Background
Solitary plasmacytoma (SP) is a rare plasma cell neoplasm characterized by clonal malignant proliferation of plasma cells at a single site, accounting for approximately 5% of all plasma cell disorders [ 1 ]. Based on the site of involvement, SP can be classified into solitary bone plasmacytoma (SBP) and solitary extramedullary plasmacytoma (SEP) [ 2 ]. SEP accounts for only approximately 30% of all SP cases [ 1 ]. SEP predominantly affects middle-aged and elderly males, involves only soft tissues, and can occur at any site or organ, most commonly in the head and neck region [ 2 ]. Primary SEP of the kidney is exceedingly rare, with only a handful of cases reported in the pediatric population. We describe a case of primary renal SEP in a child and comprehensively discuss its clinical presentation, diagnostic evaluation, management strategies, and prognosis in the context of the existing literature.
Case presentation
A 10-year-old boy was admitted with a 7-day history of intermittent, painless gross hematuria. He was otherwise asymptomatic, with no history of immunodeficiency and no family history of solid or hematological malignancies. Physical examination was unremarkable. Laboratory investigations, including complete blood count (CBC) and serum biochemical parameters, were normal. Serum tumor markers (alpha-fetoprotein, carcinoembryonic antigen, and neuron-specific enolase) were within reference ranges. Chest computed tomography revealed no abnormalities. Contrast-enhanced renal CT demonstrated a heterogeneously enhancing cystic–solid mass arising from the lower pole of the right kidney, measuring approximately 4.8 × 4.7 × 4.5 cm. The lesion showed exophytic growth with extension into the renal pelvis, resulting in local filling defects, with no radiographic evidence of retroperitoneal lymphadenopathy (Fig. 1 ). Based on these findings, a preliminary diagnosis of right renal cell carcinoma (RCC) was made, and the patient subsequently underwent right radical nephrectomy. Intraoperative exploration revealed no grossly enlarged regional lymph nodes and no tumor extension beyond the renal capsule; representative regional lymph nodes were sampled for pathological evaluation.
Histopathological examination of the resected specimen revealed diffuse infiltration of the renal parenchyma by sheets of plasmacytoid tumor cells, which varied in size and degree of differentiation (Fig. 2 A). Immunohistochemistry (IHC) demonstrated positivity for plasma cell markers CD138 (Fig. 2 B), CD38 (Fig. 2 C), and MUM1. Immunoglobulin light chain staining showed diffuse kappa positivity (Fig. 2 D) with lambda negativity (Fig. 2 E), confirming kappa light chain restriction. B-cell markers CD20 (Fig. 2 F), CD10, CD19, CD21, and PAX5 were negative; T-cell marker CD3 was negative; precursor marker TdT was negative. Epithelial, neuroendocrine, and myogenic markers (CK, Syn, S100, SMA, Myogenin) were negative, as were CD56, Cyclin D1, ALK, and WT-1. The Ki-67 proliferation index was approximately 20%. Epstein–Barr virus–encoded small RNA (EBER) in situ hybridization was negative. Surgical margins were histologically negative (R0 resection), and sampled regional lymph nodes were free of metastatic disease. These findings supported the diagnosis of plasmacytoma composed of monoclonal plasma cells and effectively excluded other renal malignancies, including lymphoma, Wilms tumor, and epithelial carcinomas.
Given the unusual diagnosis of primary renal SEP, an extensive postoperative systemic evaluation was undertaken to rule out disseminated plasma cell dyscrasia. Bone marrow aspiration and biopsy revealed normal morphology, and flow cytometric analysis demonstrated no aberrant immunophenotypic cell population. CBC and serum biochemistry (including creatinine, albumin, and serum calcium) remained normal. Serum protein electrophoresis (SPEP) and immunofixation electrophoresis (IFE) detected no monoclonal immunoglobulin (M-protein), and urine Bence-Jones protein was negative. However, serum free light chain (sFLC) testing showed κ 54.5 mg/L (reference 3.3–19.4 mg/L) and λ 13.1 mg/L (reference 5.71–26.3 mg/L), with a κ/λ ratio of 4.16 (reference 0.26–1.65), confirming monoclonality. Urine free light chain (uFLC) testing was normal. Positron emission tomography–computed tomography (PET/CT) revealed no osteolytic lesions or evidence of malignant disease activity at other sites. Based on these results, a final diagnosis of right renal SEP was established. As the tumor was completely resected and no residual disease was detected, the multidisciplinary team recommended no additional therapy. At 26 months of follow-up, the patient remains free of recurrence or progression and continues under long-term surveillance, with sFLC ratio remaining within the normal range throughout.
