Granular cell tumour is a rare tumour with Schwann cell origin which can occur anywhere, including in the breast (5-8%). It presents as a small (1-3 cm) firm nodule in the upper inner quadrant of the breast and can mimic invasive carcinoma on imaging. The majority of the granular cell tumors are benign, and complete excision is curative.

On gross examination, it presents as a yellow-white circumscribed or ill-defined firm/hard tumour resembling invasive carcinoma. On microscopy, it appears as solid sheets or small nests of round to slightly spindle shaped cells with small nuclei and abundant eosinophilic granular cytoplasm. The cytoplasmic granules are PAS positive and the tumour cells shows the following immunoprofile: S100(+), calretinin(+), CD68(+), NSA(+), cytokeratin(-), hormone receptors(-).

Granular cell tumors require more than three of the following histologic features to be considered malignant: spindle cells, pleomorphism, high nuclear-cytoplasmic (N/C) ratio, vesicular nuclei with prominent nucleoli, mitosis > 2/10HPF and necrosis. Differential diagnoses include: benign inflammatory lesions with histiocytic infiltrate, invasive carcinoma with histiocytoid or apocrine features, melanoma, renal cell carcinoma and alveolar soft part sarcoma.