Multimodality Dosimetric Evaluation for the Treatment of Recurrent Brain Tumors

PURPOSE/OBJECTIVE(S)

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.

MATERIALS/METHODS

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.

RESULTS

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.

CONCLUSION

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.

Clinical characterization of a proton beam continuous uniform scanning system with dose layer stacking

A proton beam delivery system on a gantry with continuous uniform scanning and dose layer stacking at the Midwest Proton Radiotherapy Institute has been commissioned and accepted for clinical use. This paper was motivated by a lack of guidance on the testing and characterization for clinical uniform scanning systems. As such, it describes how these tasks were performed with a uniform scanning beam delivery system. This paper reports the methods used and important dosimetric characteristics of radiation fields produced by the system. The commissioning data include the transverse and longitudinal dose distributions, penumbra, and absolute dose values. Using a 208 MeV cyclotron's proton beam, the system provides field sizes up to 20 and 30 cm in diameter for proton ranges in water up to 27 and 20 cm, respectively. The dose layer stacking method allows for the flexible construction of spread-out Bragg peaks with uniform modulation of up to 15 cm in water, at typical dose rates of 1-3 Gy/min. For measuring relative dose distributions, multielement ion chamber arrays, small-volume ion chambers, and radiographic films were employed. Measurements during the clinical commissioning of the system have shown that the lateral and longitudinal dose uniformity of 2.5% or better can be achieved for all clinically important field sizes and ranges. The measured transverse penumbra widths offer a slight improvement in comparison to those achieved with a double scattering beam spreading technique at the facility. Absolute dose measurements were done using calibrated ion chambers, thermoluminescent and alanine detectors. Dose intercomparisons conducted using various types of detectors traceable to a national standards laboratory indicate that the measured dosimetry data agree with each other within 5%.