S1966
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
1 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic. 2 Department of Oncology, Krajská Zdravotní, a.s. – Chomutov, o.z., Chomutov, Czech Republic. 3 Department of Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, Stockholm, Sweden. 4 Department of Oncology-Pathology, Karolinska Institutet, Stockholm, Sweden Purpose/Objective: Clinical implementation of synthetic computed tomography (sCT) requires dosimetric performance testing of the sCT against planning CT. As clinics adopt new dose calculation algorithms, the dosimetric tests performed during validation need to be re-evaluated to ensure a continuous high standard of MR-only treatments.This work presents a dosimetric comparison between Acuros XB (AXB) and Anisotropic Analytical Algorithm (AAA) on a commercial brain sCT solution previously implemented for treatment of glioblastoma [1]. The impact that these algorithms have on known sCT artefacts [2] around the operated skull was also inspected. Material/Methods: Ten cases from the MR-only validation cohort were chosen for re-optimization with AXB. These sCT plans were recalculated into the backup CT as in [1]. Two strategies were followed for dosimetric evaluation: 1) the re-optimized AXB were recalculated to AAA, and 2) the original AAA optimized plans were recalculated on AXB. Dose–volume histogram points for the target and organs-at-risk (OAR) were obtained and their difference calculated as Δ D[%] = (Doptimized - Drecalculated)/Doptimized. Cavity artefacts caused by metal screws in the vicinity of the PTV were identified and dose coverage was visually inspected. Results: The PTV dose differences between sCT and CT for AXB re-optimized plans are equivalent to those observed previously (i.e. AAA optimized). The OARs also presented similar dose differences except for the left cochlea (-2.2% for AAA and 0.71% for AXB), probably due to changes in optimization strategies (Table 1).Plans recalculated with AXB are generally colder with loss of target coverage towards the skull (see Figure 1 c and d). Similar dose differences for PTV and OAR, albeit with opposite sign, are found for both comparison strategies (i.e. recalculated AAA from AXB plans vs recalculated AXB from AAA plans) regardless of planning image. Figure 1 shows the impact of AXB on cavity artefacts found on the sCT. For cavities with diameters of less than 1 cm, hotspots may appear (Figure 1e) thus when recalculated on CT there is minimal loss of dose coverage (Figure 1f). For larger cavities, an obvious coverage loss is observable both in sCT and CT (Figure 1 g and h).
Conclusion: AXB can be safely introduced in MR-only treatment for glioblastoma, as the dosimetric performance of the sCT remains equal to that observed when first validated with AAA dose calculation. However, artefacts in form of missing pieces of the skull will now have to be identified and density corrected prior to optimization to avoid hotspots. References: [1] Emin S, Rossi E, Rooth EM, Dorniok T, Hedman M, Gagliardi G, Villegas F. Clinical implementation of a commercial synthetic computed tomography solution for radiotherapy treatment of glioblastoma. J Physics and Imaging in Radiat Oncol 2024;30:100589.https://doi.org/10.1016/j.phro.2024.10 0589.[2] Rossi E, Emin S, Gubanski M, Gagliardi G, Hedman M, Villegas F. Contouring practices and artefact management within a synthetic CT - based radiotherapy workflow for the central nervous system. J Radiat Oncol 2024;19:27. https://doi.org/10.1186/s13014-024- 02422-9. Keywords: dose calculation algorithm, synthetic CT
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