Case Report: Primary gastric extraskeletal osteosarcoma and literature review.

Extraskeletal osteosarcoma is a rare and aggressive malignant mesenchymal tumor that accounts for approximately 1% of all soft tissue sarcomas and usually arises in the deep soft tissues of the extremities. Primary visceral involvement is extremely uncommon. We report a rare case of primary gastric extraskeletal osteosarcoma in a 65-years-old woman who presented with progressive abdominal pain, distension, nausea, vomiting, and cessation of flatus and defecation. Contrast-enhanced abdominal computed tomography revealed a large cystic-solid mass in the left middle and lower abdomen, with linear calcifications and central non-enhancing areas suggestive of necrosis and cystic degeneration. Because of worsening obstructive symptoms, an emergency radical distal gastrectomy was performed. Histopathological examination showed spindle-shaped tumor cells arranged in sheets, with multifocal lace-like malignant osteoid formation. Immunohistochemical findings, together with exclusion of metastatic skeletal osteosarcoma, supported the diagnosis of primary gastric extraskeletal osteosarcoma. Adjuvant chemotherapy was recommended after surgery, but was declined because of the patient's poor general condition. No evidence of local recurrence or distant metastasis was observed during 9 months of follow-up. This case highlights that primary gastric extraskeletal osteosarcoma, although extremely rare, should be considered in the differential diagnosis of atypical gastric or intra-abdominal masses with calcification, necrosis, and spindle-cell morphology. Complete surgical resection remains the main treatment, and close long-term surveillance is essential because of the high risk of recurrence and metastasis.