ABSTRACT
Congenital mesoblastic nephroma (CMN) is the most common renal tumor of early infancy, and infants with renal masses suspicious for CMN often undergo upfront radical nephrectomy with proximal ureterectomy. Infective mimics, including renal abscess, acute lobar nephronia, and xanthogranulomatous pyelonephritis, are considerably less common, particularly when typical clinical signs of infection are absent or muted. We report the case of a 2-month-old boy with a painless, progressively enlarging left abdominal mass first noted at 20 days of age, accompanied by intermittent low-grade fever and constipation. Contrast-enhanced computed tomography showed an ill-defined, heterogeneously enhancing left renal mass with perinephric extension and regional lymphadenopathy, raising suspicion for CMN. The infant underwent radical left nephroureterectomy with splenic flexure resection, the latter of which was required because of dense inflammatory adhesions. Histopathological examination revealed a chronic suppurative renal abscess with palisading histiocytes and multinucleated giant cells, with no evidence of malignancy, fungal organisms, or mycobacteria. Postoperative recovery was uneventful, and the child remains well during follow-up. This case illustrates a rare but clinically important diagnostic pitfall: infective renal masses can masquerade as malignant renal tumors in early infancy, particularly when systemic inflammatory features are subtle.
-
Keywords: Congenital mesoblastic nephroma; Mesoblastic nephroma; Nephrectomy; Abscess; Case reports
INTRODUCTION
Congenital mesoblastic nephroma (CMN) typically presents as a painless abdominal mass in a neonate or young infant and may occasionally be accompanied by fever or hematuria [
1]. On imaging, CMN usually appears as a solid, heterogeneously enhancing renal mass, sometimes with cystic change; however, these features overlap substantially with those of malignant and inflammatory renal lesions [
1]. Less familiar infective mimics, including renal abscess, acute lobar nephronia (ALN), and xanthogranulomatous pyelonephritis (XGP), can closely resemble CMN or Wilms tumor on both clinical assessment and imaging [
2-
6]. We report a 2-month-old boy whose renal abscess was treated under a working diagnosis of CMN until histopathological examination established the final diagnosis.
CASE REPORT
Ethics statement
Institutional review board approval was not sought because this was a single case report. Written informed consent for publication of this case report and the accompanying clinical images was obtained from the patient’s parent. The report was prepared in accordance with the ethical principles of the Declaration of Helsinki and followed the CARE guidelines.
Patient information
A 2-month-old boy, born at 38 weeks by vaginal delivery with no significant antenatal history, was brought for evaluation of a painless left abdominal lump that had slowly enlarged since it was first noticed at approximately 20 days of age (
Fig. 1A). During the following weeks, he had intermittent low-grade fever and constipation but no vomiting, hematuria, dysuria, or other urinary symptoms. His birth weight was 2.5 kg, and his weight at presentation was 4.2 kg. He was adequately nourished. Family history was noncontributory, and the psychosocial history was unremarkable.
On examination, the infant had normal vital signs. A firm, nontender mass was palpable in the left hypochondrium and lumbar region; the mass did not move with respiration, suggesting a retroperitoneal origin. The absence of tenderness was notable in the context of an elevated total leukocyte count of 17.6×10³/µL.
Blood tests showed anemia, leukocytosis, and thrombocytopenia: hemoglobin, 9.1 g/dL (reference range, 11–14 g/dL); total leukocyte count, 17.6×10³/µL (reference range, 6–17.5×10³/µL); and platelet count, 97×10³/µL (reference range, 150–400×10³/µL). Renal function, electrolyte levels, liver function, and coagulation results were normal. Blood and urine cultures were not obtained preoperatively. C-reactive protein, erythrocyte sedimentation rate, and procalcitonin were also not measured preoperatively because the imaging findings strongly favored CMN. Abdominal ultrasonography revealed a predominantly solid, heterogeneous left renal mass measuring approximately 4.8×4.3×4.0 cm; the mass arose from and replaced most of the left kidney and contained internal cystic or necrotic areas with ill-defined margins. Contrast-enhanced computed tomography (CT) of the abdomen showed an ill-defined, heterogeneously enhancing, partly exophytic mass replacing the left kidney and measuring approximately 4.3×4.0×4.8 cm (
Fig. 1B). The mass appeared to infiltrate the pelvicalyceal system and extend beyond the perinephric space, with fat stranding and thickened pararenal fascia that distorted the collecting system. It abutted the spleen, descending colon, and abdominal wall muscles without clear invasion, and a few enlarged left para-aortic nodes were identified. The infiltrative margins, perinephric extension, and lymphadenopathy favored CMN rather than a straightforward abscess, which typically has a thick, irregular, enhancing wall surrounding a nonenhancing liquefied center [
4,
5]. Because no rim-enhancing collection was present, a solid neoplastic process was favored over a drainable abscess. The working radiological diagnosis was a left renal mass, most likely CMN.
