Gastric metastasis of malignant melanoma with osteosarcomatous differentiation: a rare and unusual presentation

Gastric metastasis of melanoma is a rare event associated with poor prognosis and short survival. Patients usually present with gastrointestinal symptoms, and in the absence of a prior history of primary tumor elsewhere, this can pose a diagnostic challenge. Melanoma has an inherent property to show transdifferentiation, which is retained even at the site of metastasis. Osteosarcomatous differentiation in melanoma is extremely uncommon, restricted to isolated case reports in the literature. A 79-year-old male patient, a known case of gastric mucosa-associated lymphoid tissue lymphoma, presented with generalized weakness and dysphagia of 15 days duration. Clinical impression was of lymphoma progression. Upper gastrointestinal endoscopy showed a 4 × 5 cm proliferative growth in the gastric fundus. Biopsy of the lesion was suggestive of melanoma with osteosarcomatous differentiation. It was regarded as metastasis as patient had a history of acral lentiginous melanoma 2 years ago. Patient was, however, willing only for palliative treatment. Patient succumbed to septic shock within 2 months of diagnosis. Herein, we report a case of gastric metastasis of melanoma with osteosarcomatous differentiation and discuss the clinical presentation, radiologic findings, challenges in histopathological diagnosis, and treatment options for this unusual entity.