# Efficacy and Limitations of Flow Cytometry for the Rapid Diagnosis of Primary Central Nervous System Lymphoma

## TL;DR

This study shows that flow cytometry can quickly and accurately help diagnose primary central nervous system lymphoma, complementing traditional methods.

## Contribution

This is the first study to provide detailed subset analyses of lymphocytes in PCNSL using flow cytometry in correlation with pathology.

## Key findings

- Flow cytometry showed high sensitivity and perfect specificity for diagnosing PCNSL.
- Discordant cases were associated with elevated T-cell markers, possibly due to reactive T-cell infiltration.
- FCM provides rapid diagnostic guidance, accelerating treatment decisions for PCNSL.

## Abstract

Primary central nervous system lymphoma is a form of aggressive brain tumor that requires rapid diagnosis to enable timely treatment. Although accurate, the results of conventional histopathology often take several days. In this study, we evaluated the usefulness of flow cytometry, which can deliver diagnostic information within hours. We retrospectively analyzed 67 patients with suspected primary central nervous system lymphoma and compared the flow cytometry findings with the final pathology; most cases confirmed by pathology were correctly identified by flow cytometry, which showed its high sensitivity and perfect specificity, although some cases were discordant due to reactive T-cell infiltration. Importantly, this is the first study to provide detailed subset analyses of lymphocytes in primary central nervous system lymphoma using flow cytometry. These results demonstrate that flow cytometry is a valuable complement to standard pathology, offering rapid and reliable diagnostic guidance that can accelerate treatment decisions and improve patient care.

Background/Objectives: Primary central nervous system lymphoma (PCNSL) has a markedly high proliferation rate, making early diagnosis and prompt therapeutic intervention essential. To accelerate diagnosis, our institution adopted flow cytometry (FCM) in conjunction with conventional histopathology, and this study therefore evaluated the diagnostic performance of FCM for PCNSL. Methods: We retrospectively analyzed 67 consecutive patients with suspected PCNSL who underwent intraoperative FCM between 2010 and 2023 based on preoperative imaging. B-cell clonality was defined as ≥20% CD19/CD20-positive cells with a κ/λ ratio of >3.0 or <0.5. Results: Using histopathology, we confirmed the presence of PCNSL in 42 patients, all diagnosed as having diffuse large B-cell lymphoma. Six cases (14.3%) were discordant (FCM-D). The sensitivity, specificity, and positive predictive value of FCM were 85.7%, 100%, and 100%, respectively. T-cell markers were significantly elevated in FCM-D cases (p < 0.01), although these were pathologically diagnosed as diffuse large B-cell lymphoma based on histology and immunohistochemistry. Conclusions: FCM yields reliable diagnostic information within hours of tissue collection and supports early therapeutic decisions in PCNSL. Discordant results may reflect reactive T-cell infiltration. This is the first study to present detailed subset analyses in PCNSL using FCM in correlation with pathology, underscoring its utility as a rapid diagnostic tool.