Incidence of benign pathologic lesions at nephrectomy for renal masses presumed to be stage I renal cell carcinoma in Japanese patients: Impact of sex, age, and tumor size.

374 Background: The widespread use of modern imaging techniques has resulted in the increased detection of small, asymptomatic renal tumors. Some recent studies from Western countries have reported that the incidence of benign lesions is approximately 15% in patients undergoing definitive surgery for renal masses presumed to be clinical stage I renal cell carcinoma (RCC). The high level of noncancerous lesions is, to some extent, due to the fact that no imaging feature can accurately distinguish either oncocytoma or lipid-poor angiomyolipma (AML) from RCC. This study attempts to determine the incidence of benign pathologic findings for such renal masses in Asian patients. Methods: Between 1991 and 2009, 711 consecutive patients (218 women and 493 men) underwent partial (n=206) or radical (n=505) nephrectomy for renal masses presumed to be stage T1N0M0 (T1a/T1b= 503/208) sporadic RCC on preoperative imaging in two Japanese centers. The mean size of the lesions was 3.3 cm (range 0.3-7.0). The pathologic features were reviewed by an experienced pathologist. Results: Of the 711 masses, 53 (7.5%) revealed benign pathologic findings. Twenty-two (3.1%) were AMLs, 13 (1.8%) were oncocytomas, 8 (1.1%) were complicated cysts, and 10 were others. Twenty-eight (12.8%) of the 218 females and 25 (5.1%) of the 493 males had benign lesions (p=0.0005). Of the 357 patients aged 60 years or younger, 37 (10.4%) had benign lesions while only 16 (4.5%) of the 354 patients over 60 years did (p=0.024). Forty six (9.2%) of the 503 T1a and 7 (3.4%) of the 203 T1b masses were benign (p=0.0071). A multivariate logistic regression model showed that sex, age and tumor size were all independently predictive of benign histology, particularly of AML. Conclusions: The present incidence (7.5%) of benign lesions in presumed clinical stage T1N0M0 RCC masses at nephrectomy was lower than the incidence of approximately 15% previously reported from Western countries, probably because of the low incidence of oncocytomas in Japanese patients. Female gender, young age and small tumor size are all independently predictive of benign lesions, particularly of AML in Japanese patients. No significant financial relationships to disclose.