Merkel cell carcinoma metastasis and dermatofibrosarcoma protuberans presenting as a collision tumour: a case report and review of the literature

INTRODUCTION: Merkel cell carcinoma and dermatofibrosarcoma protuberans are two very rare neoplasms. The simultaneous occurrence of two different tumour entities at the same anatomical site, collision tumours, is a rare phenomenon. CASE PRESENTATION: We present a rare case of a 74-year-old woman with a previous history of a recurrent dermatofibrosarcoma protuberans presenting with a metastatic Merkel cell carcinoma. Further investigation revealed a collision tumour of a metastatic lesion of the Merkel cell carcinoma within a tumour relapse of a dermatofibrosarcoma protuberans. CONCLUSION: Synchronous occurrence of two different tumour entities is extremely rare and has not been described for Merkel cell carcinoma and dermatofibrosarcoma. Merkel cell carcinoma, a tumour of the elderly or immunocompromised patients, leads to early metastasis and can be expected to be the limiting factor for prognoses.