No evidence for depletion of circulating lymphocyte populations in primary brain tumor patients receiving radiotherapy alone.

Abstract

To investigate the impact of radiotherapy (RT) for brain tumors on circulating lymphocyte populations and understand this from a dosimetric perspective. This prospective study enrolled primary brain tumor patients treated with either photon or proton RT. To specifically study the effect of RT, only patients without concurrent chemotherapy were eligible. For each patient, blood samples were collected before, during and after RT, as well as at the first follow-up, for flow cytometry analysis of blood cells. We used a stochastic model to compute blood dose distributions from treatment plans and patient-specific blood flow simulations. Dose to the lymph nodes in the head and neck region was also assessed. Dose distributions and lymphocyte trends from patients treated with photon versus proton therapy were compared. Thirty-five patients with a median age of 50 were included: 27 patients with benign tumors (mostly meningiomas) and 8 with malignant tumors (mostly gliomas). There were no significant RT-induced changes in either the total lymphocyte count or its subpopulations in this patient cohort. In line with this finding, dosimetric analyses showed that total blood doses were ∼0.1-0.2 Gy and that most lymph nodes in the head-and-neck area are spared. Proton RT was associated to a greater dosimetric sparing of both blood and lymph nodes. In this exploratory analysis, we found no evidence that RT alone results in significant changes in peripheral lymphocyte counts in primary brain tumor patients treated to limited intracranial volumes. The lack of apparent RT-induced cytotoxicity is consistent with the limited dose received by either the blood or the regional lymph nodes.