Can renal oncocytoma be distinguished from chromophobe renal cell carcinoma by the presence of fibrous capsule?

The most important differential diagnosis of chromophobe renal cell carcinoma (CRCC) is renal oncocytoma. Due to overlapping morphological characteristics of renal oncocytoma and CRCC, particularly its eosinophilic variant, making a correct diagnosis can be challenging. To date, no data are available on the presence of the tumor fibrous capsule as a diagnostic feature in differentiating these tumors. The main purpose of this study was to establish the presence and compare the thickness of the tumor fibrous capsule between two tumor groups. A total of 37 tumors--18 cases of CRCC (three eosinophilic and 15 classic) and 19 cases of renal oncocytoma--were analyzed. Four slides of each tumor stained with hematoxylin and eosin were first scanned at low-power magnification (x40) to assess the presence of the capsule. If present, the capsule was measured in three different thickest areas at higher magnification (x200). The mean value of capsule thickness was calculated and taken into consideration. The capsule was present in 12 (66.7%) cases of CRCCs and in only two (10.5%) cases of renal oncocytomas. Statistical analysis showed significant difference between the presence of fibrous capsule in these two observed tumor groups (P = 0.001). Average thickness of capsule in CRCCs was 337.7 microm, and 115.4 microm in renal oncocytomas, but the median was not statistically significant (P = 0.198). Studies with a larger number of cases are needed to conclude if this characteristic could be a low-cost, reliable microscopic feature in differentiating between CRCC and renal oncocytoma.