Characterization of Fibromuscular Pseudocapsule in Renal Cell Carcinoma

Pseudocapsule in renal cell carcinoma (RCC) has been described but little is known about its prevalence and extent. Pseudocapsule was analyzed in 105 RCCs (44 clear cell, 44 chromophobe, 17 papillary). Pseudocapsule was graded as follows: grade 1, thickness comparable to adjacent muscular arteries; grade 2, thickness more than twice the diameter of adjacent muscular arteries; grade 3, grade 2 findings with vasculopathy. Tumor size, tumor regression, and International Society of Urologic Pathology (ISUP) nucleolar grade were recorded. Cases with grade 3 pseudocapsule were stained with elastic silver stain, Alcian blue, smooth muscle actin, and CD31. More clear cell RCCs had pseudocapsule (89%, 39/44) than chromophobe (30%, 13/44) and papillary (35%, 6/17). Average tumor size with pseudocapsule was 3.9 cm; average tumor size without pseudocapsule was 3.8 cm (P = .77). Grade 2 pseudocapsule was common in clear cell RCC (56%, 22/39). Chromophobe and papillary RCC had grade 1 pseudocapsule in 77% (10/13) and 83% (5/6) of cases. Grade 3 pseudocapsule was only seen in clear cell RCC (10%, 4/39). No correlation was noted between degenerative tumor changes, tumor size, ISUP nucleolar grade, and presence and grade of pseudocapsule. Smooth muscle actin and CD31 showed abundant smooth muscle component and rich vasculature within the pseudocapsule. Arterial elastic membrane disruption and/or fibrointimal mucin deposits were present in grade 3 pseudocapsule. Thus, pseudocapsule is rather characteristic and more prominent in clear cell, less frequent in chromophobe, and rare in papillary RCC. Its presence may be evaluated radiologically or in biopsy specimens with scant tumor fragments.