Optimal Timing of Initial Surveillance MRI Following Treatment of Brain Metastases With Stereotactic Radiation: A Comparison of Six Weeks Versus Three Months.

Patients with brain metastases are increasingly being treated with stereotactic brain radiation, in one fraction termed stereotactic radiosurgery (SRS) or two-five fractions termed fractionated stereotactic radiotherapy (FSR), due to its sparing of neurocognitive dysfunction without compromising survival benefit. However, distant brain failure rates remain high, and the optimal interval for post-radiation MRI surveillance is not well established. This study evaluates the potential clinical benefit of early MRI surveillance compared with a typical three-month timeline. The primary objective of this study was to compare radiographic and clinical outcomes associated with early six-week versus 12-week post-treatment MRI surveillance following stereotactic radiation for brain metastases. In this retrospective study of patients with one or more brain metastases treated with SRS or FSR at a single tertiary center, initial post-treatment surveillance MRI and clinical follow-up were compared between patients who underwent follow-up at a six-week interval versus a 12-week interval after radiation. Of 309 patients reviewed, 232 (75.1%) completed surveillance imaging; 123 (53%) underwent later timeline 12-week MRI with a median time to MRI of 92 days, and 109 (47%) underwent early timeline six-week MRI with a median time to MRI of 43 days (p < 0.001). Among patients in the 12-week MRI group, 52 (42.3%) had radiographical findings on MRI concerning new and/or progressive disease compared to 36 (33.0%) in the early MRI group (p = 0.18). There was similarly no statistically significant difference in patients with new or worsening neurologic symptoms during clinical follow-up in the 12-week (28 (22.8%)) versus early (28 (25.7%)) groups (p = 0.46). Following MRI surveillance, completion and types of subsequent clinical interventions were documented; the frequency of patients undergoing additional interventions did not differ significantly between groups (49 (39.8%) vs 33 (30.3%), p = 0.17). While the overall distribution of intervention type did not statistically differ among patients in either cohort, a higher proportion of those who underwent intervention in the early MRI group went on to receive a short-interval MRI based on equivocal findings at the time of initial MR, while a smaller proportion went on to receive additional radiation therapy. These data support current practice guidelines recommending routine MRI surveillance following SRS or FSR at three-month intervals, as early MRI surveillance did not significantly impact outcomes in this cohort. However, additional studies to identify patients at increased risk for early, or "high velocity", distant brain progression who may benefit from earlier surveillance are warranted.