To investigate the incidence and histopathological factors of perirenal fat invasion (pT3a) in cT1a renal cell carcinomas. The implication for postoperative perirenal fat invasion, as a prognostic factor in patients with tumors less than 4cm tumor in size is also discussed.

Of 503 patients who underwent an operation for a renal cell carcinoma at our institution, between June 1995 and April 2004, we retrospectively reviewed the records of 176 with T1a renal cell carcinomas. We evaluated the pathological grade, cell type, tumor size, location and incidence of perirenal fat invasion using the Fisher's exact test. The overall survival was estimated using the Kaplan-Meier method and log-rank test, and the prognostic factors influencing the survival were estimated using the Cox proportional hazard regression model.

In this study, a radical nephrectomy was performed in 128 men and 48 women, with a mean age of 54.6 years, ranging from 23 to 77. The mean follow-up was 31.3 months, ranging from 6 to 106 months. The average size of the renal cell carcinomas was 3.0cm, ranging from 1 to 4cm. The incidence of perirenal fat invasion in the T1a renal cell carcinomas was 5.7% (n=10). The nuclear grade (p<0.001) was a statistically significant factor in the incidence of postoperative perirenal fat invasion.

In the patients with a renal cell carcinoma less than 4cm in size (cT1a), those in the postoperative perirenal fat invasion group had a significantly poorer prognosis. The tumor size and Fuhrman nuclear grade were implicated in the incidence of perirenal fat invasion in the T1a renal cell carcinomas. Therefore, in the case of nephron sparing surgery, more precise preoperative staging of the primary tumor is required.