Are small renal tumors harmless? Analysis of histopathological features according to tumors 4 cm or less in diameter

Small renal tumors detected incidentally are considered to have less aggressive potential. This assumption is mainly based on the low tendency to increase in size on serial imaging studies, but histopathological parameters of progression in larger patient series are scant.

We reviewed data of 287 tumor bearing kidneys in which solid tumors 4 cm or less in diameter were detected by cross-sectional imaging and subsequently removed surgically. Tumor size as documented by preoperative computerized tomography was correlated to histological diagnosis, and in cases of malignancy correlated to tumor type, pathological TNM stage and nuclear (Fuhrman) grade. With multifocal lesions the largest single tumor was considered the reference lesion but multifocality was also considered a separate parameter.

At a mean tumor diameter of 2.94 +/- 0.87 cm 65 (22.6%) tumors were 2 cm or less, 103 (35.9%) were 2.1 to 3.0 cm and 119 (41.5%) were 3.1 to 4 cm in diameter. A total of 56 (19.5%) tumors were benign with no correlation to tumor size (Pearson test p = 0.660). Renal cell cancer was found in 227 (79.1%) patients with 159 (70.0%) clear cell, 47 (20.7%) papillary, 11 (4.8%) chromophobe and 10 others with no correlation to tumor diameter. Of the kidneys 31 (13.6%) had multifocal renal cell carcinoma, with a significant correlation to larger tumor diameter (linear regression p = 0.048) and papillary renal cell carcinoma subtype (linear regression p = 0.018). Two (4.2%), 4 (5%) and 25 (25.5%) cases of renal cell carcinoma 2 cm or less, 2.1 to 3 cm and 3.1 to 4 cm in diameter had Fuhrman grade G3/4, respectively (Pearson p = 0.0007). Advanced stage (pT3a or greater) was documented in 2 (4.2%), 12 (14.9%) and 35 (35.7%) cases for the same categories, respectively (p = 0.0023). Whereas distant metastases were diagnosed in only 4 patients with renal cell carcinoma with tumors 3 cm or less, distant metastases were in 10 (8.4%) patients with tumors 3.1 to 4 cm (p = 0.045).

The aggressive potential of small renal cell carcinoma increases dramatically beyond a tumor diameter of 3 cm. Given the difficulty in measuring tumor diameters reliably with sequential imaging studies, the threshold for selecting patients for a surveillance strategy should be set well under this parameter.