ESTRO 2025 - Abstract Book

S6

Invited Speaker

ESTRO 2025

4661

Speaker Abstracts In vivo dosimetry: What can it catch? Evy Bossuyt Physics, Iridium Netwerk, Antwerp, Belgium

Abstract:

In vivo dosimetry (IVD) is recommended in radiotherapy to catch treatment delivery errors and to assist in treatment adaptation. Large scale clinical implementation however is not always easy. Recently, commercially automated systems have made it feasible to perform transit dosimetric quality assurance on a very large scale. We started using such a system for all our machines and all our patients in 2018 using transit EPID images. Over the years, we analyzed results for more than 23000 patients in total, including causes and actions for failed fractions. We were able to catch not only deviations in patient positioning and anatomy changes, but also errors in planning, imaging, treatment delivery, simulation, breath hold and positioning devices. The detectability of a specific type of error however depends very much on the specificity and sensitivity of a particular IVD system for that type of error including also dependency on treatment site, delivery technique, and/or indicator used. This talk will NOT give an extensive overview of all possibilities for IVD systems discussing advantages and limitations. It will refer however to some excellent references and will give some recommendations showing case examples of EPID based IVD to illustrate the wide variety of detectable deviations.

4662

Speaker Abstracts The ability of PSQA systems to detect errors Joerg Lehmann

Institute of Medical Physics, University of Sydney, Sydney, Australia. Department of Radiation Oncology, Calvary Mater Newcastle, Newcastle, Australia. School of Information and Physical Sciences, University of Newcastle, Newcastle, Australia Abstract: The ability of clinical patient specific quality assurance (PSQA) systems to detect errors has been prominently questioned over 10 years ago: Comparing the outcomes of the irradiations of their intensity modulated radiation therapy (IMRT) head and neck audit phantoms with corresponding results of the institutions in-house PSQA, the IROC Houston team found that the clinical PSQA systems showed poor ability to predict a failing IROC Houston phantom result with an overall sensitivity of 2-18%. [1] One approach to formally assess the sensitivity of clinical PSQA systems involves purposefully introducing errors into test plans and asking centres to perform PSQA for such plans relative to the dose from the original error-free plan [2]. This can be applied to many phantom based PSQA methods and those involving the Electronic Portal Imaging device (EPID). Matching expected errors in the treatment planning and delivery process, the purposefully introduced errors can be grouped in three categories: treatment planning system (TPS) beam model errors, machine related treatment delivery errors and patient related treatment delivery errors. While some of the latter can be included with pre treatment PSQA (e.g. a setup error resulting in a shift of isocentre), others require an extended definition of PSQA, which includes in vivo assessment. For relevance, purposefully introduced errors should be selected based on a process such as a Failure Mode and Effect Analysis (FMEA) [3]. To be realistic the magnitude of introduced errors should be such that they would be relevant to PSQA and not be already caught with other measures such as routine