S1818
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
deliver a BED10 of 50 Gy to each metastasis, corresponding to 18 Gy in a single fraction or 27 Gy in three fractions. Planning objectives include minimizing the mean brain BED2 and minimizing V60, the brain volume receiving a BED2 above 60 Gy (corresponding to 10 Gy in a single fraction). The method is investigated for two patients with 29 and 27 metastases, respectively, with total GTV volumes of 14.7 and 22.6 cc. Results: Figure 1 illustrates a 3-fraction STF treatment emphasizing minimization of the mean brain BED2. The treatment delivers high single-fraction doses to small metastases in alternate fractions, whereas larger lesions receive more uniform doses. Thereby, the total physical dose required to achieve the prescribed BED10 to the metastases is reduced, while some degree of fractionation is exploited in between the lesions. Compared to uniformly fractionated 3-fraction treatments, STF treatments resulted in 11% and 10% lower mean brain BED2 for the two patients, at the cost of slightly larger V60 (+0.7 and +0.3 cc). Different trade-offs can be achieved by emphasizing the objective minimizing V60. For instance, STF treatments can be achieved which reduce V60 by 0.7 and 0.3 cc compared to uniformly fractionated SRS treatments, while also improving on the mean brain BED (-5.7% and -7.6%). In this scenario, the STF plans deliver more similar doses to most metastases, enhancing the fractionation effect (Figure 2).
overall gamma passing rate was 95% (3%/3 mm). In solid phantom verification using EBT3 film at 1.5 cm depth, the optimized bolus achieved a 98.13% gamma passing rate (3%/3 mm), confirming excellent dosimetric accuracy of the developed dose engine and optimization algorithm. In anthropomorphic phantom tests, the proposed ECT bolus maintained comparable target coverage while substantially reducing normal tissue exposure. The normal tissue volume receiving 50% of the prescribed dose (V50%) decreased by over threefold (8.2 cc vs. 32–63 cc in conventional bolus plans), demonstrating improved conformality and healthy tissue sparing. Conclusion: This study successfully developed and validated a bolus optimization system for ECT integrating an electron dose calculation engine, optimization algorithm, and 3D printing–based fabrication. The system demonstrated accurate dosimetric performance and significant reductions in normal tissue dose, highlighting its strong potential for clinical implementation while eliminating the need for
hazardous cerrobend block fabrication. Keywords: Bolus Optimization System
Digital Poster 1136 Treatment of multiple brain metastases with spatiotemporal fractionation Ruben Bosschaert, Nathan Torelli, Lena Kretzschmar, Nicolaus Andratschke, Jan Unkelbach Radiation Oncology, University Hospital Zürich and University of Zürich, Zürich, Switzerland Purpose/Objective: Stereotactic radiosurgery (SRS) has become an established treatment for patients with multiple brain metastases, and is often applied repeatedly when new lesions occur. However, repeated SRS increases the risk of radionecrosis, warranting new approaches that lower the dose to the healthy brain. Spatiotemporal fractionation (STF) is a novel SRS technique that delivers distinct dose distributions in different fractions, with the goal of achieving extreme hypofractionation within the metastases along with more uniform fractionation in the healthy brain. Previous work demonstrated its potential to improve upon uniformly fractionated SRS [1]. This study further investigates STF and characterizes the trade-off between minimizing mean brain dose versus minimizing the brain volume receiving high doses. Material/Methods: STF treatments are generated using an in-house software that simultaneously optimizes multiple dose distributions based on their cumulative biological effective dose (BED). All plans are constrained to
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