Surgery was performed 6 days after the CT examination, and no repeat imaging was obtained during this interval. Because CMN was the working diagnosis, the infant underwent radical left nephroureterectomy with splenic flexure resection, primary colonic anastomosis, and retroperitoneal lymph node sampling. Preoperative biopsy was not performed, consistent with the usual practice of proceeding directly to resection when renal masses have imaging features typical of CMN, because CMN is treated surgically regardless of subtype. Intraoperative frozen-section examination was also not requested.
An approximately 5×4 cm mass arose from the mid and lower poles of the left kidney and was densely adherent to the splenic flexure, descending colon, and mesentery (
Fig. 1C). Large parahilar and para-aortic lymph nodes were present, but there was no thrombus in the inferior vena cava or renal vein. The adrenal gland was separate from the mass, although the mass was posteriorly adherent to the psoas. During exploration, careful attempts were made to separate the descending colon and mesocolon from the renal mass. These attempts were unsuccessful because the severe inflammatory process had completely obliterated the normal tissue planes, with dense fibrotic adherence to the mesocolon and serosa. Because the colon could not be safely separated despite these efforts, en bloc resection of the involved colonic segment was performed to allow safe removal of the mass. Subsequent histopathology confirmed inflammatory adhesions without true colonic invasion.
On gross examination, the capsular surface was bosselated, and the cut section showed a gray-white lesion measuring 6.5×4.3×3.2 cm. The lesion had replaced nearly the entire renal parenchyma from the interpolar region to the lower pole, with hemorrhagic discoloration of the perinephric fat. The perinephric fat at the lower pole was adherent to the resected colonic segment. Microscopy showed multiple suppurative abscesses, rimmed by palisading histiocytes and numerous multinucleated giant cells, scattered throughout the renal parenchyma without a clear relation to the pelvicalyceal system. The pelvicalyceal lining itself was not ulcerated. Adjacent tissue showed extensive fibrosis, and inflammation extended beyond the kidney into the perinephric and periadrenal soft tissue, accounting for the dense adhesions observed at surgery, including those involving the mesocolon. Background glomeruli and tubules were appropriate for gestational age. Periodic acid–Schiff, Gram, and Ziehl–Neelsen stains were negative for fungal organisms, bacteria, and mycobacteria, respectively; acid-fast bacilli and fungal cultures of the tissue were also negative (
Fig. 2). On the basis of the available histopathological findings, XGP was ruled out because characteristic lipid-laden foamy macrophages were absent. Mycobacterial nucleic acid amplification testing, however, was not performed, which is acknowledged as a limitation.
Postoperative recovery was uneventful, and the infant was discharged in stable condition. At 6 months of follow-up, he was thriving, had age-appropriate growth and development, and had no recurrent urinary infection or fever.
DISCUSSION
Renal masses presenting during the first year of life are uncommon and encompass a broad differential diagnosis that includes both neoplastic and nonneoplastic conditions. CMN is the most frequently encountered renal tumor in neonates and infants younger than 3 months, whereas Wilms tumor becomes increasingly common later in infancy [
1]. Less frequent neoplastic entities include clear cell sarcoma of the kidney and multilocular cystic nephroma. Infective and inflammatory renal lesions, including renal abscess, ALN, and XGP, may also closely mimic renal malignancy on clinical assessment and imaging [
2,
3,
6-
9].
The differential diagnosis of an infantile renal mass is therefore extensive and often difficult to resolve preoperatively. CMN typically appears as a heterogeneous solid renal mass with variable enhancement and may occasionally show the characteristic concentric “ring” sign. Imaging findings alone, however, are insufficient to distinguish CMN reliably from Wilms tumor or inflammatory renal lesions [
1]. Although Wilms tumor predominantly affects older infants and young children, it can rarely present in the neonatal period and usually appears as a well-circumscribed mass with a pseudocapsule. Clear cell sarcoma of the kidney is similarly uncommon but aggressive, lacks distinctive imaging characteristics, and is frequently indistinguishable from Wilms tumor. In contrast, multilocular cystic nephroma generally appears as a well-defined, multiloculated cystic lesion without enhancing solid components.
