Clinical and Treatment Factors associated with Lymphopenia after Palliative Radiation Therapy in Cancer Patients.

PURPOSE: Conventionally fractionated radiation therapy (RT) is associated with treatment-induced lymphopenia and reduced survival. The impact of shorter-course palliative RT on lymphopenia remains unclear. We evaluated the relationship between characteristics of common palliative RT regimens and lymphopenia in a large institutional cohort. METHODS: We retrospectively identified patients treated with palliative RT over 12 years. Patients with available absolute lymphocyte counts (ALC) before and after RT were included. Lymphopenia was graded per CTCAE criteria. A paired t-test was used to compare pre- and post-treatment absolute lymphocyte count (ALC). Univariable and multivariable logistic regression was used to determine the effect of palliative RT dose/fractionation on the likelihood of inducing lymphopenia. RESULTS: Out of the 2002 patients who received palliative radiation, 877 had lymphopenia prior to treatment. Of the remaining 1125, 748 developed lymphopenia following palliative RT. There was a significant decline in absolute lymphocyte counts (ALC) following palliative RT (p<0.01). The magnitude of ALC decline varied by fractionation. Median change in ALC was -0.27, -0.49, and -0.66 for patients receiving 8 Gy in 1 fraction (Fx), 20 Gy in 5 Fx, and 30 Gy in 10 Fx, respectively (p<0.01). Treatment site (bone/spine vs. other sites) was not significantly associated with differences in ALC change (p=0.09). In the multivariable analyses, subjects who received 5 Fx (OR=1.85; 95% CI: 1.41 - 2.44) or 10 Fx (OR=3.12; 95% CI: 2.19 - 4.43) were approximately 2 and 3 times, respectively, more likely to develop lymphopenia as compared to those who received 1 Fx.