Simple cyst-appearing renal masses at unenhanced CT: can they be presumed to be benign?

To determine renal cancer incidence in simple cyst-appearing renal masses detected at unenhanced computed tomography (CT).

Institutional review board approval and an informed consent waiver for this retrospective HIPAA-compliant study were obtained. Patients who had renal masses with homogeneous water attenuation, hairline-thin smooth walls, and no calcifications or septations were identified by applying a validated natural language processing algorithm to radiology reports for 15 695 unique patients who underwent unenhanced abdominal CT at our institution between 2000 and 2005. Reports that included renal masses were selected, then categorized through manual report review as pertaining to simple cyst-appearing renal masses, nonsimple or solid renal masses, or no renal masses. Medical records were reviewed for subsequent renal cancer diagnoses. Patients without renal cancer were evaluated for a minimum of 5 years (mean, 8 years; range, 5-12 years). The Cox proportional hazards regression model was used to compare renal cancer incidence for patients who had simple cyst-appearing renal masses with those who had nonsimple cystic or solid renal masses and those who had no renal masses.

Simple cyst-appearing renal masses were identified in 2669 patients (17%), no renal masses in 11844 (75%), and nonsimple cystic or solid renal masses in 1182 (8%). Of 1159 patients with simple cyst-appearing renal masses and a minimum of 5 years of follow-up, six (0.52%) subsequently developed renal cancers, all of which were separate from the simple cyst-appearing renal mass, rather than within it. Of 446 patients with nonsimple or solid renal masses and sufficient follow-up, 50 (11%) developed renal cancer. There was no difference in renal cancer incidence in patients with simple cyst-appearing renal masses versus those without renal masses (P = .54). The incidence of renal cancer was significantly lower in patients with simple cyst-appearing renal masses than that in nonsimple cystic or solid renal masses (P < .0001).

Simple cyst-appearing renal masses are unlikely to be malignant. These data support foregoing further imaging evaluation of these common masses.