Clinical significance of occult cerebrospinal fluid involvement assessed by flow cytometry in non‐Hodgkin’s lymphoma patients at high risk of central nervous system disease in the rituximab era

Background and aim: Flow cytometry (FCM) analysis of cerebrospinal fluid (CSF) is more sensitive than conventional cytology (CC) for diagnosis of lymphomatous meningeosis, but the clinical significance of occult central nervous system (CNS) disease (positive FCM with negative CC) remains unknown. Patients and methods: CSF samples from 105 patients with newly diagnosed aggressive lymphomas at high risk of CNS involvement were prospectively studied by both CC and FCM, and results were correlated with cumulative incidence of CNS relapse and overall survival (OS). Patients were divided into three groups: 1) patients without CNS involvement (CC?/FCM?; n = 83); 2) individuals with occult CNS disease (FCM+/CC?; n = 15); and 3) cases with CNS disease (CC+/FCM+; n = 7). Results: Six cases showed CNS relapse or progression: two in Group 1 (2.4%), two in Group 2 (13%) and two in Group 3 (28.5%) (Group 2 vs. 1, P = 0.04; Group 3 vs. 1, P < 0.001). Patients from Groups 2 (P = 0.05) and 3 (P < 0.001) also showed a higher cumulative incidence of CNS relapse than those from Group 1. Significant differences were observed in OS between FCM?/CC? and FCM+/CC+ cases (P = 0.02), while patients with occult CNS disease (FCM+/CC?) displayed intermediate OS rates, although differences did not reach statistical significance. Conclusions: The presence of occult CNS involvement at diagnosis in patients with NHL at high risk of CNS disease is associated with a higher probability of CNS relapse.