Granular Cell Tumour of Breast Mimicking Malignancy: A Series of Six Cases

Granular Cell Tumour (GCT) of the breast is a rare, benign neoplasm of presumed Schwann cell origin, accounting for less than 0.1% of all breast tumours. Despite its benign nature and low recurrence risk, it often mimics malignancy both radiologically and histologically, leading to diagnostic uncertainty. This case series describes six histologically confirmed cases of breast GCT diagnosed over an 11- year period, highlighting key clinical, radiological, histopathological, and immunohistochemical features. All patients were women between 32 and 42 years, presenting with solitary, unilateral breast lesions. Radiological findings in several cases were suspicious for malignancy. Tumours ranged from 1 to 4.1 cm, frequently located in the medial quadrant, and exhibited ill-defined margins in most cases. Histologically, tumours were infiltrative and composed of polygonal cells with granular eosinophilic cytoplasm and round hyperchromatic nuclei. Periodic Acid Schiff-Diastase (PAS-D) positivity and positive immunohistochemical stains for S100 (solubility in 100% saturated ammonium sulfate at neutral pH), SRY-related HMG box 10 protein (SOX10), Transcription Factor Binding to IGHM Enhancer 3 (TFE-3), and Cluster of Differentiation 68 (CD68) positivity helped confirm the diagnosis. Common histological findings included adipose and skeletal muscle infiltration, peritumoural lymphocytic infiltrates, rare features such as intranuclear inclusions and stromal elastosis. No mitoses, necrosis, or co-existing carcinoma were noted. No recurrences or metastases were observed during follow-up. Given its ability to mimic malignancy, especially on core biopsies, GCT must be considered in the differential diagnosis of breast lesions with granular cytoplasm. Misinterpretation may lead to overtreatment. Awareness of its histopathologic profile and immunohistochemical signature is essential to avoid diagnostic pitfalls.