S1945
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
used. Across the patient cohort the maximum change to any dosimetric criteria when applying a bulk density override was 2.15%, 0.37% and -1.52% for soft tissue, bone and lung respectively. The acceptable ranges of
and shorter delivery time compared to IMRT. Both modalities are feasible and safe for clinical APBI application, with dosimetric advantages favouring CK. Keywords: CyberKnife, Stereotactic, Radiotherapy
RED values and the maximum change to any dosimetric criteria are presented in Table 1.
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Don’t let perfect be the enemy of good: Feasibility study on the use of a diagnostic CT scan for dose calculation in the palliative setting Anna J Mason 1 , Jonathan Dunning 2 , Valery Peng 3 , Arek Mazurek 4 1 Lake Macquarie Private Hospital, GenesisCare, Newcastle, Australia. 2 St Vincent's Hospital, GenesisCare, Melbourne, Australia. 3 Lake Macquarie Private Hospital, Genesis Care, Newcastle, Australia. 4 Mandurah, GenesisCare, Perth, Australia Purpose/Objective: Standard radiotherapy planning pathways require a dedicated CT scan with the Hounsfield Unit (HU) to electron density characterised in the treatment planning system (TPS). This study aims to assess the suitability and establish a workflow for using a diagnostic CT scan for dose calculation in the palliative setting, to avoid the need for a dedicated simulation appointment and reduce the time from referral to treatment. An overall additional uncertainty of up to 5% was considered acceptable given the clinical aims of the treatment. Material/Methods: Palliative patients (19 pelvis and 17 thorax) using local simulation and planning technique were selected for this study. Tissues were defined as bone, lung or soft tissue. For each type, a bulk density override at the upper and lower ends of a proposed acceptable relative electron density (RED) range were applied to the planning CT scan and the dose was recalculated. The change in D95%, D2% and mean dose to PTV was assessed. To meet the required accuracy, the native mean HU of the diagnostic scan is required to be within a range of expected values. Any scan with mean HU outside the acceptable range will have a bulk density assigned, established from the population mean for each tissue type. All dose calculations were performed in Monaco v6.2.3.The dosimetric impact of applying a bulk density override was also assessed using the same criteria as for the density range. Results: The population mean RED for bone, lung and soft tissue were 1.262, 0.261 and 0.961 respectively, in good agreement with published values (1). The population mean for lung was very close to the ICRU46 value of 0.258, and therefore the ICRU value was selected. Bone and soft tissue did not map to any one tissue type in ICRU46, so the population averages were
Conclusion: Dose calculation on a diagnostic CT scan was shown to be accurate to within 4% additional uncertainty, either using native HU or by applying a bulk density override. This was considered acceptable, and the simless workflow has subsequently been introduced in our centres. References: 1.ICRU report 46: Photon, Electron, Proton and Neutron Interaction Data for Body Tissues Keywords: simless, pallaitive Comparative Dosimetric Analysis of Total Body Irradiation Using VMAT and Tomotherapy – a Multicentre Experience Jasmin Munchar Elias 1 , Syaza Zafirah Ab Rahman 2 , Xin Yee Chiew 2 , Swee Shing Leong 3 , Munira Mohd Rejab 4 , Ibtisam Muhamad Nor 3 , Hany Ariffin 2 , Nisha Shariff 1 1 Department of Clinical Oncology, Universiti Malaya Medical Centre, Kuala Lumpur, Malaysia. 2 Department of Paediatrics, Faculty of Medicine, Kuala Lumpur, Malaysia. 3 Department of Radiotherapy and Oncology, Hospital Kuala Lumpur, Kuala Lumpur, Malaysia. 4 Department of Medical Physics, Universiti Malaya Medical Centre, Kuala Lumpur, Malaysia Purpose/Objective: Total body irradiation (TBI) remains integral to conditioning for hematopoietic stem cell transplantation. Advanced techniques such as Digital Poster 3733 volumetric-modulated arc therapy (VMAT) and helical tomotherapy (HT) offer improved dose conformity and organ sparing. This study compared patient and dosimetric outcomes of VMAT and HT-based TBI in two tertiary public hospitals in Malaysia. Material/Methods: A retrospective comparison was performed between patients who received TBI with VMAT at Universiti Malaya Medical Centre (UMMC), a university hospital, and those treated with HT at Hospital Kuala Lumpur (HKL), a large public hospital. All patients who were treated with myeloablative TBI 12 Gy in 6 fractions were included. Definitions for planning target volume (PTV) differed between centres. For VMAT, PTV = (Body-3mm) - (Lungs-3mm) - (Kidneys-3mm) – lenses. For HT, PTV = body-lung-heart-kidneys. PTV coverage
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