S2402
Physics - Quality assurance and auditing
ESTRO 2026
a multicentre trial ofstereotactic body radiotherapyofc entrallylocatedlungtumours. PhysImagRadOncol,8:57- 62. https://doi.org/10.1016/j.phro.2018.10.003 Keywords: Dynamic Thorax Phantom, Motion Management, 4DCT
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Assessment and optimisation of stereotactic ablative radiotherapy plan complexity using an aperture-based plan complexity metric Chris Stepanek 1 , John Cope 2 , Alison Stapleton 1 , Jack Aylward 1,3 1 Department of Medical Physics and Bioengineering, Bristol Haematology & Oncology Centre, Bristol, United Kingdom. 2 Cancer Services and Clinical Haematology, 2Northern Centre for Cancer Care, Newcastle upon Tyne, United Kingdom. 3 Medical Physics, School of Applied Sciences, University of the West of England, Bristol, United Kingdom Purpose/Objective: The ‘M’ Plan Complexity Metric (M PCM) developed by Younge et al. [1, 2] was adapted into C-python and applied to modern Stereotactic Ablative Radiotherapy (SABR) Volumetric Modulated Arc Therapy (VMAT) treatment plans to create a tool for predicting pre- treatment quality control (PTQC) results and improve plan accuracy. Material/Methods: Raystation (Raysearch, Sweden) was used as the treatment planning system, Compass (IBA Dosimetry, Germany) was used as an independent dose calculator, and the MatrixX 2D array detector (IBA Dosimetry, Germany) was used for dose measurements.M PCM scores for 156 treatment plans for SABR sites (Spine, Liver, Prostate, Pancreas) were correlated against their PTQC results. A Receiver Operator Characteristics (ROC) analysis was performed to establish an appropriate M PCM threshold to predict PTQC results for SABR spine plans. Three clinical SABR spine plans were then retrospectively re-optimised while constraining leaf motion, monitor units, and delivery time to produce variant plans of clinical quality and with a range of M PCM scores. The effect of minimising M PCM score during optimisation and the resultant deliverability was then studied through independent dose recalculation and dose measurement. Results: M PCM scores for SABR Spine, SABR Liver, SABR Prostate and SABR Pancreas plans demonstrated medium – very strong correlation with PTQC results (Figure 1). SABR Spine plans were observed to have the strongest correlation (Spearman’s Rank Correlation Coefficient = 0.79) and have the highest
Figure 1:Target volume deviation values(1a) and target HU deviation values (1b) for the different respiratory phases. DA (Double Amplitude), IF (Irregular Frequency), IB (Irregular Breathing). Yellow and orange dashed lines on panels a and bindicate tolerance intervals for deviations from geometric target volume and target HU mean value measured on the 3DCT, respectively. Conclusion: Targeted tests were performed to establish baseline performance, and a routine QA schedule was proposed to ensure consistent and reliable imaging procedures.Measured values for imaging-based tests were within the recommended tolerances reported in the literature for regular respiratory curves, but not for irregular curves. These findings highlight that the complexity of the respiratory pattern directly impacts the precision of extracted parameters, reinforcing the importance of including irregular motion tests in the QA protocol and setting clinically representative tolerance levels, to ensure robust system performance. References: [1] Burghelea, M., Tahiri, J. B., et al. (2023). Results of a multicenter 4D computed tomography quality assurance audit: Evaluating image accuracy and consistency. Phys Imag Rad Oncol, 28, 100479. https://doi.org/10.1016/j.phro.2023.100479[2] Després, P., & Gaede, S. (2018). COMP report: CPQR technical quality control guidelines for CT simulators. Journal of Applied Clin Med Phys, 19(2), 12- 17. https://doi.org/10.1002/acm2.12213[3] Lambrecht M., Sonke J. J.,et al. (2018).Qualityassuranceoffour- dimensional computedtomography in
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