Abstract: Background and purpose: Immune checkpoint inhibitors targeting programmed death-1 (PD-1)/programmed death ligand-1 (PD-L1) have been approved for the treatment of a variety of malignant tumors, and their therapeutic effects depend on the activation of T lymphocytes. Radiotherapy-induced immunogenic cell death can promote immune response, but at the same time, radiation-induced lymph node cell reduction may reduce the effect of immune checkpoint inhibitors. At present, the effects of different radiotherapy techniques, radiotherapy sites and radiotherapy ranges on lymphocytes are still unclear. The purpose of this study was to investigate the effects of different segmentation methods and doses of lung stereotactic body radiotherapy (SBRT) on patients' peripheral blood lymphocytes. Methods: The clinical data of 64 patients receiving pulmonary SBRT at the Affiliated Cancer Hospital of Zhengzhou University from 2016 to 2018 were retrospectively analyzed. The total prescribed dosage ranging from 40 to 65 Gy was completed for no more than 5 times a week, with 4 to 13 fractions prescribed at the 90% isodose lines for planning target volume (PTV). The biological effective dose (biological equivalent dose, BED) was 60-116 Gy. The differences of absolute lymphocyte counts in peripheral blood of patients with different segmentation methods and doses in various parameters of SBRT were analyzed before radiotherapy, 1 week, 1 month and 3 months after radiotherapy. Results: The lymphocyte counts decreased at 1 week and 1 month after radiotherapy, and basically returned to the levels before radiotherapy at 3 months after radiotherapy. Among the various parameters of SBRT, the number of fractions had a prominent effect on lymphocyte counts. The the reduction of peripheral blood lymphocyte count was significantly less in the group of ≤5-fraction radiotherapy schedule than in the group of >5-fraction radiotherapy, respectively (1.68±0.63 vs 0.93±0.34, P=0.021; 1.6±0.68 vs 0.95±0.41, P=0.004). However, there was no significant difference in lymphocyte counts between groups before and after radiotherapy for dose per fraction, total dose and BED values (P>0.05). Conclusion: The reduction of peripheral blood lymphocyte count was significantly less in the group with less than 5-fraction radiotherapy schedule than in the group with more than 5-fraction radiotherapy schedule 1 week and 1 month after SBRT. The lymphocyte count was basically restored to the level before radiotherapy at 3 months after the end of SBRT.

Key words: Stereotactic body radiotherapy, Lymphocyte count, Immunotherapy