Primary Gastric Melanoma: Wait, I Thought the Stomach Did Not Have Any Melanocytes!

Abstract

Introduction: We report a case of a patient who presented with abdominal pain and decreased ability to tolerate food, and was found to have melanoma in the gastric mucosa. We highlight the difficulty of diagnosing primary gastric melanoma with certainty, given its rarity and theoretical implausibility, as normal gastric epithelium lacks melanocytes. We also briefly review several pathophysiologic and embryologic hypotheses for this phenomenon. A 50-year-old Hispanic male presented to our clinic with epigastric pain and decreased ability to tolerate food. An EGD revealed a large, irregular ulcerated gastric mass extending 1.5 to 2 cm from the gastroesophageal junction to the antrum. The mass was biopsied, and immunohistochemical stains showed the tumor cells were positive for melan-A, HMB45 and vimentin, confirming melanoma. A thorough search for another possible primary lesion, consisting of a complete skin examination and positron emission tomography (PET), did not identify a clear primary site of melanoma. The PET scan revealed uptake involving the gastric walls and lymph nodes in the perigastric, retroperitoneal and left supraclavicular regions. BRAF analysis was positive for a V600 mutation. The patient subsequently underwent palliative jejunostomy placement and was evaluated by medical oncology. Primary gastric melanoma is extremely rare (with only a handful of cases reported in the literature), likely due to the lack of melanocytes in normal gastric epithelium. Therefore, when melanoma is encountered in the stomach, it is crucial to search for a primary melanocytic lesion that may have metastasized to the stomach. While gastrointestinal involvement by melanoma has been found in up to 60% of autopsies of patients who have died with primary cutaneous melanoma, melanoma located primarily in the gastric mucosa is an uncommon occurrence. Several pathophysiologic mechanisms have been suggested. Ectopic migration of melanocyte precursors or differentiation of the APUD cells to melanocytes have been suggested as possible mechanisms. Spontaneous regression of primary melanoma has also been recognized, and it is possible that our patient’s gastric melanoma may have represented a metastasis from primary lesion that had spontaneously regressed. The median overall survival for primary melanoma of the GI tract is 17 months; for primary melanoma of the stomach, 5 months. There is little consensus on therapy for patients with primary GI melanoma, but treatment with complete excision may provide long-term disease-free survival. Vemurafenib, a BRAF kinase inhibitor, has been shown to improve both overall and progression-free survival in patients with metastatic melanoma with the BRAF V600E mutation