Clinicopathological characteristics of peripheral primitive neuroectodermal tumour of skin and subcutaneous tissue

Aims: To study the clinical and pathological features of primary malignant peripheral primitive neuroectodermal tumours (PNETs) of the skin and subcutaneous tissue, to discuss the differential diagnosis, and to review the existing literature on these tumours. Methods and results: Eight cases of PNETs presenting in the skin and subcutaneous tissue were identified from the pathology records of the Christie Hospital, Manchester. Detailed immunohistochemical studies were performed on all cases and seven tumours were subjected to electron microscopic examination. Detailed clinical and follow‐up information was obtained on seven cases. Six tumours occurred in children and adolescents and two were seen in young adults (age range, 8–36 years). No sex or site predilection was observed. Five tumours occupied the dermis and subcutis and three were entirely located in the subcutaneous tissue. Microscopically, they were composed of small round cells and seven tumours contained glycogen. Only one tumour focally exhibited Homer–Wright rosettes and neuropil. Two tumours contained rhabdoid or plasmacytoid cells in places and all cases showed microcystic and pseudovascular spaces. Immunostains revealed MIC2 (8/8), NSE (7/8), PGP9.5 (7/8), β2 microglobulin (7/8), neurofilament protein (6/8), S100 protein (3/8), synaptophysin (2/8) and Leu‐7 (1/8) positivity. Anomalous cytokeratin (4/8), desmin (2/8), myoglobin (2/8), NKIC3 (4/8) and GFAP (1/8) staining was also noted. Ultrastructurally, neuroendocrine granules were detected in five cases and one case exhibited microtubules in processes. Adequate follow‐up information was available in four cases. One patient died of metastatic disease. One child developed axillary lymph node metastasis but is alive with no evidence of disease 96 months after treatment. Two other patients are alive with no residual or recurrent disease 44 and 52 months after excision and radio/chemotherapy. Conclusion: PNETs are rare malignant small round cell tumours of the skin and subcutaneous tissue which are probably underdiagnosed. A correct diagnosis can be made on light microscopic features, demonstration of neuroendocrine granules on electron microscopy and a combination of MIC2, β2 microglobulin, and more than one neural marker positivity. These neoplasms should be differentiated from other cutaneous neoplasms composed of small round cells. The number of cases of cutaneous PNETs studied so far is rather small, and no firm conclusion can be drawn abou