Clinicopathologic Features of Renomedullary Interstitial Cell Tumor Presenting as the Main Solid Renal Mass

Objective To review the clinical, pathologic, and radiographic features of renomedullary interstitial cell tumor (RMICT). This is a rare benign renal tumor formerly known as medullary fibroma and is indistinguishable from other renal cortical tumors by imaging. Methods After institutional review board approval, we reviewed data on patients from the Memorial Sloan-Kettering Cancer Center kidney tumor database from 1989 to 2012 (4898 patients) with a pathologic diagnosis of RMICT or medullary fibroma as the main resected tumor. Data collected included procedure, age, gender, presentation, preoperative tumor characteristics (size, location, nearness to collecting system, and RENAL nephrometry score), and final pathologic size. Results Ten patients (0.2%) with RMICT were identified. All patients had undergone partial nephrectomy for 10 tumors (9 right). Clinical presentation was incidental to abdominal imaging performed for another clinical reason in 6 patients, as part of a hematuria evaluation in 2 patients, and as part of nephrolithiasis follow-up imaging in 2 patients. The mean patient age was 52 years (range, 39-73), and 8 patients were female. The mean preoperative and final pathologic tumor size was 1.65 cm (range, 1.0-2.5) and 0.96 cm (range, 0.3-1.7), respectively. The location of the tumors was medullary (0-9 mm from the collecting system) in 8 patients and cortical (2.5 cm mostly exophytic and 1.5 cm mostly endophytic tumor) in 2 patients. Conclusion Our data demonstrate a female predominance, a mean tumor size of <2 cm, and medullary location consistent with its pathologic origin. To our knowledge, this is the largest single-institution series of RMICT.