Discussion and conclusions
SEP is a solitary soft tissue tumor formed by abnormal proliferation of monoclonal plasma cells. It may present as an extramedullary manifestation of multiple myeloma (MM) or occur independently. SEP predominantly affects middle-aged and elderly males, with a median age at diagnosis of 55–60 years [ 1 ]. SEP is extremely rare in the pediatric population, with the SEER database indicating an incidence of approximately 0.015 per 1,000,000 person-years [ 3 ]. The anatomical distribution of SEP in children is similar to that in adults, primarily involving the head and neck region (73%–76%), especially the upper respiratory tract, followed by the gastrointestinal tract, lower respiratory tract, skin, and lymph nodes [ 2 – 4 ]. Notably, primary renal involvement has been reported only as isolated cases in the literature. The pathogenesis of SEP remains poorly understood. In adult series, chronic antigenic stimulation, viral infections such as Epstein–Barr virus, and immune dysregulation have been proposed as contributory factors [ 5 , 6 ]. In contrast, recent pediatric case series demonstrate an absence of underlying immunodeficiency or chronic inflammatory conditions, and tumor cells are consistently EBER-negative, suggesting that pediatric SEP more likely represents a primary clonal proliferation of plasma cells rather than a secondary, inflammation-driven process [ 7 ].
The clinical manifestation of renal SEP lacks specificity. Common symptoms include abdominal mass, flank pain, hematuria, and obstructive uropathy; some patients are asymptomatic and discovered incidentally [ 8 ]. In this case, painless gross hematuria likely reflected tumor involvement of the renal pelvis. Imaging features are also nonspecific; CT typically shows a well-defined soft tissue mass with varying degrees of enhancement, occasionally with hemorrhage, necrosis, and punctate calcification [ 9 ]. These features overlap with RCC and Wilms tumor; therefore, histopathology remains essential for definitive diagnosis. According to current Wilms tumor guidelines [ 10 ], preoperative biopsy of any form is not recommended for resectable unilateral renal tumors, as biopsy automatically upstages the local disease to stage III, thereby necessitating more intensive adjuvant therapy, including chemotherapy and radiotherapy. Furthermore, percutaneous biopsy may be limited by diagnostic inaccuracy, particularly because anaplastic histology is frequently missed in biopsy specimens. In our case, although preoperative evaluation favored a diagnosis of RCC, Wilms tumor, which predominantly affects younger children, could not be entirely excluded. In light of these considerations, we proceeded directly with radical nephrectomy, which served both diagnostic and therapeutic purposes.
Histologically, SEP comprises monoclonal plasma cells ranging from well-differentiated to anaplastic or plasmablastic morphology. Especially in unusual locations like the kidney, plasmacytoma may not be immediately recognizable by morphology alone [ 11 ]. Histological differential diagnoses include IgG4-related/reactive polyclonal plasma cell infiltration, B-cell lymphoma with plasmacytic differentiation, poorly differentiated carcinoma, and myogenic sarcomas. IHC is pivotal: plasmacytomas express plasma cell markers (CD138 and/or CD38) and typically lack B-cell markers (CD20, PAX5); kappa or lambda light chain restriction confirms clonality [ 12 ]. Our case fits these criteria, and the negativity of other lineage markers helps exclude morphologic mimics. Importantly, plasmablastic lymphoma (PBL) should be distinguished: although they may highly overlap in morphology and immunophenotype, PBL typically demonstrates an extremely high proliferation index (Ki-67 > 90%), EBER positivity, and is often associated with an immunosuppressed clinical background [ 13 ], which is inconsistent with this case.
Following histopathological confirmation of plasmacytoma, systematic assessment is crucial to exclude MM and establish lesion solitary status. According to the European Expert Panel recommendations, SEP is defined by the following criteria [ 12 ]: (1) no or minimal (< 10%) clonal plasma cells in bone marrow; (2) no additional lesions were detected on whole-body imaging. PET/CT or magnetic resonance imaging (MRI) is recommended for comprehensive evaluation; (3) absence of end-organ damage attributable to plasma cell dyscrasia (CRAB: hypercalcemia, renal failure, anemia, bone lesions); (4) absent or minimal M-protein levels were detected in the serum and urine, consistent with reported observations that any detectable paraprotein typically resolves within one year following local therapy. Comprehensive systemic evaluation in this patient fulfilled these criteria, thereby confirming the diagnosis of primary renal SEP.
Plasmacytomas can secrete M-protein and its polypeptide chain subunits (light chains or heavy chains). However, only about 20% of SEP cases have a detectable M-protein [ 14 ]. Consistently, our patient’s serum and urine protein electrophoresis were negative, yet the sFLC ratio was abnormally elevated (κ/λ = 4.16, > 1.65). Modern FLC assays increase sensitivity by detecting unbound κ or λ light chains that may not generate a distinct M-protein peak, with sFLC generally more sensitive than uFLC [ 15 ]. Importantly, interpretation of the sFLC ratio is more reliable when renal function is normal, as in our patient. Even when conventional electrophoresis is normal, an abnormal sFLC ratio at diagnosis can indicate an underlying clonal plasma-cell population and is considered an independent risk factor for progression from SEP to MM [ 16 ]. In our case, the sFLC ratio did not reach the SLiM-CRAB FLC biomarker threshold for MM (i.e., sFLC ratio ≥ 100 with involved sFLC ≥ 100 mg/L) [ 17 ] but provided serological evidence of monoclonality for the diagnosis. Importantly, normalization of the sFLC ratio during follow-up paralleled sustained clinical remission, underscoring its value for both diagnosis and longitudinal monitoring.