Among inflammatory conditions, XGP is a chronic granulomatous infection typically associated with urinary tract obstruction or nephrolithiasis in older children; however, it has occasionally been reported in infancy, where it may closely simulate a renal neoplasm [
7-
9]. ALN is a focal, nonliquefactive bacterial infection of the renal parenchyma that may similarly appear as a solid renal mass on imaging [
6]. Renal fungal infections are rare and are largely restricted to immunocompromised infants or infants with indwelling vascular catheters [
10]. These diagnostic possibilities indicate that infective and inflammatory etiologies should remain in the differential diagnosis whenever a renal mass is identified during infancy.
Renal abscess and ALN fall along the same spectrum of bacterial renal infection, ranging from nonliquefactive focal nephritis to frank liquefied abscess [
4-
6]. ALN classically appears as a focal, ill-defined lesion and has been reported to mimic Wilms tumor on ultrasonography and CT, even in infants as young as 2 months [
6]. In the present case, the infection had progressed beyond focal nephritis, replacing nearly the entire kidney rather than remaining focal and lobar as ALN typically does. ALN is discussed here for completeness, but the diffuse renal involvement in this infant is more consistent with an established chronic abscess than with classic ALN.
Renal abscess can be difficult to distinguish from malignancy because the two entities may have highly similar clinical and radiological features. In addition, renal abscesses do not always present with classic signs of infection: fever may be intermittent, inflammatory markers may be nonspecific, and microbiological cultures can be negative [
3,
4]. In the present case, CT suggested pelvicalyceal and perinephric infiltration, raising concern for an invasive malignant process. Histopathological examination, however, demonstrated an intact pelvicalyceal lining and inflammation contained by dense fibrosis, without evidence of true tissue invasion. This radiological-pathological discrepancy highlights an important diagnostic clue: inflammatory renal masses may produce dense adhesions and apparent infiltration on imaging despite the absence of tissue invasion.
Several reports published over multiple decades have described renal abscesses and other inflammatory renal lesions that were mistaken for renal tumors (
Table 1) [
2,
3,
6-
9]. Across these cases, recurring features include few overt infective symptoms, a striking radiological resemblance to malignancy, and definitive diagnosis only after nephrectomy. Together, these reports show that inflammatory renal lesions remain a persistent diagnostic pitfall rather than isolated diagnostic anomalies. Greater awareness of these uncommon entities, combined with careful interpretation of clinical, radiological, and laboratory findings, may broaden the differential diagnosis and, when feasible, help avoid unnecessary radical surgery in infants with renal masses.
Image-guided core biopsy is not routinely recommended for renal masses with imaging features highly suggestive of CMN, because surgical excision remains the standard treatment regardless of histologic subtype and biopsy carries potential risks in young infants, including bleeding and tumor seeding. However, when clinical or laboratory findings raise suspicion for infection—such as fever, leukocytosis, elevated inflammatory markers, or positive microbiological cultures despite atypical imaging appearances—targeted image-guided aspiration or biopsy may help establish an infective diagnosis and potentially avoid unnecessary radical surgery [
2,
6].
In the present case, the infiltrative appearance, pelvicalyceal involvement, perinephric extension, and lymphadenopathy collectively favored CMN. Dense adhesions to the splenic flexure also prevented safe separation of the mass from the bowel, necessitating en bloc colonic resection.
Although infective and inflammatory renal masses are rare, this case shows that they should remain in the differential diagnosis for CMN and Wilms tumor when a renal mass in early infancy is accompanied by even subtle systemic inflammatory features. Urine microscopy and culture, blood culture, and inflammatory markers are inexpensive and readily available; accordingly, these tests should be considered before planned radical surgery in infants with renal masses. When clinical, laboratory, and radiological findings are discordant, a lower threshold for image-guided biopsy or a trial of conservative management may be reasonable.
This report highlights the diagnostic and management challenges posed by renal masses in early infancy. Preoperative recognition of infective mimics may help clinicians anticipate inflammatory adhesions, interpret discordant clinical and radiological findings more cautiously, and avoid unnecessary radical surgery in selected cases.
NOTES
-
Authors’ contributions
Conceptualization: RAJ, NJP. Data curation: RAJ, NJP. Formal analysis: RAJ, NJP, RPK, SM, SS, JKM. Investigation: NJP, RPK, SM, SS, JKM. Methodology: RAJ, NJP. Writing–original draft: RAJ, NJP, JS. Writing–review & editing: all authors. All authors read and approved the final manuscript.
-
Conflict of interest
None.
-
Funding
None.
-
Data availability
Not applicable.
