Delayed Imaging Changes 18 months or Longer after Stereotactic Radiosurgery for Brain Metastases: Necrosis or Progression

Imaging changes after stereotactic radiosurgery (SRS) can occur for years after treatment, though the available data on the incidence of tumor progression and adverse radiation effects (ARE) are generally limited to the first two years after treatment. A single institution retrospective review was conducted for patients who had greater than 18 months of imaging follow-up. Patients who had one or more metastatic brain lesions treated with Gamma Knife SRS were assessed for time to radiographic progression; those who progressed 18 months or longer out from the time of initial treatment were included in the study. The lesions that progressed were characterized as either ARE or tumor progression based on tissue diagnosis or imaging characteristics over time. The cumulative incidence of delayed imaging radiographic progression was 35% at 5 years from time of initial SRS. The cumulative incidence curves of time to radiographic progression for lesions determined to be ARE and lesions determined to be tumor progression were not statistically different. Cumulative incidence of delayed ARE was 17% and cumulative incidence of delayed tumor progression was 16% at 5 years. A multivariate analysis indicated that the number of metastatic brain lesions at the time of initial SRS was the only factor associated with late radiographic progression. The timing of late radiographic progression does not differ between ARE and tumor progression. The number of metastatic brain lesions at the time of initial SRS is a risk factor for late radiographic progression.