Soluble interleukin-6 receptor mediated fatigue highlights immunological heterogeneity of patients with early breast cancer who undergo radiation therapy

Purpose This study aimed to explore the associations between dose-volume parameters of localized breast irradiation, longitudinal interleukin-6 soluble receptor (sIL-6R), and leukocyte counts as markers of an immune-mediated response and fatigue as a centrally-driven behavior. Methods and Materials This prospective cohort study recruited 100 women who were diagnosed with stage 0-IIIa breast cancer, prescribed 40 Gy in 15 fractions over 3 weeks adjuvant radiation therapy, and had no prior or concurrent chemotherapy. Dose-volume parameters were derived from treatment plans and related to serum sIL-6R concentrations, leukocyte counts, and a validated measure of self-reported fatigue at baseline, after 10 and 15 fractions, and 4 weeks after radiation therapy. Results sIL-6R concertation was significantly higher in patients with a total volume of tissue irradiated within the 50% isodose >2040 cm3 (P = .003). When controlling for body mass index, this result only remained significant after treatment. The volume of liver irradiated within the 10% isodose correlated with the sIL-6R concentration during and after radiation therapy (ρ = .3-.4; P = .03-.007). The 38% of the cohort that was classified as fatigued had a higher mean sIL-6sR concentration at all observation points, but the differences were only statistically significant during radiation therapy: Mean (standard deviation [SD]) after 15 fractions for fatigued patients was 47.6 ng/dL (11.2 SD) versus 41.6 ng/dL (11.4 SD) for nonfatigued patients (P = .01). Cohort leukocyte counts and leukocyte subsets decreased consistently from baseline and the values for the fatigued group were 4% lower at baseline and between 7% and 9% lower during and after treatment compared with those of the nonfatigued group but the differences were not statistically significant. Conclusions This is the first study to show that localized irradiation induces increased systemic sIL-6R during treatment in participants who reported elevated levels of fatigue before, during, and after treatment. This behavioral response appears to reflect a variation in innate host immunity, which then mediates the cellular and/or psychological stress of radiation therapy.