Apocrine encapsulated papillary carcinoma of the breast with microscopic multifocal capsular invasion: a case report.

Apocrine encapsulated papillary carcinoma (A-EPC) of the breast is exceedingly rare, having been described primarily through sporadic case reports prior to 2022 (typically fewer than 10 cumulative cases per report). The largest series to date - 28 cases from a single centre - was recently published in 2026. Whether breast-conserving surgery (BCS) is appropriate for A-EPC with frank invasion, and how the recent Rakha-Quinn (2025) staging criteria should be applied, remain unresolved. A 57-year-old postmenopausal woman presented with a 5-month history of an enlarging right breast mass. Imaging demonstrated a 27 × 17 × 17 mm well-circumscribed nodule at the 9 o'clock position, approximately 2 cm lateral to the nipple. Intraoperative frozen section identified prominent apocrine cytological features, predicting a hormone-receptor-negative phenotype; same-session sentinel lymph node biopsy (SLNB) was therefore performed during the index operation. Successful BCS with negative margins was achieved. Final histopathology demonstrated A-EPC (2.3 × 1.8 × 1.5 cm) with microscopic multifocal frank invasion (largest focus 4 mm; pT1aN0M0; 0/4 sentinel nodes). Immunohistochemistry confirmed ER-/PR-/HER2 2+ (FISH-negative)/AR+ (~90%)/GCDFP-15+/Ki-67 ~15%. A PIK3CA exon 20 H1047R/L hotspot mutation was detected. After multidisciplinary discussion, no systemic therapy was administered and the patient completed adjuvant whole-breast radiotherapy. Three points were central to this case: intraoperative frozen-section recognition of apocrine differentiation supported same-session SLNB during breast-conserving surgery; systematic application of the Rakha-Quinn (2025) staging criteria - including the 5 mm proximity rule and the assessment of whether microscopic multifocal invasion indicates a diffusely invasive EPC-like pattern - supported staging as pT1a; and the co-occurrence of strong androgen-receptor positivity and a PIK3CA mutation provides a rationale for AR-PI3K dual-pathway targeted therapy should recurrence occur.