Isolated Psoas Muscle Metastasis From Ampullary Carcinoma: A Report of a Rare Case

Ampullary carcinoma is a rare malignancy arising from the ampulla of Vater. It commonly presents with obstructive jaundice and typically metastasizes to regional lymph nodes, the liver, lungs, and peritoneum. Skeletal muscle metastasis is extremely uncommon, and involvement of the psoas muscle is particularly rare. We report the case of a 67-year-old man presenting with obstructive jaundice and cholestatic liver function abnormalities. Cross-sectional imaging demonstrated dilation of both the common bile duct and pancreatic duct, consistent with the classic double-duct sign, without a clearly identifiable obstructing lesion. In addition, heterogeneous thickening of the right psoas muscle was observed. Endoscopic retrograde cholangiopancreatography (ERCP) with biopsy confirmed poorly differentiated adenocarcinoma of the ampulla of Vater. Magnetic resonance imaging (MRI) further characterized the psoas lesion, and percutaneous biopsy demonstrated metastatic carcinoma compatible with a biliopancreatic primary tumor. The patient was diagnosed with metastatic ampullary carcinoma with psoas muscle involvement, and multidisciplinary evaluation also determined the primary tumor to be locally unresectable due to advanced locoregional disease. The patient was therefore referred for palliative systemic chemotherapy. This case highlights an exceptionally rare metastatic presentation of ampullary carcinoma and emphasizes that atypical muscular lesions in patients with suspected or confirmed malignancy should prompt consideration of metastatic disease and histological confirmation, as accurate diagnosis is essential for appropriate staging and treatment planning.