Primary digestive melanoma in association with tubular adenoma: a case report illustrating the distinction from metastatic colonic melanoma.

We report here an exceptional pattern of atypical lentiginous melanocytic proliferation within an adenoma, leading to focal lamina propria infiltration and pulmonary metastasis, which was considered as primary colonic mucosal melanoma (MM) in a Caucasian patient. Such case illustrates the diagnosis criteria required to differentiate primary MM from colonic metastasis of melanoma, including the absence of past history of other primary melanoma, a unique colonic and abdominal lesion with predominant features of in situ lentiginous MM and a very focal and unique invasive area without other digestive tract or abdominal localization. This tumor displayed a KIT exon 11 mutation leading to a unique combination of p.I571M and p.D572G deleterious amino acid changes. Such pattern also favors the diagnosis as KIT appears as a master oncogenic player in MM oncogenesis.