Ultrahypofractionated radiotherapy better preserves B- and T-cell immunity than conventional fractionated radiotherapy in prostate cancer

Ultrahypofractionated radiotherapy (UHRT) has emerged as an effective treatment option for localized prostate cancer (PCa), but the systemic immunological consequences of different radiotherapy (RT) regimens remain largely overlooked. In particular, the relative contribution of fractionation schedule and irradiation volume to long-term immune remodelling has not been systematically investigated. We conducted a longitudinal immune-monitoring study in peripheral blood from PCa patients treated with five different RT schedules, including three UHRT regimens. Complete blood counts and multiparametric flow cytometry were performed before treatment and longitudinally up to 12 months after RT completion to assess lymphocyte counts, immune-cell composition, and differentiation status of T- and B-cell compartments. B-cell functionality was evaluated by flow-cytometric analysis following CpG stimulation. In a subset of patients, tertiary lymphoid structures (TLS) were investigated in prostate tissue by immunohistochemistry using anti-CD20 and anti-CD3 antibodies, together with CD21 and CD23 staining to assess TLS maturation. Conventional fractionated RT was associated with persistent lymphopenia, increased neutrophil-to-lymphocyte ratio, reduction of CD3⁺ and conventional CD4⁺ T cells, enrichment of activated regulatory T-cell subsets, and prolonged depletion of memory B cells. Functional analyses demonstrated a marked impairment of B-cell responsiveness following conventional RT, whereas patients treated with UHRT maintained substantially preserved B-cell function after CpG stimulation. In contrast to conventional RT, UHRT-based regimens induced limited and largely reversible immune alterations, with progressive restoration of both T- and B-cell compartments and recovery of B-cell functionality. Importantly, peripheral immune profiles returned close to baseline values approximately one year after treatment completion, even in patients receiving pelvic irradiation. Comparison between pelvic conventional RT and pelvic UHRT further suggested that cumulative radiation exposure contributes to systemic immune disruption beyond the effect of irradiation volume alone. Different RT schedules exert distinct effects on long-term systemic immune proficiency. Compared with conventional fractionated RT, UHRT preserves immune-cell homeostasis, maintains B-cell functional competence, and supports recovery of peripheral immune compartments within one year after treatment. These findings provide a strong rationale for optimizing RT fractionation strategies and for the future integration of UHRT with immunotherapeutic approaches in prostate cancer. Clinicaltrials.gov NCT04774133, registered February 23 2021, retrospectively registered; https://clinicaltrials.gov/ct2/show/NCT04774133