2868 - Changes in Systemic Immune Response and Its Dosimetric Correlates in Patients with Non-Small-Cell Lung Cancer Treated with Stereotactic Body Radiation Therapy

J. Rutkowski¹, Z. Nowicka², T. Slebioda³, B. Sobocki¹, B. Tomasik¹, Z. Kmiec³, and R. Zaucha¹; ¹Department of Clinical Oncology and Radiotherapy, Medical University of Gdansk, Gdansk, Poland, ²Department of Biostatistics and Translational Medicine, Medical University of Lodz, Lodz, Poland, ³Department of Histology, Medical University of Gdansk, Gdansk, Poland

Purpose/Objective(s): Radiotherapy delivered to non-small-cell lung cancer (NSCLC) causes tumour antigen release potentially stimulating immune response against cancer; however, radiation-induced lymphocyte depletion negatively impacts patient survival. Lymphocyte subtypes are characterized by different radiosensitivity, with CD8+ cytotoxic lymphocytes more resistant than CD4+ helper T cells. We evaluated changes in lymphocyte subtypes in patients undergoing SBRT for NSCLC and their associations with dosimetry parameters. Materials/

Methods: Ninety-three treatment-naïve patients with NSCLC stage T1/2aN0M0 were prospectively recruited. Peripheral Blood Mononuclear Cels (PBMC) were collected before treatment, at 2 and 12 weeks after RT completion, and the levels of T lymphocytes expressing CD4, CD8, CD25, CD28, PD-1 or CTLA-4 were evaluated using flow cytometry. Dosimetric data were converted to equivalent dose in 2 Gy fractions using an a/ß ratio of 10 Gy and missing values were imputed using k nearest neighbors. Cell counts were compared across timepoints using Friedman ANOVA with Nemenyi post-hoc test; associations between cell counts and dosimetry parameters were assessed using Spearman test.
Results: Lymphocyte count decreased from median 1.99 (25-75%: 1.55-2.35) K/µL at baseline to 1.55 (1.23-1.86) K/µL 2 weeks after RT. At 12 week follow-up 41 patients (43.2%) had sustained lymphopenia <1.5 K/µL. The decrease was not observed uniformly across all cell subtypes as cytotoxic CD8+ lymphocytes increased during RT (p=0.002), with persistently high levels three months after RT completion (p=0.004). The CD4+ to CD8+ ratio decreased during RT (p=0.011). The T lymphocytes expressing exhaustion markers (CD8+/PD-1+, CD4+/PD-1+, CD4+/CTLA4+) showed a transient but significant increase at 2 weeks (p=0.003, 0.001 and 0.001, respectively), followed by a decline to baseline levels at week 12 (p=0.001 for all). A delayed increase of cytotoxic CD8+ lymphocytes expressing activation markers CD25 or CD28 was observed between week 2 and 12 post-SBRT (p=0.009 and p=0.001) while the level of CD4+ helper lymphocytes with those surface antigenes remained stable (p=0.394 and 0.724). Mean lung dose was negatively correlated with lymphocyte count at week 12 (R=-0.53, p<0.001) and CD4+ lymphocyte count at week 12 (R=-0.25, p=0.016), but not with CD8+ lymphocyte count at week 12 (p=0.997). In general, dosimetric parameters for the sum of lungs, heart and whole body were predictive of CD4+, but not CD8+ lymphocyte counts at week 12. Lung and whole-body volume exposed to higher radiation doses (>10Gy) was relatively more important than low doses (1-5 Gy) for predicting CD4+/PD-1+ lymphocyte counts at 12 weeks post-RT.
Conclusion: Radiotherapy elicits dosimetry-dependent changes in the immune profile of patients with NSCLC consistent with an activation of cellular immune response counterbalanced by increase in peripheral tolerance markers, followed by a delayed increase in activated cytotoxic T cells.