Changes in peripheral blood lymphocyte subsets in patients undergoing radiotherapy

Purpose: To investigate the changes in peripheral blood lymphocyte subpopul ations in patients undergoing radiotherapy. Materials and methods: In 8 patients undergoing external beam radiotherapy to the pelvis, the different lymphocyte subpopulations were followed during treatment. The lymphocyte populations were determined using two-colour flo w cytometry. The study comprises the T-helper, T-suppressor/cytotoxic cells , the B-lymphocytes and natural killer (NK) cells. Results: The B-cells were characterized by a steep decrease at the beginnin g of the radiotherapy. They reached their lowest level at an equivalent tot al body dose of similar to 1.5 Gy and remained constant during the rest of the therapy (10% of the initial level). In T-cells (both T-helper and T-sup pressor subsets) the steep decrease was less pronounced. T-lymphocytes reac hed a base level at 2.5 Gy equivalent total body dose (20% of the initial l evel). No significant differences between the T-helper and the T-suppressor /cytotoxic cells were observed. NK cells were characterized by a weak decli ne during the first weeks of therapy, being less pronounced than in the oth er populations. Near the end of therapy, the NK cells reached the level of the T-lymphocytes. Conclusion: In vivo, NK cells were the most radioresistant and B-cells the most radiosensitive lymphocytes. No significant differences between T-helpe r and T-suppressor/cytotoxic cells were observed. These data are in agreeme nt with the differences in apoptosis induction in peripheral blood lymphocy te subpopulations after in vitro gamma-irradiation of whole blood lymphocyt es.