The impact of HER2 status in ductal carcinoma in situ (DCIS) on the risk of progression to invasive ductal carcinoma (IDC) has been debated. We aim to use a national database to identify patients with known HER2 status to elucidate the effect of HER2 overexpression on ipsilateral IDC (iIDC) development. We performed survival analysis on patient-level data using the U.S. NCI's Surveillance Epidemiology and End Results program. We identified patients diagnosed with DCIS who underwent lumpectomy and had known HER2 status. Competing risks analysis was performed. A total of 1,540 patients had known HER2 status and met inclusion criteria. Median age at diagnosis was 60, median follow-up time was 44.5 months. A total of 417 (27.1%) patients were HER2 positive and 1,035 (67.2%) were HER2 negative. Twenty-two (1.4%) patients developed iIDC and 27 (1.8%) developed ipsilateral in situ or contralateral disease. The estimated cumulative incidence of iIDC at 5 years was 1.9% for all patients, 1.2% for HER2-negative and borderline patients, and 3.9% for HER2-positive patients. On multivariate competing risks regression, two factors were significant for iIDC: radiation (protective) therapy within 24 months (HR, 0.05; P = 0.00006) and HER2 overexpression (increased likelihood; HR, 2.72; P = 0.044). Patients with HER2-positive DCIS were more likely to have recurrences with receptor discordance. HER2 may serve as a prognostic factor for invasive recurrence and was the only lesion-related factor to significantly relate to iIDC development. It may also be associated with receptor discordance of recurrences. Further large studies will be needed to confirm these results.

Histopathological growth distribution of ductal carcinoma in situ: tumor size is not “one size fits all” Thomas J. O’Keefe, Olivier Harismendy, Anne M. Wallace

Ductal carcinoma in situ (DCIS) incidence has risen rapidly with the introduction of screening mammography, yet it is unclear who benefits from both the amount and type of adjuvant treatment (radiation therapy, endocrine therapy) versus what constitutes over-treatment. Our goal was to identify the effects of adjuvant radiation therapy, or endocrine ± radiation therapy versus breast conservation surgery alone in a large multi-center registry of retrospective DCIS cases (N = 1916) with median follow up of 7.0 (IQR: 8.43) years. We show that patients with DCIS who took less than 2 years of adjuvant endocrine therapy alone have a similar second event rate as breast conservation surgery. However, patients who took more than 2 years of endocrine therapy show a significantly reduced second event rate, similar to those who received either radiation or combined endocrine + radiation therapy, which was independent of age, tumor size, grade, or period of diagnosis. This highlights the importance of endocrine therapy duration for risk reduction.