Adult metanephric adenoma presumed to be all benign? A clinical perspective

Background: In most documented literature, metanephric adenoma (MA) is described as a benign tumour. Nevertheless, the nature of MA remains unclear and the clinical criteria of different MA subtypes are not well established. In the present study, we investigated the clinicopathological characteristics of MA, especially those of the uncommon histological subtypes. Methods: A cohort study was performed on 18 patients with pathologically proven MA in our institute from January 2004 to June 2014. The patients' clinicopathological and radiological data were retrospectively analysed and evaluated with an emphasis on the corresponding subtypes. Results: The patient population had a female: male ratio of 1: 1 and mean age of 50 years (range, 18-66 years). The mean tumour size was 3.9 cm (range, 1.4-9.0 cm). There were no pathognomonic radiological features that posed a challenge for a preoperative diagnosis of MA. Fourteen patients underwent radical nephrectomy, and the other four underwent partial nephrectomy. Three histological subtypes were observed: classic MA (n = 10), malignant MA (n = 2), and composite MA with coexistence of different malignant components (n = 6). Despite the presence of atypical histological features and malignant components among the patients, only one patient developed distant metastasis (median postoperative follow-up, 56 months; range, 30-86 months). Conclusions: MAs are a heterogeneous group of neoplasms with different biological characteristics. The correct identification of this entity and its subtypes would facilitate stratification of optimal management protocols and accurate assessment of the prognosis.