Differential Immunologic Effects of Short-Course and Long-Course Radiotherapy in Locally Advanced Rectal Cancer.

[PURPOSE] Two dominant neoadjuvant radiotherapy (RT) regimens are in common use for locally advanced rectal cancer (LARC): long-course RT (LCRT; 25 × 1.8 Gy) with concomitant chemotherapy and short-course RT (SCRT; 5 × 5 Gy), typically followed by systemic chemotherapy. This study uses serial sampling to investigate the evolution of systemic and local immune responses to different RT regimens in LARC.

[EXPERIMENTAL DESIGN] We conducted a serial sampling study involving patients receiving RT for LARC, in which longitudinal blood and tumor biospecimens were collected at baseline and at 2, 6, and 12 weeks after treatment initiation. Leukocyte concentrations via full blood count and a multiplex cytokine ELISA of plasma samples measured systemic changes. Multiplex immunofluorescence (CD8 and FOXP3) and RNA sequencing of tumor biopsies were used to assess local changes.

[RESULTS] Circulating lymphocyte concentrations tended to increase in SCRT patients and decrease in LCRT patients between week 2 and week 6 and were significantly lower in LCRT patients compared with SCRT patients at week 6 (P < 0.0001) and week 12 (P = 0.019). Additionally, we report higher densities of CD8+ and FOXP3+ tumoral lymphocytes from SCRT patients compared with LCRT patients at week 2 (CD8 P = 0.053; FOXP3 P = 0.023) and week 6 (CD8 P = 0.035; FOXP3 P = 0.0016).

[CONCLUSIONS] SCRT is less lymphodepleting and induces more frequent increases in intratumoral T-cell infiltration compared with LCRT. These results are relevant to the field of radiation-immune-oncology combination studies in LARC. Furthermore, these findings may underpin early trial results, in which higher rates of response to RT-immune checkpoint inhibitor combinations have been reported with SCRT-based regimens.