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Background  Melanoma of the anal region is a very uncommon disease, accounting for only 0.2-0.3% of all melanoma cases. Mutations of the  BRAF  gene are usually absent in melanomas occurring in this region as well as in other sun-protected regions. The development of a tumour in a longstanding perianal fistula is also extremely rare. More frequent is the case of a tumour presenting as a fistula, that is, the fistula being a consequence of the cancerous process, although we have found only two cases of fistula-generating melanomas reported in the literature.    Case Presentation  Here we report the case of a 38-year-old male who presented with a perianal fistula of four years of evolution. Histopathological examination of the fistulous tract confirmed the presence of malignant melanoma. Due to the small size and the central location of the melanoma inside the fistulous tract, we believe the melanoma reported here developed in the epithelium of the fistula once the latter was already formed. Resected sentinel lymph nodes were negative and the patient, after going through a wide local excision, remains disease-free nine years after diagnosis. DNA obtained from melanoma tissue was analysed by automated direct sequencing and the  V600E  ( T1799A ) mutation was detected in exon 15 of the  BRAF  gene.    Conclusion  Since fistulae experience persistent inflammation, the fact that this melanoma harbours a  BRAF  mutation strengthens the view that oxidative stress caused by inflammatory processes plays an important role in the genesis of  BRAF  gene mutations.

Malignant melanoma arising from a perianal fistula and harbouring a BRAFgene mutation: a case report