S108
Brachytherapy - Head & neck, skin, eye
ESTRO 2026
1 Department of Radiation Oncology, University of Minnesota, Minneapolis, USA. 2 Department of Neurosurgery, Brown University, Providence, USA Purpose/Objective: A framework for integrating intracranial low-dose-rate (LDR) brachytherapy with external beam radiation therapy (EBRT) for glioblastoma (GBM) has been recently described1. Initiating radiation therapy at resection via Cs-131 collagen-tile brachytherapy has demonstrated potential to mitigate rapid early progression2. This study evaluates practical implementation of a biological effective dose (BED)- based approach to plan EBRT boosts following Cs-131 implants. Material/Methods: Ten patients with newly diagnosed GBM received maximal safe resection and intraoperative GammaTile Cs-131 collagen-tile implantation. 24 ± 5 days after surgery and implant, EBRT was delivered to T2/FLAIR regions, excluding volumes sufficiently covered by brachytherapy. Brachytherapy BED was computed voxel-by-voxel in MIM using α / β = 8 Gy for targets and α / β = 2 Gy for organs at risk (OARs). This BED map served as a base dose for EBRT optimization. Each EBRT boost was planned to a total (LDR + EBRT) BED of 55.6 Gy to high-risk PTV (HR-PTV) and 44.9 Gy to low- risk PTV (LR-PTV) in 20 fractions, converted from standard 30 fraction EBRT-only target doses of 60 Gy and 46 Gy, respectively. OAR constraints followed RTOG limits converted to equivalent 20-fraction BED. Results: Clinically acceptable EBRT boost plans were achieved in all 10 patients. Composite BED targets were met in 7/10 cases for both HR-PTV (D95% ≥ 55.6 Gy; range 54.14–56.14 Gy, median 55.04 Gy) and LR-PTV (D95% ≥ 44.9 Gy; range 44.0–48.1 Gy, median 45.84 Gy), with failures in 3 patients due to OAR proximity. Mean PTV_LR volume was 99.7 cc (range 59.5–193.5 cc); mean PTV_HR volume was 72.3 cc (range 40.1–131.5 cc). All OAR constraints were fulfilled (100% compliance), including brainstem PRV D0.03cc ≤ 47.95 Gy (max 40.67 Gy, median 31.6 Gy), optic chiasm PRV D0.03cc ≤ 47.95 Gy (max 33.13 Gy, median 27.4 Gy), spinal cord D0.03cc ≤ 43.77 Gy (max 3.69 Gy, median 0.83 Gy), retina (L/R) D0.03cc ≤ 39.58 Gy (max 13.48 Gy), and lens (L/R) D0.03cc ≤ 6.7 Gy (max 5.70 Gy). Brain non-target D5% ≤ 56.27 Gy was achieved in all cases (max 41.86 Gy, median 37.8 Gy). Dose homogeneity was excellent, with 0 cc receiving ≥ 110% of prescription dose in every PTV. Conclusion: BED-based integration of LDR brachytherapy and EBRT is complex but feasible, enabling composite dose distributions equivalent to EBRT-only regimens while respecting normal-tissue tolerances and initiating therapeutic radiation immediately post-resection.
calculated to extract mean tumor dose, D99% (minimum dose to 99% of tumor volume), and volumetric coverage (V100). Additional analyses were performed for the notched CIB plaque across a range of tumor base sizes (8 × 8–13 × 13 mm ² ) to assess the interplay between base area, shape, and dose coverage. Results: For unnotched plaques (CCB, CCC, CCD) and the notched COB plaque volumetric coverage (V100) remained 100% across all shapes and thicknesses. However, D99% and mean dose varied significantly with tumor shape: conical tumors received the highest values, whereas mushroom configurations exhibited lower D99%, particularly at greater thicknesses. Differences between mushroom- stem and mushroom-ball geometries were negligible. In contrast, the notched CIB plaque demonstrated a strong dependency on tumor morphology. For thin (< 5 mm) mushroom- shaped tumors, V100 decreased below 95%, indicating partial underdosage, while dome and conical tumors maintained adequate coverage. Statistical analysis confirmed significant differences in V100 and D99% between dome and mushroom-stem geometries (p < 0.05). Expanded analysis across tumor base sizes revealed that large, shallow mushroom-shaped tumors ( ≥ 11 × 11 mm ² , ≤ 3 mm thick) were particularly prone to underdosage, potentially compromising local control. Conclusion: Tumor morphology exerts a measurable and clinically relevant influence on dosimetric outcomes in Ru-106 ocular IRT, especially when using asymmetric (notched) plaques such as the CIB model. Assuming an idealized dome-shaped geometry may lead to systematic dose inaccuracies and inadequate tumor coverage in irregular or mushroom-shaped lesions. Incorporating three-dimensional, morphology-aware treatment planning is therefore essential to ensure accurate dose delivery and optimize therapeutic outcomes in ocular brachytherapy. Keywords: Brachytherapy, interventional radiotherapy, ocular
Digital Poster Highlight 4763
EBRT Boost Planning Considerations Following Tumor Resection & Permanent Brachytherapy: Voxel-by-Voxel Based BED Dose Calculation for GBM Treatment Jessica Vadas, PhD 1 , David Sterling, MS 1 , MaKenna Zapzalka 1 , Lindsey Sloan, MD, PhD 1 , Margaret Reynolds, MD 1 , Kaili Ranta, MD, PhD 1 , Clark Chen, MD, PhD 2 , Clara Ferreira, PhD 1
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