Case Report: Granular cell tumor of the axillary tail mimicking node-positive breast carcinoma - two cases with imaging-pathology correlation.

Granular cell tumor of the breast (GCTB) is a rare Schwann-cell-derived neoplasm whose multimodal imaging features overlap extensively with those of invasive carcinoma, frequently exposing patients to unnecessarily extensive surgery in the absence of preoperative tissue diagnosis. We report two institutional cases with detailed imaging-pathology correlation. In Case 1, a 53-year-old postmenopausal woman with a 2.5-cm left axillary tail mass displayed convergent malignant-pattern features across all three imaging modalities - a BI-RADS 5 spiculated mass with skin retraction on mammography, marked posterior acoustic shadowing on ultrasound, and heterogeneous enhancement with a type II (plateau) kinetic curve on contrast-enhanced MRI (a pattern associated with intermediate-to-high suspicion for malignancy) - together with an ipsilateral axillary lymph node demonstrating type III (washout) kinetics. The composite preoperative picture was clinically indistinguishable from cT2N1M0 invasive carcinoma. After the patient declined preoperative core needle biopsy (CNB), breast-conserving surgery and sentinel lymph node biopsy were performed; final pathology confirmed benign GCTB (S-100+, SOX10+, CD68+, Pan-CK-, GATA3-, ER/PR/AR-; 0 of 6 Fanburg-Smith criteria) with reactive nodal hyperplasia. In Case 2, a 38-year-old premenopausal woman with a 0.9-cm right breast nodule underwent ultrasound-guided vacuum-assisted biopsy (VAB) yielding benign GCTB; histology demonstrated tumor cells encircling peripheral nerve bundles within the lesion - direct morphological evidence supporting the established Schwann-cell origin of the tumor. Both patients remained disease-free at 12 and 36 months. These cases reinforce that preoperative CNB with a targeted immunohistochemistry panel - explicitly distinguishing GCTB from ectopic axillary breast carcinoma - should be strongly recommended for any suspicious axillary tail mass; Case 1, in which the patient declined CNB and proceeded directly to surgery, illustrates by counter-example the diagnostic uncertainty that results when this step is omitted. We present these cases in accordance with the CARE reporting guidelines.