2241 - Dosimetric Comparison between Surgically Targeted Radiation Therapy (STaRT) and External Beam Stereotactic Radiation Therapy for Glioblastoma Resection Cavities
Presenter(s)
C. L. Teng1, M. Trager1, J. S. Jung1, J. Starner2, C. Pham3, E. Calugaru1, J. Liu1, A. Sharma4, Y. Cao1, J. Baker2, J. Chang1, and G. Wernicke5; 1Northwell, New Hyde Park, NY, 2Northwell, Lake Success, NY, 3Department of Physics and Astronomy, Hofstra University, Hempstead, NY, 4Zucker School of Medicine at Hofstra/Northwell, Hempstead, NY, 5Department of Radiation Medicine, Northwell Health, New Hyde Park, NY
Purpose/Objective(s): Our hospital offers multiple treatment options for recurrent GBM, including surgically targeted Cs-131 brachytherapy (BT) and external beam stereotactic radiotherapy (SRT) using Varian TrueBeam (TB), Halcyon (HAL), or Gamma Knife (GK). This study aimed to analyze the dosimetric characteristics of each modality.
Materials/Methods: Eight patients with recurrent GBM treated with Cs-131 STaRT were retrospectively replanned for SRT. The BT prescriptions were 60Gy (n=6; Gamma Tile) and 80Gy (n=2; stranded seeds) at 5mm distance in tissue from the cavity surface. Postoperative resection cavities (RC) were contoured on post-implant CT scans and replanned for fractionated SRS (6Gy x 5) using TB, HAL and GK. The planning target volume (PTV) included the RC plus a 1mm margin. Plan quality followed institutional planning directives, and doses were normalized to PTV V100%=99.0% for TB and HAL, and PTV V100%=99.5% for GK. Characterization of dosimetric parameters included conformity index (CI), gradient index (GI), heterogeneity index (HI), and the minimum/maximum/median biological effective dose (BED). BED8Gy of a/ß = 8Gy for GBM and BED2Gy of a/ß = 2Gy for normal brain were computed at distances of 0-1mm, 5-6mm and 10-11mm away from the cavity surface. The dose volume of BED2Gy 81.6Gy, equivalent of V24Gy in 5fx, were computed for brain-RC. Statistical significance is determined using Wilcoxon signed rank test with p<0.05.
Results: The median RC volume is 13.1cc (8.1-40.6cc). For RC, median BED8Gy for BT (170.6 Gy8, range 56.6-219.6 Gy8) is significantly higher than TB (58.9 Gy8, range 57.7-60.2 Gy8, p = 0.008), HAL (63.0 Gy8, range 60.5-66.2 Gy8, p = 0.008), and GK (90.0 Gy8, range 81.1-137.1 Gy8, p = 0.008). At 0-1mm distance from RC, median BED8Gy to residual tumor with BT (78.6 Gy8, range 63.1-126.5 Gy8) is also significantly higher than TB (57.4 Gy8, range 56.7-59.6 Gy8, p = 0.0015) and HAL (58.0 Gy8, range 57.1-65.2 Gy8, p = 0.015), and higher than GK (66.6 Gy8, range 60.3-80.0 Gy8, p = 0.11). At the same 0-1mm distance, median BED2Gy to the normal brain tissue with BT (98.9 Gy2, range 75.3-177.2 Gy2) is significantly lower than GK (160.4 Gy2, range 142.1-199.9 Gy2, p = 0.02), and lower than TB (133,8 Gy2, range 131.8-140.2 Gy2, p = 0.2) and HAL (135.6 Gy2, range 132.9-156.4 Gy2, p = 0.2). BED8Gy remained highest with BT up to 10-11mm distance from the RC, but BED2Gy became lower with TB at 10-11mm. Furthermore, dose distribution is most heterogeneous with BT (SDBT = 100.3 Gy8, range 46.3-162 Gy8, p = 0.008), most conformal with TB (CITB = 1.03, range 1.00-1.12, p = 0.008), and has the fastest dose fall-off with TB (GITB = 2.24, range 2.09-2.35, p = 0.008).
Conclusion: This study investigated dosimetric characteristics between BT and various EBRT modalities for treating recurrent GBM. Preliminary analyses of BED8Gy and BED2Gy distribution suggest that Cs131 brachytherapy could deliver higher biological doses to GBM cells and lower biological doses to normal brain cells than EBRT in tissues immediately outside the RC.