We presented a case of diffuse-type tenosynovial giant cell tumour (DTSGCT) of foot masquerading as Langerhans cell histiocytosis. Preliminary diagnosis by needle biopsy was difficult due to the major involvement of bones and the overshadowing effect of the accompanying Langerhans cells. The complete curettage specimen with relevant immunohistochemistry and molecular tests made the final diagnosis of DTSGCT possible. The biomolecular mechanism for the masquerading phenomenon was explained by CSF1 overexpression in the neoplastic cells attracting migration and proliferation of CSF1R-positive Langerhans cells.

This a preprint and has not been peer reviewed. Data may be preliminary.

When faced with a slowly enlarging firm mass in the parotid gland accompanied by a histological picture of unusual sclerosis with abundant Langerhans cells and eosinophilic infiltrates, sclerosing mucoepidermoid carcinoma with eosinophilia should be considered as one of the differential diagnoses. Further studies are warranted for accurate diagnosis and appropriate treatment.