Morphologic Spectrum of Renal Cell Carcinoma, Unclassified: An Analysis of 136 Cases

Aims Renal cell carcinoma, unclassified (RCCU) is a category that includes a morphologically and biologically heterogeneous group of tumors that are unable to be diagnosed as other well-defined entities. We aim to describe the morphologic findings of tumors within this category and to determine the most frequent morphologic features leading to classification difficulty. Methods and results One hundred and thirty-six cases of RCCU were examined. Patients ranged in age from 23 to 87 years. Seventy-seven patients were men and 59 were women. International Society of Urological Pathology (ISUP) grade was most commonly 3 (n=66), followed by 2 (n=42) and 4 (n=28). Tumor size ranged from 0.6 cm to 24.9 cm. The AJCC pathologic T categories included pT1a (n=50), pT1b (n=14), pT2a (n=7), pT2b (n=4), pT3a (n=50), and pT4 (n=9). Forty-four cases included lymph node(s), of which 41% (n=18) had metastases. Tumors were assessed for a variety of histologic features and assigned to the following morphologic groups: predominantly oncocytoma/chromophobe RCC-like; clear cell RCC-like; papillary RCC-like; collecting duct-like; and pure sarcomatoid differentiation. The majority of the oncocytoma/chromophobe and clear cell RCC-like phenotypes were low stage (pT1 or pT2). The papillary RCC-like, collecting duct-like, and pure sarcomatoid phenotypes were mostly high stage (pT3 or pT4). Conclusions RCCU is a term that encompasses tumors with a variety of morphologic features and a wide biologic spectrum. The most common source of diagnostic difficulty was tumors composed of predominantly eosinophilic cells