e12620 Background: Breast cancer is a leading cause of cancer-related mortality. Advances in screening and imaging have increased detection of early-stage disease, including lesions along the spectrum of ductal carcinoma in situ (DCIS) and invasive breast cancer. Microinvasive breast cancer (T1mi), defined as ≤1 mm of stromal invasion, represents a small subset of cases and demonstrates favorable outcomes that more closely resemble DCIS than larger invasive tumors. Sentinel lymph node biopsy (SLNB) is standard for invasive breast cancer but not recommended for DCIS, rendering its role in T1mi controversial. We evaluated SLNB positivity in T1mi using published clinical outcomes. Methods: A meta-analysis of seven published studies identified through a systematic PubMed search was performed. Studies reporting the number of T1mi patients, SLNB procedures, and SLNB-positive cases were included. When available, data on macrometastasis, micrometastasis, and isolated tumor cells (ITCs) were extracted. A single-arm random-effects meta-analysis was conducted. The primary outcome was pooled SLN positivity. Results: Seven studies including 11,243 patients with T1mi were analyzed. Overall, 500 patients (4.4%) were SLN positive, with reported rates ranging from 2.9% to 21.5%. The pooled SLN positivity rate was 9% (95% CI, 6–13%). Four studies reporting ITCs (n = 110 SLN-positive cases) demonstrated substantial heterogeneity, with ITCs comprising 9%–56% of nodal metastases; the pooled ITC proportion was 39% (95% CI, 24–55%). Five studies reporting micrometastatic and macrometastatic disease (n = 186 SLN-positive cases) showed pooled proportions of 40% (95% CI, 29–51%) and 29% (95% CI, 14–47%), respectively. Conclusions: SLN positivity in T1mi is uncommon but not negligible. When nodal metastases are present, they are most often low-volume, although macrometastatic disease occurs in a subset of patients. These findings highlight heterogeneity in axillary involvement and support individualized SLNB decision-making in T1mi breast cancer. Sentinel lymph node outcomes in microinvasive breast cancer (T1mi). Outcome No. of Studies No. of Patients Range (%) Pooled Estimate (95% CI) Any SLN positivity 7 11,243 2.9-21.5 9% (6-13) Isolated tumor cells (≤0.2 mm)* 4 110† 9-56 39% (24-55) Micrometastasis (0.2–2 mm)* 5 186† 17-55 40% (29-51) Macrometastasis (>2 mm)* 5 186† 16-51 29% (14-47) *Proportions calculated among SLN-positive patients† SLN-positive cases with available stratification data.
Brady et al. (2026) studied this question.