Volumetric trends of progressive in-field recurrences after stereotactic radiosurgery of metastatic intracranial tumors.

Objective: Local recurrence after stereotactic radiosurgery for brain metastasis is a well-known problem. We analyzed volumetric trends from the time of radiosurgery to time of treatment to understand progression behavior. Methods: A retrospective review of patients who underwent treatment for post-radiation progressive lesions was performed. Volumetric trends were obtained by plotting individual lesion volumes from the post-radiation nadir volume to volume at treatment and then fitted to exponential decay or linear regressions. Results: Twenty-eight post-radiation recurrences demonstrated exponential growth and thirteen followed a linear pattern. For lesions exhibiting exponential growth, the average nadir volume was 0.26cm3 (SEM=0.06) at an average of 298 days before treatment and mean volume at treatment was 2.39cm3 (SEM=0.33). The average adjusted R2 was 0.94 (SEM=0.013) and doubling factor was 68.60days (SEM=12.55). In the linear growth cohort, the mean nadir volume was 1.43cm3 (SEM=0.25) at an average of 158 days before treatment and average volume at treatment was 6.90cm3 (SEM=1.43). The mean R2 was 0.92 (SEM=0.02) and average growth rate was 0.034cm3/day. Majority of lesions from primary non-small cell lung cancer (81%) and breast cancer (63%) followed exponential growth. Conclusions: Exponential and linear regressions are accurate representations of post-radiation progression behavior and may be valuable in understanding the growth patterns for recurrences ultimately requiring treatment.