S2134
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
applied to the original planning CT of clinical plans (20 breast, 20 lung and 19 prostate plans) of the associated protocol treatment site. The change in PTV volume coverage at 95% of the prescription dose (V95%) was evaluated per CBCT imaging protocol for
indicate relevant tumor deformation, making isotropic GTV-to-ITV expansion disproportionate in this setting. Adaptive treatment techniques should therefore be considered in external-beam boost concepts to ensure adequate target localization. References: 1. Hofheinz, R.D., et al., Localised rectal cancer: ESMO Clinical Practice Guideline for diagnosis, treatment and follow-up. Annals of Oncology, 2025. 36(9): p. 1007-1024.2. Hearn, N., et al., Neoadjuvant Radiotherapy Dose Escalation in Locally Advanced Rectal Cancer: a Systematic Review and Meta-analysis of Modern Treatment Approaches and Outcomes. Clinical Oncology, 2021. 33(1): p. e1-e14.3. Slevin, F., et al., Overview of patient preparation strategies to manage internal organ motion during radiotherapy in the pelvis. Journal of Radiotherapy in Practice, 2020. 19(2): p. 182-189. Keywords: GTV, volume reduction, motion analysis The effect of CBCT imaging protocol parameters on the accuracy of a commercial CBCT conversion algorithm Max O. Koch, Steven J. M. Habraken, Frank J. W. M. Dankers, Marjan Dwarswaard, Chrysi Papalazarou Radiotherapy, Leiden University Medical Center, Leiden, Netherlands Purpose/Objective: Dose recalculation on CBCT is a valuable tool for daily plan evaluation and offline or online replanning. Its accuracy is, however, limited by reduced image quality, artefacts and lack of proper calibration. The CBCT conversion algorithm for direct dose calculation offered in RayStation 2023B (RaySearch Laboratories, Stockholm, Sweden) has been previously validated [1- 3]. However, the potential effect of CBCT imaging Digital Poster 2135 protocol settings on the corrected CBCT (cCBCT) conversion remains unexplored. This study aims at investigating this effect, focusing on (i) consistency of the conversion of CT HU to electron densities (CT-to- ED), and (ii) clinical evaluation based on fulfillment of planning goals. Material/Methods: CBCT imaging of the Gammex 467 density phantom (Gammex Inc., Middleton, WI, USA) on an Elekta XVI CBCT imaging system (Elekta, Stockholm, Sweden) was performed for five different protocols: Breast, Thorax, Thorax-high mAs, Pelvis and Pelvis-fast. The average HUs for the phantom inserts were determined and compared to CT-based values. To examine the effect of HU errors on the resulting dose distributions, new protocol-specific CT-to-ED calibration curves, based on measured CBCT values per insert, were created and
all plans. Results:
CT-to-ED curves remain within 15 HU of the reference CT for adipose and water-like materials (densities: 0.6- 1.2 g/cm3). Larger differences were observed for lung (0.29 g/cm3) and bone-like materials with densities>1.82 g/cm3 (Figure 1). In general, PTV-V95% was minimally affected by the choice of protocol (Figure 2). No significant differences in mean coverage values were observed for the mamma and pelvis treatment sites. For the Thorax site, when employing the high-mAs protocol, coverage is on average 0.5% higher, possibly due to the presence of bone and lung tissue for which higher HU discrepancies were observed. Thorax plans with targets in the mediastinum near the vertebrae exhibited the largest deviations, up to 0.9%. Conclusion: HU errors in the CBCT conversion were found to increase for lung- and bone-like materials. No clinically relevant effect was found on PTV coverage; however, changes in planning goal fulfillment may occur if the target area contains a high proportion of such materials. We recommend further investigation using clinical reference data, e.g. based on replanning CTs. References: [1] W. Lechner, S. Haupt, D. Kanalas, L. Zimmerman, and D. Georg, “Evaluation of a novel CBCT conversion method,” ESTRO 2022 - Abstract Book, 2022. [Online].Available: https://user- swndwmf.cld.bz/ESTRO-2022-Abstract-Book/808/[2] C. O’Hara, D. Bird, R. Speight, S. Andersson, R. Nilsson, and B. Al-Qaisieh, “Assessment of CBCT based synthetic ct generation accuracy for adaptive radiotherapy planning,” ESTRO 2022 - Abstract Book, 2022. [Online]. Available: https://user- swndwmf.cld.bz/ESTRO-2022-Abstract-Book/356/[3] R. Thing, R. Nilsson, S. Andersson, M. Berg, and M. Lund, “Evaluation of two generic algorithms for CBCT based dose calculation,” ESTRO 2022 Abstract-Book, 2022. [Online]. https://user-swndwmf.cld.bz/ESTRO-2022- Abstract-Book/1267 Keywords: CBCT, CT calibration, offline adaptation Mini-Oral 2218 Robustness of online adaptive MRI-guided pancreatic cancer SBRT: Baseline vs adaptive plan quality in 152 patients Uffe Bernchou 1,2 , Rana Bahij 3 , Pia Braagaard Hartfelt 3,2 , Mathilde Weisz Ejlsmark 3,2 , Anders Smedegaard
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