Limitations of outcome prediction based on interfractional volume changes of large (≥ 10cm³) brain metastases during fractionated gamma knife radiosurgery

Purpose: This study investigated the interfractional volume changes of large (≥ 10 cm³) brain metastases (BMs) during fractionated gamma knife radiosurgery (FGKRS) to assess its predictive value for tumor control outcomes. Methods: The patients who underwent FGKRS for large BMs between January 2017 and December 2022 in our center were reviewed. The interfractional volume change was defined as the disparity in tumor volume (TV) measured between the magnetic resonance images acquired on the first treatment day and those obtained after 2 or 3 fractions during the course of FGKRS. Results: A total of 73 lesions in 70 patients with various primary pathologies were included. Over a median follow-up period of 11 months (range 1–77), the tumor control rate was 63%. Initial TV (cm³) was associated with progression free survival (PFS) and overall survival (OS) in both univariate and multivariate analyses (p = 0.01). Interfractional TV changes revealed an increase in 13 (17.8%) lesions, no change in 14 (19.2%) lesions, and a decrease in 46 (63.0%) lesions, with a mean volume reduction of 5% ± 0.12. Three cut-offs (5%, 10% and 15% volume decrement) were established and patients were divided into two groups based on each reference point. However, there were no significant differences in PFS and OS between the two groups, irrespective of the chosen cut-off value used. Conclusion: Interfractional volume changes of large BMs were not found to be associated with tumor control outcomes. Neither significant interfractional volume reduction nor significant volume increase necessarily predicts the tumor control, making early close monitoring essential after FGKRS.