LAUSR

e13564 Background: Standard glioblastoma (GBM) management includes radiotherapy, chemotherapy, and steroids; all of which can result in immunosuppression and a low absolute lymphocyte count (ALC). Previous literature identified an association between low CD4 and worse progression free survival (PFS) and overall survival (OS). There remains a lack of research addressing predictors of immunosuppression in patients with GBM. The primary objective of this study is to identify the degree of immunosuppression, measured by ALC, in GBM patients receiving concurrent temozolomide chemoradiation (CRT). Secondary objectives include associations between ALC, PFS, and OS, and whether there are any predictors of immunosuppression in patients with GBM. Methods: We retrospectively reviewed 231 newly diagnosed GBM patients who underwent surgery followed by standard of care CRT. We also analyzed the association between ALC and age, sex, MGMT methylation status, and extent of surgical resection. ALC was collected at the time of surgery, CRT start date, and two, four, six, and ten weeks post-CRT start date. Common Terminology Criteria for Adverse Events (CTCAE) protocol version 5.0 was then used to grade low ALC as grade 0, 1, 2, 3, or 4. Results: Of the 231 patients analyzed, 139 were males, 74 underwent gross total resection of the tumor, 129 patients were less than 65 years, and 79 (42.5%) were MGMT methylated. 37 patients had grade 3-4 low ALC. In a univariate analysis, grade 3-4 low ALC at 4 weeks (±14 days) post-CRT start was associated with higher mortality (HR 1.54, P = 0.028) but had no significant association with PFS (HR 1.22, P = 0.29). Logistic regression analysis was used to identify risk factors for grade 3-4 low ALC and its association with survival. None of the risk factors that we tested such as age, gender, type of surgery, or molecular markers including MGMT, IDH, or EGFR were associated with low ALC. Conclusions: Our study demonstrated that patients with ALC grade 3 or 4 at 4 weeks (±14 days) of CRT had a significantly higher mortality (HR 1.54, P = 0.028) but had no significant association with PFS (HR 1.22, P = 0.29).