Low grade malignant eccrine spiradenoma with florid squamous differentiation-a potential diagnostic pitfall

Malignant eccrine spiradenoma is an exceedingly rare tumor of cutaneous adenxal origin. It usually arises from a pre-existing eccrine spiradenoma and typically manifest as rapid enlargement in long-standing spiradenomas. It has two distinct patterns; high-grade carcinoma and low-grade tumor imitating benign spiradenoma. Low grade MES have subtle microscopic features that are more difficult to diagnose and are said to have a better prognosis. MES frequently shows focal squamous differentiation, which may be florid in rare instances. We describe the diagnostic difficulties encountered in a case of low grade malignancy in a eccrine spiradenoma with florid squamous differentiation. The tumor was removed 2 months after rapid increase in size of a long standing nodule over the left forearm was noted by the patient. Histopathological examination showed the earliest stage of malignant transformation within eccrine spiradenoma, characterized by mild nuclear pleomorphism, loss of two cell types, increased mitotic activity with atypical mitoses and florid squamous differentiation. PAS stain outlined a partial loss or destruction of PAS positive basement membrane in the malignant area. There was no evidence of vascular invasion. P53 was not expressed in benign areas, whilst in malignant areas occasional nuclear reaction was noted. The motive behind presenting this case is to highlight importance of recognizing some of the histological features of early malignant transformation in a rapidly increasing eccrine spiradenoma and diagnostic difficulties that can arise due to florid squamous differentiation in these tumors.