Primary gastric plasmacytoma evolving to non-secretory multiple myeloma

RATIONALE: Primary gastric plasmacytoma is a rare form of extramedullary plasmacytoma that may mimic gastric adenocarcinoma or lymphoma. Although it is generally considered a localized disease, delayed histologic remission after radiotherapy and subsequent systemic progression remain poorly characterized. PATIENT CONCERNS: A 75-year-old man was referred after screening endoscopy revealed an approximately 3-cm protruding gastric mass initially suspected to represent advanced gastric cancer. DIAGNOSES: Repeat biopsy demonstrated atypical plasmacytoid cell proliferation with CD138 positivity and kappa light-chain restriction. Comprehensive systemic evaluation, including bone marrow biopsy, serum and urine electrophoresis, immunofixation, serum free light-chain analysis, and PET/CT, revealed no evidence of systemic disease, supporting a diagnosis of primary gastric plasmacytoma. INTERVENTIONS: Definitive radiotherapy was delivered to the stomach at a total dose of 45 Gy in 25 fractions. OUTCOMES: Follow-up endoscopy at approximately 3 months demonstrated marked regression of the lesion; however, biopsy revealed residual monoclonal plasma cells with persistent kappa light-chain restriction. Repeat biopsy performed approximately 9 months after radiotherapy demonstrated loss of light-chain restriction with a polytypic kappa/lambda pattern, consistent with histologic remission. During subsequent follow-up, multifocal osteolytic skeletal lesions developed, and biopsy of the L5 vertebral lesion confirmed plasma cell neoplasm. Following multidisciplinary reassessment, the patient was ultimately managed as non-secretory multiple myeloma, and systemic therapy with bortezomib, lenalidomide, and dexamethasone (VRd) was initiated. LESSONS: This case demonstrates that histologic remission after radiotherapy may be delayed and that, although uncommon, systemic progression may occur despite the absence of systemic disease at initial staging. Long-term surveillance integrating clinical, radiologic, and pathologic findings is therefore essential, and early post-treatment biopsy findings should be interpreted in the context of the overall clinical course.