Pattern of recurrence after fractionated stereotactic reirradiation in adult glioblastoma.

Background: Glioblastomas all eventually relapse after initial treatment, and an option to treat these recurrences is fractionated stereotactic reirradiation (fSRT). The location of recurrences following reirradiation has not been studied for fSRT delivered by a dedicated stereotactic device. We aimed to analyze these locations to better elucidate safety margins, dose and fractionation regimens.

Methods: We retrospectively analyzed the data of patients with glioblastoma recurrence that had been reirradiated by fSRT in October 2010-December 2020, in 25 Gy in 5 fractions delivered by a CyberKnife^® at Institut de Cancérologie de Lorraine. We matched the images of the post-fSRT relapse with the stereotactic radiation treatment planning scan to determine the relapse location.

Results: The location of recurrences after fSRT was "out-field" in 43.5%, "marginal" in 40.3%, and "in-field" in 16.1% of patients (N = 62). A GTV-PTV margin of 1 mm (versus 2-3 mm, HR = 0.38 [0.15-0.95], p = 0.037) and a PTV volume of ≥ 36 cc (HR = 5.18 [1.06-25.3], p = 0.042) were significantly associated with the "marginal" recurrences. Being ≥ 60 years old at initial treatment (HR = 3.06 [1.17-8.01], p = 0.023) and having one or more previous recurrences (HR = 5.29 [1.70-16.5], p = 0.004) were significantly associated with "out-field" recurrences. The median PFS from fSRT was 3.4 months, and OS from diagnosis and from fSRT were 25.7 and 10.8 months respectively.

Conclusion: Reirradiation of glioblastoma recurrence by fSRT with 25 Gy in 5 fractions provides good local control.