Primary Gastric Extraosseous Osteosarcoma with Extensive Multivisceral Invasion: A Case Report

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Introduction: Extraosseous osteosarcoma (ESOS) is a rare soft-tissue sarcoma that produces osteoid without any anatomical relationship to the skeleton. Primary gastric ESOS is exceptionally rare and presents major clinical, radiological, and pathological diagnostic challenges.Case Presentation: A 78-year-old man with six months of nonspecific gastrointestinal symptoms was found to have a 14×16×21 cm gastric mass invading the left hepatic lobe, pancreas, duodenum, and omentum. Endoscopic ultrasound–guided fine-needle aspiration was non-diagnostic. After massive upper gastrointestinal hemorrhage with hemodynamic instability, life-saving emergency laparotomy with R2 debulking was performed. Histopathology and SATB2-positive immunohistochemistry confirmed high-grade extraosseous osteosarcoma. PET-CT showed no distant or skeletal lesion, supporting a primary gastric origin.Conclusion: Primary gastric ESOS may present as a giant, multivisceral mass with non-diagnostic biopsies. Awareness, multidisciplinary evaluation, and timely surgery are critical, as resectability remains the principal prognostic determinant.