Effect of the Time-of-Day of Radiotherapy Administration on Radiation-Induced Lymphocyte Depletion in Lung Cancer

Radiation-induced lymphopenia (RIL) negatively impacts survival of patients with lung cancer. Low baseline lymphocyte counts and high radiation dose to immune-rich organs increase the risk of RIL. As the lymphocyte trafficking in tissues is following a circadian rhythm, we hypothesized that administration time of radiotherapy (RT) impacts the rate and depth of lymphocyte depletion during RT. We retrospectively reviewed data of patients with non-small-cell lung cancer treated with radical RT alone at a single tertiary institution between 2015-2023. We included patients that had available absolute lymphocyte counts (ALCs) at baseline (3 months before to 3 days after start of RT) and during RT and had >90% treatment fractions delivered before or after noon (12 PM). Patients with treatment breaks, modification of RT plans, concurrent chemo-RT or treated with stereotactic RT were excluded. We converted dosimetric data to equivalent dose in 2 Gy fractions using an α/β ratio of 10 Gy and calculated estimated radiation dose to immune cells (EDRIC) as a function of mean heart, lung and body dose and the fraction number. Lowest ALC value before day 45 was considered as the ALC nadir and ALC<0.5 K/µL was considered severe RIL. For 162 patients included the median age was 68.0 (25-75%: 63.0-72.8) years and 107 (66.0%) were male. Twenty-six (16.0%) patients were treated with intensity-modulated RT and the rest with volumetric modulated arc therapy. The median ALC at baseline was 1.86 (25-75%: 1.42-2.42) K/µL and the median RT dose was 60.0 (25-75%: 55.3-66.0) Gy delivered in 30 (25-75%: 21-33) fractions. Severe RIL was observed in 58 (35.8%) patients, including 36 (39.6%) treated before 12 PM vs 22 (31.0%) of those treated in the afternoon (p = 0.259). After accounting for baseline ALC and EDRIC, morning RT predicted for a higher rate of severe RIL (p = 0.041; OR 2.40, 95% CI: 1.10-5.26). In multivariable analysis baseline ALC (p<0.001), EDRIC (p<0.001) and RT timing (p = 0.003) were significant predictors of ALC nadir value, with patients treated in the morning experiencing lower ALC nadirs than patients treated in the afternoon (Table 1). Circadian timing of RT may impact the rate and depth of RIL, consistent with the time-of-day changes in peripheral blood lymphocyte counts. While the present study was not powered to consider different cut-offs for the morning and afternoon definition, future studies should explore more biologically adequate divisions or model ALC by assessing RT dose to lymphocytes that undergo circadian trafficking, as it has important translational implications for patients treated with RT-immunotherapy combination. Table 1. Multivariable linear regression model predicting ALC nadir value.