Abstract 1374: Influence of radiation target volume on circulating lymphocyte counts in patients with malignant glioma receiving radiation therapy and temozolomide.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Introduction: Treatment induced lymphopenia (TRL) is an independent negative prognostic factor for survival in patients with malignant glioma (MG). Radiation therapy (RT) is a likely contributor to TRL through the irradiation of circulating lymphocytes. Previous modeling studies suggested that RT target volume (PTV) would not have an appreciable impact on total lymphocyte count (TLC) at the end of RT given the large PTVs in patients with MG. In this study, we attempted to validate the model by studying % changes in total lymphocyte count (TLC) during brain RT in patients with MG. Methods: This was an IRB-approved retrospective chart review study. All patients received 59.4-60Gy given in 1.8-2Gy daily fractions with concurrent temozolomide (TMZ; 75 mg/m2). Data on baseline patient characteristics were collected along with data on weekly TLC obtained through the clinical laboratory and data on PTV obtained from the RT planning system (Pinnacle®). Pearson correlation was used to test the relationship between PTV size and TLC after RT. Results: This study included 63 patients, 30 (45%) female and 37 (55%) male. Median age was 58 years (range 22-79 years). Median baseline KPS was 90 (range 30-100). Most patients (n=58; 87%) had GBM. Gross total resection was performed in 25 patients (37%), subtotal resesction in 34 (50%) and biopsy in 8 (12%). Median PTV size was 337 cm3 (range 103-879 cm3). Median baseline lymphocyte count was 1230 cells/uL (range, 331-4790). Median lymphocyte counts decreased during and after the course of treatment. Median TLCs during the last week of RT/TMZ, one month after RT/TMZ, and two months after RT/TMZ were 730 cells/uL (range 84-2270), 910 cells/uL (range 331-4790), and 820 cells/uL (range 183-1900), respectively. PTV was not correlated with % decrease in the lymphocyte count at the end of treatment nor at one or two months after completing treatment (r = 0.005, 0.11, and 0.16; p = 0.99, 0.42, and 0.27, respectively). There was no significant difference in mean PTV size among patients who developed grade 3-4 lymphopenia compared with those who did not. Conclusions: Brain radiation induces lymphopenia which is progressive over the course of therapy. PTV size was not correlated with lymphocyte count at the end of treatment nor with the risk of developing grade 3-4 lymphopenia. Although modeling predicts that differences in PTV size may impact lymphocyte counts very early in treatment or when very small PTVs are used, it is unlikely that changing PTV size will have a major effect on lymphocyte counts given a six-week course to a large field such as is used in malignant glioma. These conclusions may not apply to patients treated with very small PTVs or with only a few RT fractions (i.e., stereotactic radiosurgery). Alternative strategies are needed to prevent or treat lymphopenia in patients receiving protracted RT courses to a large target volume. Citation Format: Susannah Yovino, Eric Ford, Prakash Ambady, Stuart Grossman, Lawrence Kleinberg, Xiaobu Ye. Influence of radiation target volume on circulating lymphocyte counts in patients with malignant glioma receiving radiation therapy and temozolomide. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1374. doi:10.1158/1538-7445.AM2013-1374