Metanephric adenoma with low apparent diffusion coefficient value mimicking renal cell carcinoma: A case report

Abstract

Rationale: Metanephric adenoma (MA) is a rare and often benign tumor. Most MAs were misdiagnosed as renal cell carcinomas (RCCs) preoperatively. Diffusion weighted imaging (DWI) and apparent diffusion coefficient (ADC) mapping can help to differentiate benign and malignant tumors. However, there are still pitfalls in using DWI and ADC to discriminate benign and malignant lesions.

Patient concerns: A 56-year-old woman had a right renal metanephric adenoma. The tumor showed very low ADC value preoperatively and was misdiagnosed as a renal cell carcinoma.

Diagnosis: Intraoperative ultrasound-guided percutaneous biopsy of tumor was performed. Based on the histopathological findings and immunohistochemical stains, a diagnosis of metanephric adenoma was suggested.

Interventions: The patient received percutaneous cryoablation of this tumor. Five years later, she underwent right partial nephrectomy because local recurrence was revealed on a follow-up computed tomography (CT).

Outcomes: MA was confirmed again by histological examination. The patient was uneventful after surgery.

Lessons: ADC mapping can be used for differentiating RCCs from other benign tumors by their lower ADC values. However, some benign and malignant lesions have overlapped low ADC values. This case illustrated that a benign lesion such as MA could mimic RCC on ADC, by its highly cellular component. Cryoablation is an optional treatment, which has an increased risk of local recurrence. Follow-up CT or MRI is useful and necessary for detection of local recurrence by depicting enhancing solid parts in a tumor over time.

Figure 1

Computed tomography (CT) of the right kidney. (A) Non-enhanced CT shows an exophytic nodule at the mid-pole of the right kidney. The tumor was measured 40 Hounsfield unit (HU), slightly dense than adjacent renal parenchyma. (B) Medullary and 1C) delayed phase contrast CT shows a well-circumscribed, hypovascular tumor with cystic component. Enhancement of the tumor was less than that of right renal cortex and medulla in all phases.

Figure 2

Magnetic resonance imaging (MRI) of the right kidney. (A) T2 weighted imaging shows a well-demarcated, hypointense nodule with cystic area at mid-pole of the right kidney. (B) Diffusion weighted image with b-values of 1000 s/mm² and (C) apparent diffusion coefficient mapping confirmed fluid restriction in the nodule, implying highly cellular nature of the nodule.

Figure 3

Sequential follow up computed tomographic (CT) images of a middle-aged woman with recurrent metanephric adenoma after cryoablation. (A) Post-contrast CT on the first day after cryoablation. It shows faint enhancement (arrow) in the ablated lesion, indicating hyperemic change or viable tumor. Perirenal hematoma is also noted. (B) and (C) CT scan at 4th and 5th year after thermal ablation. Gradually increased solid component of the lesion as well as mild irregular, nodular enhancement at periphery of the tumor (arrow) can be observed after comparison of sequential follow up images. Recurrence of metanephric adenoma is highly suspected.

Figure 4

Histological features of metanephric adenoma (H&E, ×100). The tumor is tightly packed, composed of small uniform epithelial cells forming small acini and glomeruloid structures.