2064 - Upfront Intracranial Brachytherapy Using Cs-131 Followed by External Beam Radiation Therapy vs. Stupp Protocol: A Pilot Voxel-by-Voxel Biological Effective Dose Comparison Accounting for Tumor Repopulation in Glioblastoma Patients
Presenter(s)
C. Ferreira1, J. Vadas2, C. J. Tien3, M. Zapzalka4, D. Sterling1, S. Mishra2, B. Gomes5, V. Chaswal6, R. P. Tolakanahalli7, M. J. Rivard8, L. Sloan2, and C. Chen9; 1Department of Radiation Oncology, University of Minnesota Medical School, Minneapolis, MN, 2University of Minnesota: Department of Radiation Oncology, Minneapolis, MN, 3Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT, 4University of Minnesota, Minneapolis, MN, United States, 5University of Minnesota, Minneapolis, MN, 6Herbert Wertheim College of Medicine, Florida International University, Miami, FL, 7Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL, 8Brown University, Providence, RI, 9Department of Neurosurgery at the Warren Alpert Medical School of Brown University, Providence, RI
Purpose/Objective(s): To compare the voxel-by-voxel biologically effective dose (BED) delivered by upfront intraoperative intracranial brachytherapy (ICBT) via surgically targeted radiation therapy (STaRT) with Cs-131 embedded in collagen-tiles followed by external beam radiation therapy (BT+EBRT) versus conventional Stupp-protocol EBRT alone in glioblastoma patients.
Materials/Methods: Ten consecutive glioblastoma patients previously treated with BT+EBRT per the GT-103 protocol (upfront post-resection BT implantation followed by EBRT boost ˜1 month later) were retrospectively replanned per the Stupp Protocol. BT+EBRT received BT using Cs-131 (half-life of 9.689 d) and then treated with EBRT to a total of 44.9 Gy to the low-risk PTV (LR-PTV) and an integrated boost to 55.6 Gy to the high-risk PTV (HR-PTV) in 20 fractions. Patients were retrospectively replanned per the Stupp protocol (EBRT-only: 60 Gy in 30 fractions of 2 Gy). All EBRT plans used VMAT (6 MV) in Eclipse TPS. Physical doses were converted to BED using the linear-quadratic model (tumor a/ß = 8 Gy, late normal tissue a/ß = 2 Gy) and Dale's generalized formulation for continuous low-dose-rate brachytherapy, incorporating repair kinetics (T½ = 0.5 h tumor, 1.5 h normal tissue) and repopulation (tumor doubling time Tp = 15.4 days, a = 0.12 Gy?¹) parameters per the GT-103 protocol. BED dose profiles were created from voxel-by-voxel BED calculations and compared.
Results: When repopulation is included in BT+EBRT plans demonstrated higher summed BED in resection cavity and high-risk regions due to the immediate initiation of low-dose-rate dose delivery from BT, partially offsetting repopulation losses compared with delayed EBRT-only schedules. Our model predicts that a paradigm with immediate brachytherapy delivery ultimately such as BT+EBRT produces roughly 10–20 Gy8 advantage in the resection cavity and high-risk regions compared to Stupp protocol. BED profiles comparisons in the target region reveal even sharper local differences near the cavity surface.
Conclusion: Upfront BT followed by hypofractionated EBRT appears to deliver superior biological effective dose to residual tumor volumes in glioblastoma compared with standard Stupp EBRT, primarily by reducing the impact of tumor repopulation through earlier initiation of intensive local therapy. These dosimetric advantages support further clinical investigation of combined BT–EBRT regimens.