2589 - Impact of Tumor Volume on Survival in Patients with Five or More Brain Metastases Undergoing Stereotactic Radiosurgery

M. Pasli¹, S. O. Dudzinski², M. K. Rooney², M. C. Tom², A. J. Ghia², D. N. Yeboa², M. F. McAleer², T. A. Swanson², S. L. McGovern², C. Wang², C. Chung², T. M. Briere³, R. An⁴, B. De², B. Y. S. Kim⁵, S. Ferguson⁵, J. Li², W. Jiang Jr², D. Mackin³, and T. Beckham²; ¹Brody School of Medicine, East Carolina University, Greenville, NC, ²Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, ³Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, ⁴The University of Texas MD Anderson Cancer Center, Houston, TX, ⁵Department of Neurosurgery, The University of Texas MD Anderson Cancer Center, Houston, TX

Purpose/Objective(s): Stereotactic radiosurgery (SRS) is typically preferred over whole brain radiation therapy (WBRT) for patients with limited brain metastases (BM) due to improved neurocognitive outcomes without compromising survival. ASTRO guidelines support SRS in patients with up to 10 BM with favorable performance status. While treatment of larger numbers of BM is technically feasible, there are mixed data on the influence of tumor volume on outcomes, with some reporting reduced overall survival (OS) for patients with higher volume of BM. Our aim was to analyze the relationship of BM volume and OS in a large retrospective cohort of patients with 5 or more BM receiving SRS. Materials/

Methods: On an IRB approved protocol, we performed a retrospective review of 558 patients who received SRS to five or more BM at our institution with no prior history of brain radiotherapy. Descriptive statistics were used to compare groups based on quartile of summed BM volume: Q1: <600 mm³, Q2: 600-1700 mm³, Q3 1701-4800 mm³, Q4: >4800 mm³. Regression analysis was done to determine a relationship between overall tumor volume and number of BM. Kaplan Meier method with Log-Rank testing was utilized to determine univariate outcomes stratified by quartile of brain metastasis volume (both for total volume and average volume). Cox proportional hazards regression models were applied to multivariate analyses.
Results: Quartiles comprised a range of 134-145 patients, and the most common histology in each was non-small cell lung cancer (NSCLC). There were no significant differences in quartile cohorts with respect to measured covariates including age, sex, KPS, number of lesions, and histology. Overall, when considered as continuous variable in a multivariate model and measured as either volume sum of all lesions or average lesion volume, there was no clear monotonic relationship between tumor volume and OS. However, Kaplan-Meier analysis demonstrated that patients in Q3 demonstrated inferior OS when compared to other cohorts (p=0.019).
Conclusion: In this large patient population, there was not a clear monotonic relationship between tumor volume and OS. The finding that patients in a middle quartile of summed BM volume have worse OS suggests the presence of features associated with both SRS target volume and OS that are not accounted for in our data at this time. Additional covariates such as status of systemic disease and the presence of surgical cavities at the time of SRS will be explored in future analysis.