Effect of Radiotherapy Variables on Circulating Effectors of Immune Response and Local Microbiome

Abstract

Effective radiation (RT) induced immunogenicity requires sufficient numbers of effector and memory T-cells in order to generate anti-tumor immune response. Standard RT regimens, with daily exposure of active hematopoietic tissue and circulating lymphocytes over 4-8 weeks result in treatment-induced lymphopenia. Improvements in RT technology allow for hypofractionated regimens, potentially mitigating the risk of lymphopenia. In this study, we hypothesize that differences in RT fractionation (short versus protracted) and field size (whole breast vs partial breast) may result in different effects on circulating immune mediators. To test this hypothesis, we are prospectively studying patients with breast or prostate cancer undergoing standard protracted vs short-course hypofractionated RT. This is an ongoing prospective non-randomized study assessing 4 distinct cohorts of cancer patients: localized prostate cancer undergoing standard fractionation RT to at least 70 Gy (cohort 1A) vs stereotactic body RT in 5 fxs (cohort 1B); or localized breast cancer patients undergoing standard fractionation RT (cohort 2A) vs partial breast RT in 5 fxs (cohort 2B). Blood samples are collected at baseline, at midpoint during RT, at completion of treatment and at 3-6 months after completion of RT. Lymphocyte counts are obtained and characterization of T-cell subsets is performed using multi-parameter flow cytometry. Inflammatory cytokines in the plasma are measured using bead-based multi-analyte immunoassay. The study has thus far accrued 33/60 planned patients, 13 of whom have completed all 4 blood draws (Cohorts 1A n=3, 1B n=4, 2A n=0, 2B n=6). A significant (P=0.023) decline in absolute lymphocyte count (ALC) for protracted RT patients (n=3) was detected, which persists at 3 month follow up (mean decline 0.93, SD 0.25), with both CD4 and CD8 populations impacted. Conversely, patients undergoing short course radiation (n=10) do not have depletion of ALC, with 8/10 patients maintaining stable ALC during RT. A more detailed characterization of immune subsets, including NK cells, dendritic cells and myeloid cells is ongoing, as well as analysis of inflammatory cytokines. An increase of IL-2 was noted during protracted, but not short, RT in one patient; additional datasets will be acquired. Elucidating the impact of different regimens of RT on circulating immune cells provides valuable information on how RT can best be harnessed to potentiate its immunogenic effects. These preliminary results confirm that shorter radiation courses result in less, or no lymphopenia, whereas protracted radiation courses result in persistent lymphopenia, sustained 3 months after RT is completed. Variables of RT techniques such as fractionation and field size likely impact the availability of key effectors of immune rejection, and warrant further ongoing study.