Primary cutaneous Ewing sarcoma/primitive neuroectodermal tumour: a clinicopathological analysis of seven cases highlighting diagnostic pitfalls and the role of FISH testing in diagnosis

Aims: To perform a clinicopathological analysis of a series of primary cutaneous Ewing sarcomas/primitive neuroectodermal tumours (ES/PNET) to highlight the pathological features, discuss the differential diagnosis, emphasise the role of molecular testing (particularly fluorescence in situ hybridisation, FISH) in diagnosis and outline the patients' clinical course. Methods: Seven cases of primary cutaneous ES/PNET were identified from the authors' consultation files. Results: The patients were aged 16-61 years (median 25). Five were female and two were male. Five cases involved the limbs and two the trunk. Five were initially misdiagnosed (three as carcinoma and two as melanoma). All cases were characterised histologically by sheet-like growth of small round cells with little cytoplasm and showed strong membranous staining for CD99 and positive but variable staining for FLI-1. Six patients showed an EWS rearrangement (five on FISH analysis and one on RT-PCR). All tumours were completely excised. Three patients received adjuvant chemotherapy, one of whom also received radiotherapy. Follow-up was available in all cases (range 11-57 months; median 41). No recurrences or metastases occurred. Conclusions: Although rare, primary cutaneous ES/PNET should be considered in the differential diagnosis of cutaneous "small blue cell tumours''. Immunostaining for FLI-1 and molecular testing for evidence of an EWS rearrangement are useful ancillary investigations to confirm the diagnosis. The prognosis of primary cutaneous ES/PNET appears to be more favourable than extracutaneous ES/PNET.