PHSOP5  Presentation Time: 9:20 AM: Feasibility of Optimizing External Beam RT While Accounting for LDR Brachytherapy Based on Biologically Effective Dose for Glioblastoma

• Published in: Brachytherapy Vol. 24; no. 4; pp. S37 - S38
• Main Authors: Turner, Adam Christopher, Garcia, Michael, Patel, Sita, Johnson, Kimberly, Brachman, David
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.07.2025
• ISSN: 1538-4721
• DOI: 10.1016/j.brachy.2025.06.064

The standard of care (SOC) for newly diagnosed glioblastoma (GBM) consists of maximal safe resection (MSR) followed by external beam radiation therapy (EBRT) and chemotherapy and subsequent courses of adjuvant chemotherapy. The prognosis for GBM is typically poor, with a 5-year survival rate of <10% and median survival ranging from 12 to 16 months with SOC treatment. Rapid early progression (REP) is the regrowth or progression of tumor between MSR and start of EBRT, which occurs in ∼45% of patients, and is associated with worse survival. A multicenter trial is underway (GESTALT, NCT05342883) to investigate the safety and feasibility of immediately initiating radiation delivery at time of resection using permanently implanted low dose rate brachytherapy (LDR-BT) administered with 3.5 U cesium-131 seeds embedded in collagen (GammaTile, GT Medical Technologies, Tempe, AZ, USA) followed by a 20-fraction course of EBRT. The goal of EBRT is to provide the remaining dose, on a voxel-by-voxel basis, so that the composite biologically effective dose (BED) from LDR-BT and EBRT closely approximate the BED from SOC radiation. The trial employs an experimental method to convert LDR-BT dose to a “base dose” which was derived so that the sum of base dose and dose from an ideal EBRT plan, optimized using standard treatment planning software (TPS), results in a composite BED that matches the BED from the SOC. This aim of this work is to demonstrate the feasibility of generating acceptable EBRT plans using the experimental method. For 22 consecutive patients enrolled on the GESTALT trial, MIM Software (MIM Software Inc., Cleveland, OH, USA) was used to fuse EBRT planning CT and MRI images, contour a low-risk target (PTV-LR), a high-risk target (PTV-HR), and organs at risk (OARs), calculate dose from LDR-BT, and convert LDR-BT dose to base dose using a custom MIM Workflow. The enrolling site’s TPS was used to optimize an EBRT plan using TPS-specific base or background dose planning tools so that the composite of the base dose and EBRT dose satisfactorily fulfilled composite dose objectives specified as part of the experimental method. The summations of base dose and EBRT dose for each subject were analyzed to investigate feasibility. Specifically, the number of subjects was determined for which the composite doses covering 95% (D95%) of the PTV-LR and PTV-HR were within 5% of the prescribed composite doses of 44.90 Gy and 55.65 Gy, respectively. Similarly, the number of subjects was obtained for which the composite dose met the composite dose objective for all specified OARs. Composite D95% for both PTV-LR and PTV-HR (Figure 1A) were within 5% of the prescribed composite doses for 19/22 (86%) subjects. Composite D95% was >5% below prescription for both target volumes for subject 16 and for the PTV-LR for subject 8. In both cases, the target volumes overlapped OARs and target coverage was intentionally sacrificed (i.e., OAR goals were prioritized per protocol). For subject 18, the composite D95% was >5% higher than the prescription for the PTV-HR but could have been decreased through monitor unit normalization to meet all dose objectives. For 19/22 (86%) subjects, the composite D95% met the stated composite dose objective for all OARs (Figure 1B). This is the first reported analysis demonstrating the feasibility of generating EBRT plans that account for dose from LDR-BT that results in desired composite BED distributions, as per the GESTALT trial protocol for GBM. The method resulted in high rates of compliance for target volume coverage and OAR protection. Feasibility and safety of this method will be further investigated using additional subjects enrolled in the GESTALT trial.