Metanephric adenoma with polycythaemia in an adolescent: Diagnostic and pathological challenges —A case report

Abstract

We report a 14-year-old girl presenting with flank pain, microscopic haematuria, and polycythaemia. Imaging demonstrated a well-defined upper pole renal mass with minimal enhancement, while image-guided biopsy suggested metanephric adenoma but remained inconclusive. She underwent open partial nephrectomy, and histopathology confirmed metanephric adenoma despite atypical immunohistochemistry. Postoperative recovery was uneventful with preserved renal function, and polycythaemia resolved completely. This case highlights the diagnostic challenges of differentiating benign from malignant renal tumours in young patients and supports nephron-sparing surgery as an effective management approach.

1. Introduction

Metanephric adenoma (MA) is a rare benign renal epithelial neoplasm that is encountered predominantly in the 5th and 6th decades of life and is uncommon in the paediatric population 1, 2, 3. Its clinical presentation is variable; up to 20% are asymptomatic, and lesions may be detected incidentally or during evaluation of flank pain, abdominal pain, haematuria, or a palpable mass ^1,2,4. Polycythaemia has also been reported alongside metanephric adenoma, which is reported to be more prevalent than in renal cell carcinoma.^2,5

Preoperative diagnosis of MA is often difficult because imaging features are not specific and may overlap with those of other renal neoplasms, including nephroblastoma and papillary renal cell carcinoma.^1,4,5 Even when a preoperative tissue diagnosis is attempted, limited biopsy material may not reliably distinguish metanephric adenoma from histological mimics.⁴

2. Case presentation

A 14-year-old previously healthy girl presented with right-sided flank pain for 3 months and was found to have microscopic haematuria and persistent polycythaemia, with a haemoglobin of 17.6 g/dL and a haematocrit of 56%.

Ultrasonography demonstrated a well-defined hypoechoic lesion in the upper pole of the right kidney, initially suggesting a suprarenal or renal mass. A later contrast-enhanced computed tomography scan showed a well-defined upper pole lesion that was 4.5 × 4.4 × 4.9 cm in size. It was hyperdense on pre-contrast imaging and had little post-contrast enhancement. There were no calcifications, cavitations, or perilesional infiltration and lymphadenopathy (Fig. 1). The right suprarenal gland was separate, and the presence of a claw sign confirmed renal origin.

Fig. 1.

CECT abdomen – A: CECT demonstrating a well-defined renal mass (arrow) with minimal enhancement. B: Lesion compressing the upper calyceal group (arrows), demonstrating a claw sign.

Further evaluation showed a negative JAK2 mutation, normal serum erythropoietin, and an unremarkable endocrine profile, with preserved renal function. An ultrasound-guided core needle biopsy suggested metanephric adenoma, with epithelial-predominant nephroblastoma in the differential; the sample was inadequate for immunostaining.

Following a multidisciplinary team discussion, a decision was made to proceed with an open right partial nephrectomy in view of the indeterminate diagnosis and feasibility of renal preservation.

An open partial nephrectomy was performed in a standard manner with a warm ischaemic time of 15 minutes. Intraoperatively, a well-defined upper-pole renal mass was identified and completely excised (Fig. 2). Postoperatively, she was managed according to the ERAS protocol, with an uneventful recovery and preserved renal function.

Fig. 2.

Intraoperative findings of right-side partial nephrectomy. A: well-defined upper-pole renal mass showed by the arrow; B: performing partial nephrectomy.

Histopathology showed an encapsulated tumour made up of tightly packed tubular structures lined by primitive cells with hyperchromatic nuclei and minimal stroma, with glomeruloid formations and no blastemal component, necrosis, or mitotic activity; margins were clear. Immunostaining for CK7, EMA, and WT1 was negative. Despite this unusual immunophenotype, the overall morphology was consistent with the diagnosis of metanephric adenoma. At the 6-month follow-up, there was no clinical or radiological evidence of recurrence, and the polycythaemia had resolved with a return to a normal haemoglobin level.

3. Discussion

Metanephric adenoma is an uncommon renal neoplasm that remains particularly rare in children and adolescents 1, 2, 3. In paediatric series, most reported cases have presented either incidentally or during investigation of nonspecific symptoms, making preoperative recognition difficult 1, 2, 3. Our patient was unusual in that she presented with flank pain, microscopic haematuria, and confirmed polycythaemia, with subsequent complete haematological normalisation after tumour excision.

The imaging findings in metanephric adenoma are variable and frequently non-diagnostic. These tumours are generally well circumscribed and often hypovascular, but no radiological pattern is sufficiently specific to exclude malignant renal neoplasms preoperatively.⁵ The lesion in this patient was similarly well defined and only minimally enhancing, suggesting a relatively indolent tumour but preventing a confident diagnosis. Initial ultrasonography also raised concern regarding suprarenal origin, adding further uncertainty.

The principal diagnostic challenge in this case was the overlap with other renal tumours seen in younger patients, particularly epithelial-predominant nephroblastoma. This difficulty persisted even after a core needle biopsy, which favoured metanephric adenoma but could not conclusively exclude nephroblastoma. The importance of immunohistochemical and molecular analyses in distinguishing metanephric adenoma from epithelial-predominant nephroblastoma and solid variant papillary renal cell carcinoma has been emphasised, particularly when morphology alone is insufficient.⁴

In this case, the immunostaining showed WT1, CK7, and EMA negativity, which is atypical. However, the diagnosis of metanephric adenoma was supported by the absence of blastemal elements, mitotic activity, or necrosis, along with the presence of glomeruloid structures and characteristic architecture. Although WT1 positivity is typically expected, rare atypical immunophenotypes have also been described, and morphology remains the cornerstone of diagnosis.³ Diagnosis ultimately depends on an integrated interpretation of morphology, immunophenotype, and, where available, molecular findings such as BRAF alterations.³

Polycythaemia is recognised but uncommon in metanephric adenoma and has been described more frequently in this tumour than in many other benign renal lesions.^2,5 In our patient, JAK2 testing was negative, and serum erythropoietin was reportedly normal; however, haemoglobin and haematocrit were normalised after tumour removal, suggesting a tumour-related cause despite the absence of elevated erythropoietin. The case may indicate that mechanisms other than overt erythropoietin elevation contribute to tumour-associated erythrocytosis.

Considering the patient's age, the small size of the upper pole lesion, the preserved renal function, and the potential for a benign tumour, nephron-sparing surgery was the most appropriate option.⁶ Paediatric studies support partial nephrectomy when it is technically feasible, particularly for localised lesions where preserving the renal function is important.⁶ In this case, open partial nephrectomy achieved complete resection with preservation of renal function and an uncomplicated recovery.

4. Conclusion

Metanephric adenoma should be a differential diagnosis in children and adolescents who have a well-defined renal mass, especially if it is accompanied by haematuria or unexplained polycythaemia. Preoperative diagnosis using imaging and a core needle biopsy is often uncertain, so a careful evaluation is necessary to rule out other tumours, such as papillary renal cell carcinoma and nephroblastoma. A definitive diagnosis depends on histopathological examination. Nephron-sparing surgery is an effective treatment option that helps preserve renal function.

CRediT authorship contribution statement

Suthakaran Vivian Trishan: Writing – original draft, Resources, Formal analysis, Data curation, Conceptualization. Jesuthasan Mithushan: Writing – review & editing, Software, Methodology. Peethambaram Jeepara: Validation, Supervision, Project administration.