No abstract

Materials/Methods: Patients treated with SRS for BM between July 2017 and January 2020 were included. Clinical data was retrieved from patient files. Patients were categorized into below and above median MRI-to-SRS interval. Sub-analysis was done by number of BM and initial brain metastasis velocity (IBMV). IBMV is calculated by dividing the number of BM by the interval between primary cancer and BM diagnosis. IBMV 2 is associated with worse OS. Statistical analysis was performed with SPSS Version 26. OS and PFS was calculated with the Kaplan-Meier method and difference between groups was assessed with the Log-Rank method. Results: 256 patients were included. The median MRI-to-SRS interval was 24 days (range 7-51 days). 21-day threshold was chosen as it is 3 calendar weeks and similar to the median. 79 patients were treated within 21 days (Group <21) and 179 treated after 21 days (Group >21). The median PFS overall was 320 days. The PFS for Group <21 was 454 days and for Group >21 was 242 days. The difference approached statistical significance (p Z 0.081). There was no difference in PFS between Group <21 and Group >21 in patients with 1 BM (p Z 0.289) or 2 BM (p Z 0.806). Among patients with 3 BM, Group <21 patients had significantly longer PFS compared to Group >21 (454 days vs 125 days, p Z 0.007). Among patients with IBMV <2, Group <21 had significantly longer PFS compared Group >21 (765 days vs 235 days, p Z 0.019). There was no difference in PFS between Group <21 and Group >21 in IBMV 2 (p Z 0.325) and synchronous cancer (p Z 0.520) subgroups. Conclusion: Our study has demonstrated that MRI-to-SRS 21 days in patients with 3 BM or IBMV <2 is associated with significantly shorter duration of PFS. We hypothesize that the greater MRI-to-SRS interval the greater the probability of emergence of new BM or further increase in BM beyond planning target volume resulting SRS failure. We expected MRIto-SRS 21 days to be associated shorter PFS in patients with IBMV 2 but this was not the case. This is could be due to patients with IBMV 2 having shorter overall survival thus not surviving long enough to develop progressive brain disease. This study is significant as we identified an MRI-to-SRS interval beyond which certain subgroups are at higher risk of brain progression after SRS. These subgroups might benefit from expedited radiotherapy planning and SRS treatment.