Axillary Nodal Involvement in Microinvasive Breast Cancer: A Systematic Review and Meta-Analysis of Sentinel Lymph...

Microinvasive breast cancer represents a transitional entity between ductal carcinoma in situ and invasive disease, with generally favorable outcomes but an uncertain risk of lymph node involvement. The role of sentinel lymph node (SLN) biopsy in this population remains controversial. This study aimed to estimate the pooled rate of SLN positivity and characterize the burden of nodal metastasis in patients with microinvasive breast cancer. A systematic review and single-arm meta-analysis were conducted in accordance with PRISMA guidelines. PubMed was searched for studies published between 2015 and 2025 reporting SLN biopsy outcomes in patients with histologically confirmed microinvasive breast cancer (T1mi, ≤1 mm stromal invasion) undergoing sentinel lymph node biopsy. Eligible studies reported the number of patients undergoing biopsy and the number with positive nodes. Random-effects models were used to estimate pooled proportions of SLN positivity and metastatic burden, including isolated tumor cells (ITCs), micrometastases, and macrometastases. Seven studies comprising 11,243 patients were included. Overall, 500 patients (4.4%) were SLN-positive, with a pooled positivity rate of 8.6% (95% CI, 4.3-15.4%) and substantial heterogeneity (I² = 97%). Among SLN-positive cases, pooled proportions were 42.3% (95% CI, 29.2-56.5%) for ITCs, 41.4% (95% CI, 31.4-52.1%) for micrometastases, and 28.9% (95% CI, 15.5-47.5%) for macrometastases. SLN positivity in microinvasive breast cancer is uncommon but not negligible and is most often characterized by low-volume disease. Low-volume disease, including ITCs and micrometastases, comprised the majority of positive nodes, with macrometastases identified in a clinically meaningful subset. These findings support a selective, risk-adapted approach to axillary staging, balancing the potential diagnostic benefit of biopsy against procedural morbidity.