The protocol for this study was approved by our Institutional Review Board. We retrospectively reviewed all patients from our center from January 2017 through December 2021 who received fractionated SRS with Gamma Knife ICON for newly diagnosed primary GBM. 14After upfront biopsy and/or maximal surgical debulking, patients were selected for fractionated GK SRS after a multidisciplinary discussion with input from our institution’s neurosurgeons, radiation oncologists, and neuro-oncologists. All patients underwent magnetic resonance imaging (MRI) within 24 h after surgery/biopsy intended for frameless GKRS planning. 1 mm, thin-slice, volumetric, axial images were acquired down to C3 vertebral body, and T2 and T1 contrast-enhanced images were used for treatment planning. MRI images were imported into GammaPlan version 11.1.1, and the scalp border was defined. Both the radiation oncologist and the neurosurgeon participated in treatment planning. The post-operative resection bed and corresponding contrast enhancement in addition to residual gross disease was delineated as the gross tumor volume (GTV) on the MRI T1 contrast-enhanced image sequence. The clinical target volume (CTV) included a 1-1.5 cm margin respecting anatomical boundaries beyond the GTV to encompass microscopic disease, similar to other well-established hypofractionated radiation contouring guidelines [10]. The final planning target volume (PTV) included a 2 mm margin, with a prescription dose of 15–25 Gy to the 50-60% isodose lines in five daily fractions at the discretion of the treating radiation oncologist and neurosurgeon.
After treatment planning, patient setup and treatment delivery occurred in the ICON GK suite as previously described at our institution [14]. Briefly, while patient is in the supine position we molded a warmed thermoplastic mask over the patient’s face, folding back a rim of the mask around nasal aperture to prevent sharp edges. We then deployed the IFMM camera and placed the circular reflective marker on the patient’s nose for real-time intrafraction motion monitoring. CBCT was performed and registered with the planning MRI using Gamma Plan’s registration algorithm. Once the MRI-CBCT registration was complete, the dose distribution was recalculated to reflect the patient’s actual treatment position and geometry as defined by the reference CBCT. The updated dose distribution was reviewed, and modifications to the plan were made, if necessary, to ensure adequate dose to the target and acceptable dose sparing to the organs at risk. once the final distribution and treatment plans were approved, a second CBCT was performed for localization and registered to the reference CBCT. An updated dose distribution and dose-volume histogram (DVH) reflecting the patient geometry in the pretreatment CBCT was reviewed and if satisfactory, the treatment was delivered. Subsequent fractions required only one pretreatment CBCT for localization. Intrafraction monitoring with the IFMM is set to allow nasal tip motion of up to 3 mm during treatment, however this tolerance may be reduced at the discretion of the treating physician. Deviation beyond the threshold for > 30 s automatically aborts radiation delivery, removes patient from the GK bore, and a repeat localization CBCT is required before treatment can resume. After completion of radiation therapy, patients followed closely with their primary neuro-oncologist, radiation oncologist, and neurosurgeon.
For this study, datapoints including patient demographics, surgical margins, molecular subtyping, radiation treatment volumes, systemic therapies, and follow-up imaging findings were extracted from the electronic medical record. Median follow-up time, median PFS, median OS, and other descriptive statistics for this patient cohort were calculated using GraphPad Prism 7.0.