Critical histologic appraisal of the pseudocapsule of small renal tumors

Small renal tumors are usually enwrapped in a pseudocapsule with well-confined borders, a feature that facilitates the performance of nephron-sparing surgeries (NSS). Our study was designed to evaluate the histologic features of the pseudocapsule of small renal tumors. One hundred seventy-eight renal tumors (≤4 cm), which were surgically removed by total nephrectomy, partial nephrectomy, or enucleation procedures during 2002-2013, were re-examined microscopically. Special attention was paid to the completeness and thickness of the pseudocapsule as well as the extra-pseudocapsular extension (EPE); components of the pseudocapsule and the intra-pseudocapsular vasculature (size/number) were evaluated. The data were analyzed according to the histological tumor types, Fuhrman grades, and sizes. Student's t test and chi-square tests were used for statistical analysis. Among 178 renal tumors, clear cell renal carcinomas (RCC) showed the thickest pseudocapsule (average 0.23 mm), while oncocytoma showed the thinnest (average thickness of 0.09 mm). Chromophobe RCC had the highest rate of EPE and the highest percentage of tumors with larger (≥0.2 mm) intra-pseudocapsular arteries. The EPE rate was also related to the nuclear grade (p = 0.001). Muscular differentiation, reticulin, and collagen components were present in the fibrous stroma of the pseudocapsule. Our study suggests that clear cell RCC has the thickest pseudocapsule while oncocytoma has a poorly developed pseudocapsule, but shows the least infiltrative pattern. In small RCC (≤4.0 cm), the EPE rate is related to tumor grade but not to tumor size. Larger arterioles (≥0.2 mm) are encountered infrequently within the tumor pseudocapsule, with the highest percentage being found in chromophobe RCC and the lowest in papillary RCC.