Case Report: Gastrointestinal cancer initially presenting as postmenopausal osteoporosis.

Background: Gastric cancer usually presents late with nonspecific symptoms such as epigastric discomfort, anorexia, weight loss, nausea, anemia, or gastrointestinal bleeding. Initial presentation with osteoporosis-like manifestations and multiple bone metastases in the absence of gastrointestinal symptoms is rare. Case presentation: We report a 59-year-old postmenopausal woman who presented with thoracolumbar back pain after routine activity. Bone mineral density assessment showed osteoporosis, but laboratory evaluation revealed markedly increased bone turnover markers, including alkaline phosphatase, β-CTX, and P1NP. Because these findings were disproportionate to typical primary postmenopausal osteoporosis, secondary osteoporosis and other metabolic bone disorders were further investigated. Whole-body bone scintigraphy, SPECT/CT, and PET/CT demonstrated multiple osteoblastic skeletal lesions suggestive of metastatic disease. Gastrointestinal endoscopy subsequently identified two synchronous primary tumors: a poorly differentiated gastric adenocarcinoma with focal signet-ring cell differentiation and a moderately differentiated sigmoid colon adenocarcinoma. Bone biopsy confirmed metastatic adenocarcinoma of gastrointestinal origin, most likely from the stomach. No definite extraosseous visceral metastases were identified on imaging. Discussion: This case illustrates that disseminated gastric cancer may rarely present without gastrointestinal symptoms and may initially mimic postmenopausal osteoporosis, particularly when bone pain coexists with markedly elevated bone turnover markers. It also highlights the importance of a multidisciplinary diagnostic approach in patients initially suspected of primary osteoporosis but showing atypical biochemical or imaging findings. Clinicians should remain alert to secondary osteoporosis, including occult malignancy, in such settings.