2613 - Additional Brain Metastases Identified on SRS Treatment MRI: A Missed Opportunity?

Purpose/Objective(s): Linear accelerator based stereotactic radiosurgery (SRS) for brain metastases is a common practice in the United States. Typically, due to the workflow associated with this modality, a diagnostic MRI is obtained for treatment planning purposes several days prior to treatment delivery. Conversely, the typical non-invasive stereotactic radiosurgery instrument SRS workflow calls for a repeat MRI on the actual day of treatment. This study reports the incidence of additional brain metastases identified on the day of non-invasive stereotactic radiosurgery instrument SRS, compared with the immediate pre-treatment diagnostic MRI. The objective is therefore to provide insight into the likelihood of missed metastases when a same-day MRI is not incorporated into an SRS workflow.

Materials/Methods: Clinical records and imaging for 134 consecutive patients receiving non-invasive stereotactic radiosurgery instrument SRS at a single institution for brain metastases were reviewed under an IRB approved study. Number of metastases on initial MRI, time from initial MRI to the day of non-invasive stereotactic radiosurgery instrument MRI, and then number metastases found on non-invasive stereotactic radiosurgery instrument MRI were recorded. Fisher’s exact test was used to determine effect of time (between the 2 MRIs) on the appearance of additional metastases.

Results: Demographics, primary disease site, number of metastases, and the incidence of additional metastases at time of treatment are reported in Table 1.

Conclusion: In this study, the overall incidence of additional metastases identified for treatment at the time of non-invasive stereotactic radiosurgery instrument SRS was 44%. Likelihood of additional metastases when the diagnostic MRI is =7 days old vs. =10 days old vs. =14 days old was statistically significant and increased with increasing time to treatment. When using MRIs obtained the same week as treatment, the incidence of additional metastases on day of treatment was 6.7%. This jumped to 16.4% when the preceding MRI was up to 14 days old. Further time-to-event analyses will be evaluated on the 5-year data set which will include additional consecutive patients.

Abstract 2613 - Table 1: P-value comparing 7 days vs 14 days (0.02) and comparing all 3 timepoints (0.03)