Effect of reclassification on the incidence of benign and malignant renal tumors

The incidence of benign renal tumors has increased in recent years. This trend is commonly attributed to the increased use of cross-sectional imaging and minimally invasive surgical approaches. An alternative hypothesis is that recent changes in histological classification are responsible for the increasing incidence. To further investigate the impact of histological reclassification we reexamined all excised renal masses using the 2004 WHO criteria and compared this histological classification to the prior criteria.

We identified 1,101 consecutive partial and radical nephrectomy cases managed at our institution from 1989 to 2003. All histopathological sections were rereviewed by a single pathologist and reclassified according to 2004 WHO criteria. The percentages of benign lesions per year according to the prior histological and current WHO 2004 histological criteria were compared.

Of the 1,101 renal masses 132 (12.0%) and 165 (15.0%) were classified as benign using prior and current WHO criteria, respectively. On average the WHO criteria diagnosed more benign tumors per year than the prior criteria (p = 0.004). Linear regression demonstrated a similar, persistent increase in benign diagnoses per year of 0.69% (WHO) and 1.22% (prior) during the 14-year period (p = 0.33). All masses reclassified as benign were oncocytoma (33).

Implementation of the 2004 WHO criteria is contributing to the increase in diagnosis of benign renal lesions, specifically oncocytoma. Changes in histological classification do not account for the entire increase. Other factors, which remain to be delineated, are also contributing to the increase in the diagnosis of benign renal lesions.