Conventionally fully fractionated Gamma Knife Icon re-irradiation of primary recurrent intracranial tumors: the first report indicating feasibility and safety

OBJECTIVE With the incorporation of real-time image guidance on the Gamma Knife system allowing for mask-based immobilization (Gamma Knife Icon [GKI]), conventionally fully fractionated (1.8–3.0 Gy/day) GKI radiation can now be delivered to take advantage of an inherently minimal margin for delivery uncertainty, sharp dose falloff, and inhomogeneous dose distribution. This case series details the authors’ preliminary experience in re-irradiating 7 complex primary intracranial tumors, which were considered to have been previously maximally radiated and situated adjacent to critical organs at risk. METHODS The authors retrospectively reviewed all patients who received fractionated re-irradiation using GKI at the Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada, between 2016 and 2021. Patients with brain metastases, and those who received radiotherapy courses in 5 or fewer fractions, were excluded. All radiotherapy doses were converted to the equivalent total dose in 2-Gy fractions (EQD2), with the assumption of an α/β ratio of 2 for late normal tissue toxicity and 10 for the tumor. RESULTS A total of 7 patients were included in this case series. Three patients had recurrent meningiomas, as well as 1 patient each with ependymoma, intracranial sarcoma, pituitary macroadenoma, and papillary pineal tumor. Six patients had undergone prior linear accelerator–based conventional fractionated radiotherapy and 1 patient had undergone prior proton therapy. Patients were re-irradiated with a median (range) total dose of 50.4 (30–63.4) Gy delivered in a median (range) of 28 (10–38) fractions with GKI. The median (range) target volume was 6.58 (0.2–46.3) cm 3 . The median (range) cumulative mean EQD2 administered to the tumor was 121.1 (107.9–181.3) Gy, and the median (range) maximum point EQD2 administered to the brainstem, optic nerves, and optic chiasm were 91.6 (74.0–111.5) Gy, 58.9 (6.3–102.9) Gy, and 59.9 (36.7–127.3) Gy, respectively. At a median (range) follow-up of 15 (6–42) months, 6 of 7 patients were alive with 4 having locally controlled disease. Only 3 patients experienced treatment-related toxicities, which were self-limited. CONCLUSIONS Fractionated radiotherapy using GKI may be a safe and effective method for the re-irradiation of complex progressive primary intracranial tumors, where the aim is to minimize the potential for serious late effects.