S1922
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
by-case ‘dose remaining’ calculations: The reirradiation target was transferred to the original dataset (using rigid registration) and, for each OAR, in EQD2Gy, the maximum dose to the portion nearest the reirradiation target was subtracted from a cumulative constraint. For the background dose EQD2Gy approach, the original dose distribution was deformably mapped to the reirradiation CT and used as background dose for voxel-wise optimisation of cumulative EQD2Gy[2].Photon plans used two-arc VMAT. PBT plans used IMPT with single-field optimisation, +/-3.5% +3mm range uncertainty, with beam numbers and angles optimised per patient. Cumulative dose distributions were generated by deformable registration and mapping of original doses to the reirradiation CT with a kernel-based robustness method, to account for registration uncertainties[3]. Original and reirradiation doses were converted to EQD2Gy ( α / β =3Gy; except nerves α / β =2Gy) before summation.Dose metrics were compared using Wilcoxon signed-rank exact tests. Results: Dose metrics are shown in Tables 1 (reirradiation plans) and 2 (cumulative EQD2Gy). PBT reirradiation plans achieved lower maximum doses compared to photon plans for several OARs; but this was not the case for OARs nearest the reirradiation PTV (i.e. OARs within 0.5cm of the reirradiation PTV). Target coverage metrics were statistically significantly improved with photon background dose planning in EQD2Gy compared to PBT (median increase in PTV: D99: 2.3Gy (p=0.014), D95: 1.9Gy (p=0.037), D50: 1.6Gy (p=0.010)) and standard photon planning (median increase in PTV D50: 2.4Gy (p=0.004)). Robustly summated plans demonstrated reduced maximum doses with PBT compared to photon planning for bladder, rectum and sacral plexus.
Conclusion: For OARs near reirradiation targets, there were no significant differences between photon or PBT planning. Background dose EQD2Gy optimisation can improve coverage by facilitating voxel-by-voxel reirradiation dose placement compared to applying a maximum dose constraint across the entirety of an OAR. References: [1] Murray L, Thompson C, Pagett C, Lilley J, Al-Qaisieh B, Svensson S, Eriksson K, Nix M, Aldred M, Aspin L, Gregory S, Appelt A. Treatment plan optimisation for reirradiation. Radiother Oncol. 2023 May;182:109545. doi: 10.1016/j.radonc.2023.109545[2] Ödén J, Eriksson K, Svensson S, Lilley J, Thompson C, Pagett C, Appelt A, Murray L, Bokrantz B. Technical note: Optimization functions for re-irradiation treatment planning. Med Phys. 2024;51:476–484. https://doi.org/10.1002/mp.16815[3] Thompson C, Pagett C, Lilley J, Svensson S, Eriksson K, Bokrantz R, Ödén J, Nix M, Murray L, Appelt A. Brain Re-Irradiation Robustly Accounting for Previously Delivered Dose. Cancers (Basel). 2023 Jul 28;15(15):3831. doi: 10.3390/cancers15153831 Keywords: Reirradiation, EQD2Gy optimisation The lung-sparing effect of DIBH is overestimated in right-sided breast IMRT with nodal involvement: a deformable image registration study Mikko Mankinen 1 , Jaana Tiainen 2,3 , Tuomas Virén 4 , Jan Seppälä 4 , Kristiina Vuolukka 5 , Janne Heikkilä 4 , Tuomas Koivumäki 6 1 Department of Radiotherapy, Wellbeing Services County of Päijät-Häme, Lahti, Finland. 2 Comprehensive Cancer Centre, Helsinki University Hospital, Helsinki, Finland. 3 Department of Physics, University of Jyväskylä, Jyväskylä, Finland. 4 Center of Oncology, Kuopio University Hospital, Kuopio, Finland. 5 Department of Radiotherapy and Oncology, Wellbeing Services County of Central Finland, Jyväskylä, Finland. Poster Discussion 3206
Made with FlippingBook - Share PDF online