Misinterpretation of normal cellular elements as neoplastic in fine needle aspiration (FNA) is a well-recognized pitfall in cytopathology. In fact, the kidney is the organ with the highest false positive rate for neoplasms in FNA samples. Diagnostic accuracy is typically high for an FNA/biopsy of a localized renal mass with sensitivity and specificity over 96% when a diagnostic result is obtained¹. Correct histologic diagnosis and subclassification of renal cell carcinoma is often achieved by cytomorphology, but the accurate assessment of nuclear grade has shown to be low¹. Another limitation of renal FNA is the high non-diagnostic rate¹. Although the risk of tumor seeding theoretically exists for renal mass biopsies, this phenomenon has rarely been reported¹.

In general, FNA samples are considered satisfactory for evaluation if the smears contain representative material of the mass seen on imaging studies. Preferably, we should see a significant number of well-preserved cells characteristic of a diagnostic category. In this case, the cells in figures 1 and 2 are well organized and distributed in honeycomb arrangements. The cells are small to medium sized with granular (oncocytic) cytoplasm. Nuclei are round. The cytoplasm has small vacuoles and granules and indistinct cytoplasmic membranes. Due to the indistinct cell membranes, the granules seem to spill into the background. They represent proximal tubular epithelial cells. In other instances, fewer honeycomb epithelial groups with small, cohesive cells that have less abundant, clear cytoplasm and well-defined cell borders may be seen, representing distal tubular epithelial cells. Therefore, the on-site evaluation and final diagnosis should state that the specimen is non-diagnostic.