Distinguishing Non-Renal Cell Neoplasms on Kidney Fine Needle Biopsies

Summary: Introduction: Most lesions diagnosed through cytologic evaluation of renal mass fine needle biopsies (FNBs) are renal cell carcinomas (RCCs). Encountering non-renal-cell neoplasms is rare and not widely studied in the context of cytology, but has marked impact on subsequent treatment decisions. With recent clinical guidelines recommending a utility-based approach to renal biopsies, this study was designed to gain more information on this important topic. Materials and Methods: A five-year retrospective study of renal mass FNBs was conducted at a tertiary care medical center. All patients with malignant renal masses evaluated by Cytopathology but not diagnosed as RCC were retrieved from the electronic medical record. Patient demographics, imaging features, pathology data, and clinical follow-up were collected. Results: Of 227 renal FNBs performed over five years, 30 (13%) were malignant, but not diagnosed as RCC on cytologic evaluation. The diagnoses were solid tumor metastasis (n=10), urothelial carcinoma (UC) (n=5), carcinoma not otherwise specified (NOS) (n=5), lymphoma (n=4), carcinoma with squamous differentiation (n=3), and sarcomatoid neoplasm (n=3). Solid tumor metastases included primaries from the head and neck (n=4), lung (n=2), breast (n=1), skin (n=1), bladder (n=1), and unknown primary (n=1). Surgical pathology correlation was available in 27% of cases (n=8), with an overall concordance of 87.5% (n=7); one case diagnosed as metastatic squamous cell carcinoma was subsequently diagnosed as UC with extensive squamous differentiation. While not diagnosed with RCC initially, seven patients were subsequently deemed to have RCC, either clinically (n=4), or via additional sampling (n=3). Main challenges in these cases included scant viable material (n=4), or sarcomatoid differentiation (n = 2). Conclusions: Non-RCC malignancies, while uncommon, represent a significant proportion of renal masses sampled by FNB. Most can be diagnosed with high accuracy into categories with significantly different therapeutic implications, including solid tumor metastases, UC, and lymphoma. Challenges included tumors with squamous features, necrosis, and sarcomatoid differentiation.