Primary gastric plasmacytoma evolving to non-secretory multiple myeloma: A case report

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.

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.

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.

Definitive radiotherapy was delivered to the stomach at a total dose of 45 Gy in 25 fractions.

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.

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.