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.