ESTRO 2025 - Abstract Book

S3706

Physics - Quality assurance and auditing

ESTRO 2025

All deviations of EMQC output drift were <1% different to Farmer chamber measurements. However, due to expected sources of uncertainty such as long term panel sensitivity drift, short term panel sensitivity changes due to environmental factors and linac output changes an expanded tolerance is required for clinical practice. We recommend the adoption of the default tolerances currently provided with the test of 2% (warning) and 3% (fail).

Keywords: AQUA, Elekta machine QA, automation

References: Renaud, James, and Bryan Muir. “Assessing the accuracy of electronic portal imaging device (EPID)-based dosimetry: I. Quantities influencing long-term stability.” Medical physics vol. 49,2 (2022): 1231-1237. doi:10.1002/mp.15434

4531

Digital Poster CBCT doses from radiotherapy treatments: Dose quantification, optimization of clinical and reference CBCT protocols André Lucas 1,2 , Katia Jacob 1,2 , Cecília Borges 3 , João Fatana 3 , Dora Bernardes 1,2 , Sara Marques 1,2 , Fábio Carmo 1,2 , Maria João Rosa 4,5 , Márcia Coelho 4,5 , Iola Cardoso 4,5 , Mariana Girão 1,2 , Anthony Ribeiro 4,5 , Bruno Oliveira 4,5 , Gonçalo Fernandez 6 1 Physics, Mercurius Health, Lisbon, Portugal. 2 Physics, CUF Descobertas, Lisbon, Portugal. 3 Radiation Protection, CUF Descobertas, Lisbon, Portugal. 4 Dosimetrist, Mercurius Health, Lisbon, Portugal. 5 Dosimetrist, CUF Descobertas, Lisbon, Portugal. 6 Radiation Oncologist, CUF Descobertas, Lisbon, Portugal Purpose/Objective: Precision and accuracy in radiotherapy are crucial due to the complexity of the treatments, the rise in the dose given per fraction and the decrease in the margins of the volumes to be irradiated. CBCT images can be used to verify patient position prior or during treatment. Even with low doses, the increasing use of this type of imaging can raise the risk of induction of secondary malignant tumours. In order to provide reference protocols, for the radiotherapy service, the goal is to measure the doses and optimise the imaging protocols by combining high quality images with the lowest feasible dose. Material/Methods: In order to quantify the dose in each of the three most used clinical imaging protocols (head and neck, chest, and pelvic) measurements were made on two Elekta linear accelerators models – VersaHD (2018) and Infinity (2022) - using the IEC-60601-2-44 (2009) method relying on the CTDI methodology. Qualitative optimization through image quality check of the imaging protocols was performed utilizing a anthropomorphic phantom. Through adjustment of the acquisition settings parameters, three optimisation iterations were applied aiming at reducing the dose whilst maintaining adequate image quality for each clinical protocol. Results: Optimization was possible by quantitatively confirming the images quality for the three clinical protocols. For protocols 1 to 3, the mean dose reduction was 45%, 35% and 68%, respectively. The low contrast displays a mean Mean Pixel Value (MPV) result below 2%, 3% and 12% with a Standard Deviation (SD) below 34%, 23% and 7% for protocols 1 to 3, respectively. The uniformity presents MPV results below 4%, 2% and 15% with and SD value below 17%, 21% and 19% for protocol 1, 2 and 3 respectively. Consideration should be given to the variations in the X-ray tube brands and its lifetime cycle when comparing the outcomes of the identical image protocols. Conclusion: One radiological concept, ALARA, may be complied by optimizing imaging techniques through the use of CBCT dose measurement. The results obtained and the analysis carried out lead to the conclusion that dose reduction had