Fig. 1.(A) Clinical photograph showing abdominal distension with a palpable left-sided abdominal mass. (B) Contrast-enhanced computed tomography of the abdomen, axial view, showing an ill-defined, heterogeneously enhancing mass arising from and replacing the left kidney, with perinephric fat stranding and para-aortic lymphadenopathy. (C) Resected surgical specimen showing the left renal mass densely adherent to the splenic flexure of the colon and its mesentery.
Fig. 2.Histopathological images of the renal lesion. (A) H&E staining showing an abscess in the renal medulla, indicated by black arrows; the black asterisk marks the renal medulla. (B, C) H&E staining showing palisading histiocytes, indicated by white arrows; multinucleated giant cells, indicated by white arrowheads; and central suppuration, indicated by a white asterisk. (D) Periodic acid–Schiff staining did not highlight fungal organisms. Scale bars are shown as indicated.
Table 1.Reported infective and inflammatory renal masses mimicking renal tumors in infants and children
Table 1.
|
Study |
Age |
Sex |
Preoperative impression |
Final diagnosis |
Management |
Outcome |
|
Simonowitz and Reyes (1976) [2] |
NR |
NR |
Wilms tumor |
Renal abscess |
Nephrectomy |
NR |
|
Constantine and Kaye (2004) [3] |
9 yr |
Female |
Metastatic Wilms tumor (renal mass+pulmonary nodules) |
Renal abscess with septic pulmonary emboli |
Biopsy-directed management |
NR |
|
Marteinsson et al. (1996) [7] |
NR |
NR |
Renal tumor |
Focal xanthogranulomatous pyelonephritis |
Nephrectomy |
NR |
|
Shah et al. (2012) [8] |
5 mo |
Male |
Renal mass with fever |
Xanthogranulomatous pyelonephritis |
Nephrectomy |
NR |
|
Nandedkar et al. (2014) [9] |
8 mo |
NR |
Malignant renal tumor |
Xanthogranulomatous pyelonephritis |
Nephrectomy |
NR |
|
Masood et al. (2021) [6] |
2 mo |
NR |
Bilateral nephroblastoma/Wilms tumor |
Acute lobar nephronia (biopsy-proven) |
IV antibiotics, no surgery |
Resolution of renal masses on serial ultrasonography |
|
Present case (2026) |
2 mo |
Male |
Congenital mesoblastic nephroma |
Chronic suppurative renal abscess |
Radical nephroureterectomy+colonic resection |
Well at follow-up |
REFERENCES
- 1. van der Beek JN, Schenk JP, Morosi C, et al. Diagnostic magnetic resonance imaging characteristics of congenital mesoblastic nephroma: a retrospective multi-center International Society of Pediatric Oncology-Renal Tumor Study Group (SIOP-RTSG) radiology panel study. Pediatr Radiol 2024;54:965-76.
- 2. Simonowitz DA, Reyes HM. Renal abscess mimicking a Wilms’ tumor. J Pediatr Surg 1976;11:269-70.
- 3. Constantine S, Kaye J. Metastatic renal abscess mimicking Wilms’ tumour. Pediatr Radiol 2004;34:924-6.
- 4. Cheng CH, Tsai MH, Su LH, et al. Renal abscess in children: a 10-year clinical and radiologic experience in a tertiary medical center. Pediatr Infect Dis J 2008;27:1025-7.
- 5. Yen DH, Hu SC, Tsai J, et al. Renal abscess: early diagnosis and treatment. Am J Emerg Med 1999;17:192-7.
- 6. Masood Y, Hussain I, Khan UU, Khalid MU, Javed MU. Acute lobar nephronia in an infant presented as a renal tumor. Urol Case Rep 2021;34:101450.
- 7. Marteinsson VT, Due J, Aagenaes I. Focal xanthogranulomatous pyelonephritis presenting as renal tumour in children: case report with a review of the literature. Scand J Urol Nephrol 1996;30:235-9.
- 8. Shah K, Parikh M, Gharia P, Modi PR. Xanthogranulomatous pyelonephritis: mimicking renal mass in 5-month-old child. Urology 2012;79:1360-2.
- 9. Nandedkar SS, Malukani K, Sakhi P. Xanthogranulomatous pyelonephritis masquerading as a tumor in an infant. Indian J Urol 2014;30:354-6.
- 10. Nayagam LS, Vijayanand B, Balasubramanian S. Isolated renal mucormycosis in an immunocompetent child. Indian J Nephrol 2014;24:321-3.
, Nitin James Peters1,*
