Multimodality Dosimetric Evaluation for the Treatment of Recurrent Brain Tumors

Abstract

GammaTile Surgically Targeted Radiation Therapy (STaRT) are bioresorbable collagen tiles containing four Cesium-131 radioactive seeds which are permanently implanted into the operative bed immediately following the surgical resection of a brain tumor. Alternative radiation treatment approaches for recurrent brain tumors include external beam radiation therapy (EBRT) with either proton or photon (volumetric modulated arc therapy, or VMAT) radiation. The purpose of this study is to perform a comparative dosimetric study of these three modalities for cases of recurrent brain tumors. We hypothesize that GammaTile would be dosimetrically superior at sparing radiation dose to surrounding organs at risk. A total of 5 patients have received GammaTile StaRT therapy at our institute for recurrent brain tumors in 2021-2022. Out of these 5 patients, 3 were recurrent glioblastoma (GBM) and 2 were brain metastasis (brain-met) cases. All patients were previously treated with radiation. Typical prescription for GammaTile therapy is 60 Gy to the High Risk CTV (HR_CTV), specified by 5mm depth from the plane of the seeds. For recurrent GBM cases, the alternative methods of radiation are proton therapy and photon therapy using VMAT. The standard re-irradiation dose for proton and photon therapy is 35 Gy in 10 fractions prescribed to PTV (3 mm expansion of EBRT CTV). For brain-met cases, an alternate method is hypo-fractionation radiation therapy with Linac-based SBRT. Linac-based SBRT prescription depends on the volume of EBRT CTV. For one of the brain-met cases, the SBRT prescription was 30 Gy in 5 fractions, and for the other case it was 27 Gy in 3 fractions to PTV (2 mm expansion from EBRT CTV). Proton and photon plans were generated in a treatment planning system using a double scattering Mevion system and a VersaHD with agility head, respectively. All the physical doses were converted to biological effective dose (BED) for evaluation. The dosimetric quantities are summarized in table 1. These quantities were evaluated using HR_CTV and EBRT_CTV for GammaTile therapy and EBRT (proton/photon), respectively. GammaTile therapy reduces dose to normal brain tissue considerably. However, it may have more uncertainty in the dose delivered compared to proton and photon EBRT. It appears that all three treatment modalities are adequate for treating recurrent brain tumors. However, GammaTile therapy may allow to deliver higher dose to the targets while reducing the irradiation to adjacent normal tissue.