Granular cell tumours are tumours of Schwann cell origin that are composed of polygonal cells with abundant granular cytoplasm. These tumours are usually small (1-2 cm), solitary and are slow growing (they can be multicentric in up to 10% of cases). They can occur at any site however they are most common in the skin and subcutaneous tissue.

Granular cell tumours are typically benign and have an excellent prognosis. Malignant granular cell tumours are very rare. When malignant, granular cell tumours metastasize to distant sites in up to 50% of cases.
These tumours are more frequent in females and in African American patients.

On microscopy, granular cell tumours usually form a non-encapsulated mass with irregular borders, forming sheets, nests and cords of plump cells with a slightly spindled appearance and with abundant eosinophilic granular cytoplasm. The nuclei are small and hyperchromatic.

Malignant granular cell tumours show at least 3 atypical features such as: pleomorphism, nucleoli, high N:C ratio, spindling of cells, mitoses >2 per 10 HPF and necrosis.

Special staining for PAS highlights the cytoplasmic granules (diastase resistant). Positive immunohistochemical stains include S100 which stains strongly and diffusely, Sox-10 (nuclear), CD68, calretinin and inhibin.