Second prize: frequency of benign renal cortical tumors and histologic subtypes based on size in a contemporary series: what to tell our patients.

BACKGROUND AND PURPOSE: Historically, 85% to 90% of renal masses have been malignant, with the majority being renal-cell carcinoma. However, with the increasing frequency of incidentally discovered renal masses, only 70% to 85% of lesions are found to be malignant. Furthermore, the pathologic breakdown of these lesions on the basis of size is not well described. This study sought to determine the incidence of the various histologic subtypes of renal cortical tumors according to size in a contemporary series of surgically treated patients.

MATERIALS AND METHODS: Our prospectively collected database of all 482 patients who underwent partial or radical nephrectomy for a unilateral, unifocal, suspicious renal cortical tumor between January 2001 and October 2005 was reviewed. The frequency of benign and malignant lesions was determined according to size, as was the incidence of the various histologic subtypes.

RESULTS: Of the lesions, 228 were <or=4 cm, and 254 were >4 cm. Of all lesions <or=4 cm, 26.3% were benign, whereas only 8.3% of the lesions >4 cm were benign. For larger lesions, 10.9% of those between 4 and 7 cm and only 5.6% of those >7 cm were benign (P < 0.001). A significant difference between histologic subtypes according to size also was found (P = 0.01). Smaller lesions (<or=4 cm) that proved to be malignant were less likely than larger lesions to be of clear-cell histology (50.0% v 72.8%) and more likely to be papillary (15.8% v 9.4%).

CONCLUSIONS: This study examined the largest contemporary database evaluating the histologic type of renal lesions according to size. Approximately one fourth (26.3%) of lesions <or=4 cm and 16.8% of lesions overall were benign, percentages higher than those reported in older series. Importantly, there was a higher incidence of benign and papillary lesions and a lower incidence of clear-cell histology in smaller lesions. Given the differences in the biological behavior of the various histologic subtypes of these tumors, these data are important when counseling patients about the treated and untreated natural history of small renal masses.