Importance of Flow Cytometry in the Cytopathologic Evaluation of Lymphoid Lesions Involving the Kidney

Background: Cytomorphology alone is often insufficient for the diagnosis and subclassification of lymphomas, so flow cytometry (FC) may be used as an adjuvant test. Methods: Renal fine-needle aspirations (FNAs) performed from January 1993 to August 2014 were reviewed for FC data or a diagnosis of lymphoma. Results: A total of 586 renal FNAs were collected. Thirty-three cases (5.1%) had FC analysis. Lymphoma was diagnosed 35 times (6%), and FC was performed in 21 (60%) cases. Both cytomorphology and FC were consistent with lymphoma in 20 cases. Cytomorphology alone was diagnostic of lymphoma in 15 cases. In 28 cases, biopsy from the kidney or another site was diagnostic of lymphoma. One subsequent biopsy revealed that a kidney FNA, which showed no definitive morphologic or FC evidence of lymphoma, likely represented necrotic diffuse large B-cell lymphoma. Conclusion: FC is a useful adjuvant diagnostic test for renal FNAs, particularly for subclassification and confirmation of the diagnosis when there is insufficient material for immunohistochemistry. FC should be interpreted with caution when a sample is limited or when there is suspicion of Hodgkin lymphoma, and further work-up is warranted when cytomorphology suggests lymphoma but FC is negative.