title: Merkel cell carcinoma metastasis and dermatofibrosarcoma protuberans presenting as a collision tumour: a case report and review of the literature.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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.
authors.forename: Daniel-Johannes
authors.initials: DJ
authors.lastname: Tilkorn
authors.forename: Marcus
authors.initials: M
authors.lastname: Lehnhardt
authors.forename: Jörg
authors.initials: J
authors.lastname: Hauser
authors.forename: Adrien
authors.initials: A
authors.lastname: Daigler
authors.forename: Heinz
authors.initials: H
authors.lastname: Homann
authors.forename: Hans
authors.initials: H
authors.lastname: Steinau
authors.forename: Cornelius
authors.initials: C
authors.lastname: Kuhnen