Owing to the rarity of SEP, no standardized treatment protocol or pediatric-specific guidelines currently exist, and management is largely guided by expert consensus and retrospective case series. Local therapy is the mainstay, including radiotherapy (RT), surgery, or both. In adults, RT is considered the first-line treatment for most SEPs, achieving local control rates exceeding 80%–90% [ 18 ]. For head and neck lesions, RT should be prioritized since achieving an R0 resection is frequently challenging and surgery may lead to serious mutilations or functional limitations. For SEPs at other sites, surgical excision is a reasonable first choice when an R0 resection can be safely accomplished [ 19 ]. Some studies recommend adjuvant local RT even after R0 resection to reduce recurrence risk [ 20 , 21 ]. In children, however, RT may carry significant long-term risks, such as musculoskeletal growth retardation and secondary malignancies. In our case, radical nephrectomy achieved a histologically confirmed R0 resection, and postoperative evaluation demonstrated no evidence of residual or disseminated disease. The multidisciplinary team therefore reached consensus on a surveillance-based management strategy, supported by pediatric case series indicating that surgery alone can be sufficient for durable disease control in completely resected localized SEP, with RT reserved for cases of incomplete resection (R1/R2) or local recurrence after initial treatment [ 7 , 22 ]. Chemotherapy is generally not recommended; however, it may be considered in patients with persistent disease on PET/CT after local therapy [ 12 ].
Compared with other plasma cell neoplasms, SEP is associated with a more favorable prognosis, with reported 10-year overall survival rates exceeding 70% [ 12 ]. However, a persistent long-term risk of progression to multiple myeloma remains, with pooled analyses indicating 10-year progression rates of approximately 10%–30% [ 12 , 23 , 24 ]. Oschlies et al. [ 7 ] demonstrated that pediatric SEP tends to exhibit a more indolent biological behavior, characterized by absent CD56 and Cyclin D1 expression, a low proliferative index, and the absence of high-risk cytogenetic abnormalities commonly seen in plasma cell neoplasms, supporting its classification as a localized tumor with relatively low malignant potential. Given the scarcity of pediatric cases, these observations require validation in larger cohorts with prolonged follow-up. Reported adverse prognostic factors include age > 60 years, tumor diameter > 5 cm, non–head-and-neck primary site, persistent serum M-protein ≥ 5 g/L after treatment, minimal bone marrow infiltration by clonal plasma cells, an abnormal sFLC ratio at diagnosis, and suboptimal treatment strategy [ 12 , 16 , 19 , 25 ]. Although age was favorable in our patient, the renal primary site and abnormal sFLC ratio still indicate some risk. As most relapses or transformations to MM occur within 2–3 years after treatment, follow-up every 3 months for the first 2 years is recommended; if stable, every 6 months for years 3–5, then annually for life [ 26 ]. Follow-up should include clinical evaluation, hematologic testing (CBC, serum chemistry, sFLC, SPEP, and IFE), and imaging evaluation (CT or MRI). When relapse or evolution to MM is suspected, PET/CT and repeat bone marrow evaluation should be performed. Vigilant follow-up facilitates early detection of local recurrence or systemic progression and timely intervention.
In summary, this case underscores that pediatric renal SEP, although rare, should be considered in the differential diagnosis of renal masses in children. Complete surgical resection may provide adequate treatment for localized disease, and sFLC analysis may serve as a useful tool for establishing monoclonality and monitoring disease activity. However, as a single case report, the findings have limited generalizability. Long-term follow-up remains essential given the risk of recurrence or progression to MM, and the current 26-month follow-up period is relatively short for assessing such late events.
Acknowledgements
The authors would like to thank this family for their consent to publish this report.
Abbreviations
Complete blood count
Computed tomography
Epstein-Barr virus-encoded small RNA
Free light chain
Immunofixation electrophoresis
Immunohistochemistry
Multiple myeloma
Magnetic resonance imaging
Plasmablastic lymphoma
Positron emission tomography–computed tomography
Radiotherapy
Renal cell carcinoma
Solitary bone plasmacytoma
Solitary extramedullary plasmacytoma
Serum free light chain
Solitary plasmacytoma
Serum protein electrophoresis
Urine free light chain
Authors’ contributions
DWC and CQ designed the study and drafted the initial manuscript. CJX provided the clinical data. LYW performed the pathological and immunohistochemical analysis. YZ critically reviewed and revised the manuscript. All authors took part in writing the manuscript and approved the final, submitted version.
Funding
This work was supported by the Clinical Medicine Talent Training Project funded by Hebei Province (ZF2024187).
Data availability
No datasets were generated or analysed during the current study.
Declarations
Ethics approval and consent to participate
This study was approved by the Ethics Committee of the Children’s Hospital of Hebei Province (202407-90) and conducted in accordance with the ethical standards of the Declaration of Helsinki.
Consent for publication
Written informed consent for publication of this case report and accompanying images was obtained from the parents of the patients.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Dengwei Chu, Chaojun Xin, Can Qi and Lingyan Wang contributed equally to this work.
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Associated Data
Data Availability Statement
No datasets were generated or analysed during the current study.