ESTRO 2025 - Abstract Book

S3693

Physics - Quality assurance and auditing

ESTRO 2025

Conclusion: The virtual audit provides a representative dataset for the variety of TPSs/beams and allows the feasibility for centres to benchmark their Dm implementation. The study highlighted variability in the calculation of Dm between different TPS vendors and potentially between the way centres have implemented those algorithms since differences were observed even within the same TPS type. Put into context, the variability seen is of the order of the daily variation of output on a linac which highlights the importance of developing a framework for the validation of Dm algorithms.

Keywords: Dose-to-medium, virtual audit

4255

Digital Poster CBCT dose optimisation in prostate IGRT

Alejandro Domínguez Perea 1 , Núria Jornet Sala 1 , Agustí Ruiz Martínez 1 , Marta Barceló Pagès 1 , Javier Roda García 1 , Cristina Ansón Marcos 1 , Víctor Riu Molinero 1 , Jaime Perez-Alija Fernandez 1 , Pedro Gallego Franco 1 , Natalia Tejedor Aguilar 1 , Helena Vivancos Bargalló 1 , Eva Maria Ambroa Rey 1 , Adrian Ramos Martín 1 , Cristina Fernández Arnal 2 , Marina Vargas-Machuca Gomez 2 , Ingrid Arrieta Parreño 2 , Susana González Moreno 2 , Juan Manuel Reina González 2 , Francisco Javier Chalé Rull 2 , David Martínez Martin 2 , Ariadna Machado Deulofeu 2 , Sonia Bermejo Martínez 2 , Arantxa Mera Errasti 2 , Gemma Sancho Pardo 2 , Pablo Carrasco de Fez 1 1 Radiofísica i Radioprotecció, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain. 2 Oncología radioterápica, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain Purpose/Objective: Cone-beam computed tomography (CBCT) is widely used in modern radiotherapy for image guidance, dose recalculation, and adaptive planning. However, kV CBCT has a different dose distribution compared to megavoltage radiation and often exposes areas beyond the planned target volume (PTV), increasing dose to surrounding tissues. Multiple CBCT scans during treatment can further elevate cumulative patient dose. Optimizing CBCT dose while ensuring adequate image quality is essential [1]. We present a study on dose optimization, evaluating image quality, patient dose and dose recalculation accuracy. Material/Methods: We developed three pelvic CBCT protocols based on patient size, estimated through average diameter [2]. Optimisation occurred in two phases, each involving a 20% reduction in milliampere-seconds (mAs) to minimise patient dose while maintaining clinically acceptable image quality [3]. This stepwise reduction resulted in cumulative dose reductions of 20% and 36% in the first and second phases, respectively. Twelve prostate cancer patients, all within the Pelvis_Medium size category (average diameter = 26–36 cm), were studied using a Varian TrueBeam system. Image quality and usability for registration were evaluated in vivo by radiotherapy technicians using a RedCap form to rate daily CBCT images on a Likert-type scale, using pre-optimisation images as references. Images from pre-optimisation and both optimisation phases were acquired for each patient; however, only optimised images were evaluated. We recorded the number of CBCT repetitions for all patients to assess the impact of optimization on this metric. Additionally, we monitored the effect on in vivo dose recalculation performed on CBCTs using Sun Nuclear's PerFRACTION software. Finally, we used Varian's Velocity software to assess the impact of optimization on synthetic CT generation. sCT images were generated for pre-optimised CBCT and both optimised CBCTs phases for one patient. Results: The mAs reduction did not compromise image quality or registration usability; all images received consistent scores, indicating no perceptible changes. Increased noise levels from reduced mAs values affected CBCT Hounsfield Unit