The effects of local irradiation on circulating lymphocytes in dogs receiving fractionated radiotherapy.

Localized radiation therapy can be an effective treatment for cancer but is associated with localized and systemic side effects. Several studies have noted changes in complete blood count (CBC) parameters including decreases in the absolute lymphocyte count (ALC) and increases in the neutrophil:lymphocyte ratio (NLR). These changes could reflect immunosuppression and may contribute to decreased efficacy of immunotherapies used to treat cancer. We hypothesized that dogs would demonstrate decreased ALCs during a course of radiotherapy. A retrospective study was conducted on 203 dogs receiving definitive-intent radiotherapy. Demographic information, CBC values and details of the radiotherapy protocol were collected. The mean lymphocyte count pre-treatment was 1630.68 cells/mu L (SD +/- 667.56) with a mean NLR of 3.66 (SD +/- 4.53). The mean lymphocyte count mid-treatment was 1251.07 cells/mu L (SD +/- 585.96) and the mean NLR was 6.23 (SD +/- 4.99). There was a significant decrease in the mean lymphocyte count by 351.41 lymphocytes/mu L (SD +/- 592.32) between pre-treatment and mid-treatment (P < .0001), and a corresponding significant increase in the mean NLR of 0.93 (P = .02). Lymphopenia grade increased in 33.5% of dogs and was significant (P = .03). The ALC decrease was not correlated with the volume irradiated (P = .27), but correlated with the irradiated volume:body weight ratio (P = .03). A subset of patients (n = 35) with additional CBCs available beyond the mid-treatment time point demonstrated significant and sustained downward trends in the ALC compared with baseline. Although severe lymphopenia was rare, these decreases, especially if sustained, could impact adjuvant therapy for their cancer.