches to inform evidence-based decision-making and optimize patient care. Methods This retrospective cohort study analyzed data from two sources: the Surveillance, Epidemiology, and End Results (SEER) database (1998-2018), comprising 7,467 patients (4,458 with DCIS and 3,009 with DCISM), and a Chinese cohort of 1,132 patients (903 with DCIS and 229 with DCISM) from Sun Yat-sen Memorial Hospital and the Chinese Society of Clinical Oncology. To reduce confounding bias, risk regression analysis and propensity score matching (PSM) were applied, enabling a robust comparison of survival outcomes between DCIS and DCISM. Key variables included patient age, tumor size, molecular subtypes (estrogen receptor/progesterone receptor ER/PR status), and treatment modalities (surgical type, radiotherapy, chemotherapy, and endocrine therapy). Prognostic endpoints assessed were overall survival (OS), breast cancer-specific survival (BCSS), and disease-free survival (DFS). Results Patients with DCISM displayed more aggressive pathological characteristics than those with DCIS, including a higher prevalence of high-grade tumors (P 0.001) and elevated HER2 expression (P 0.001), alongside a greater likelihood of undergoing total mastectomy (P 0.001). Despite these differences, no statistically significant disparities were observed in OS, BCSS, or DFS between the two groups, suggesting comparable long-term prognoses. Breast-conserving surgery (BCS) combined with radiotherapy emerged as the optimal treatment strategy for both DCISM and DCIS, significantly enhancing DFS compared to BCS alone or radical mastectomy (DCIS: BCS alone vs. BCS + radiotherapy, hazard ratio HR = 2.37, 95% confidence interval CI: 1.36-4.16, P = 0.003; DCISM: HR = 15.03, 95% CI: 1.68-134.60, P = 0.015). In DCISM, this approach also improved OS (HR = 3.85, 95% CI: 1.87-7.90, P 0.001). Conversely, post-surgical chemotherapy was associated with significantly worse DFS in DCISM patients (P = 0.0062), indicating a potential detrimental effect, whereas it showed no significant impact on BCSS or OS in DCIS patients. Endocrine therapy markedly improved DFS in DCIS patients (HR = 0.46, 95% CI: 0.25-0.85, P = 0.013), but this benefit was not evident in DCISM. Targeted therapy yielded no clear survival advantage in HER2-positive cases (P 0.05). Conclusion Despite the more aggressive pathological profile of DCISM, its survival outcomes align closely with those of DCIS, highlighting the risk of overtreatment in clinical practice. BCS combined with radiotherapy is an effective therapeutic option for both conditions, significantly improving survival metrics. Chemotherapy appears contraindicated for DCISM, particularly in hormone receptor-positive cases, due to its adverse prognostic impact. Endocrine therapy is recommended for hormone receptor-positive DCIS patients, though its efficacy in DCISM remains unestablished. These findings underscore the need for tailored treatment strategies based on molecular profiles and recurrence risk, offering critical guidance for managing DCISM. Future studies should investigate long-term outcomes and molecular underpinnings to refine individualized therapeutic approaches. Citation Format: Y. Yang, X. Chen, q. Peng. Prognosis and Management of DCIS with Microinvasion: Insights from SEER Database and a Chinese Cohort abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-08-02.