Osteoblastic skeletal metastases mimicking high-bone-turnover metabolic bone disease in gastric adenocarcinoma

Markedly elevated bone-specific alkaline phosphatase with radiographic skeletal abnormalities is a common endocrine presentation, most often attributed to disorders with high bone turnover, including Paget disease of bone or osteomalacia. Rarely, tumor-driven osteoblastic metastases may closely mimic this biochemical and radiographic phenotype, thereby potentially delaying the diagnosis of occult malignancy. This report describes a 50-year-old woman presenting with progressive dyspnea and constitutional symptoms who was found to have massive pericardial effusion with cardiac tamponade requiring pericardiocentesis. Imaging revealed diffuse osteosclerotic lesions with axial skeleton involvement. Laboratory examination showed markedly elevated alkaline phosphatase with a disproportionately high bone-specific component, alongside mild hypocalcemia, secondary hyperparathyroidism, and severe vitamin D deficiency, initially suggesting a high-bone-turnover metabolic bone disorder. However, whole-body fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) demonstrated multiple sclerotic skeletal lesions with mild homogeneous marrow uptake, suggesting metastatic disease. Upper gastrointestinal endoscopy identified a lesion in gastric antrum, and biopsy confirmed poorly differentiated signet-ring cell adenocarcinoma. Bone marrow biopsy established metastasis, confirming stage IV disease with diffuse osteoblastic skeletal metastases. This case highlights tumor-mediated osteoblastic activity as an important endocrine mimic and underscores the need to integrate biochemical, radiologic, and histopathologic findings for the timely diagnosis of underlying malignancy.