Solitary extramedullary plasmacytoma of the kidney in a child: a case report

BACKGROUND: Solitary extramedullary plasmacytoma (SEP) is a rare malignant neoplasm of plasma cells that predominantly affects adults, most commonly in the head and neck region. Renal involvement in children is exceedingly rare. We report a case of pediatric renal SEP to highlight its diagnostic challenges and management considerations. CASE PRESENTATION: A 10-year-old boy presented with intermittent painless gross hematuria. Computed tomography (CT) revealed a cystic-solid mass (4.8 × 4.7 × 4.5 cm) in the lower pole of the right kidney, initially suspected to be renal cell carcinoma. Right radical nephrectomy was performed, and histopathology confirmed plasmacytoma with kappa light chain restriction. Comprehensive evaluation excluded multiple myeloma (MM) and other extramedullary lesions. Notably, despite negative serum and urine protein electrophoresis, the serum free light chain (sFLC) ratio was abnormally elevated (κ/λ = 4.16), confirming monoclonality. A final diagnosis of renal SEP was established. No adjuvant therapy was administered. The patient remains disease-free at 26 months of follow-up, with sFLC ratio remaining within the normal range. CONCLUSIONS: Pediatric renal SEP may mimic common malignant renal tumors, necessitating systematic pathological, hematological, bone marrow, and imaging assessments for accurate diagnosis. Complete surgical resection may be sufficient for localized lesions. sFLC analysis may serve as a valuable tool for diagnosis and monitoring. Long-term follow-up is essential given the risk of recurrence or progression to MM.