ArticlePhysics and imaging in radiation oncology2026
In-vivo dosimetry using electronic portal imaging in online-adaptive radiotherapy for rectal cancer.
Article in Physics and imaging in radiation oncology, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Abstract
Background and purpose: Electronic portal imaging device (EPID)-based in-vivo dosimetry (IVD) is widely used to verify radiotherapy dose delivery and detect deviations from anatomical changes, setup uncertainties, or machine-related errors. However, cone-beam computed tomography (CBCT)-guided online-adaptive radiotherapy (oART), which re-optimizes treatment plans on-couch based on daily imaging, lacks a measurement-based quality assurance framework. This study aimed to evaluate the feasibility and accuracy of EPID-based transit IVD for CBCT-based oART. Materials and methods: EPID-based IVD data from ten patients receiving short-course oART for rectal cancer on an Ethos linear accelerator were retrospectively analyzed over 50 treatment sessions using a commercial EPID-based IVD framework. Gamma pass rates (GPR) were calculated using global gamma criteria of 3 %/3 mm, 3 %/2 mm, and 2 %/2 mm. Angular dependence and field shape effects were investigated using in-air and in-phantom measurements. Results: Across all patients and fractions, the mean GPR at 3 %/3 mm with a 20 % low-dose threshold was (95.7 ± 1.8) %, with patient-specific means ranging from 91.4 % to 98.0 %. Two sessions showed notable reductions associated with large intra-fractional gas variations. Angular analysis revealed notable GPR variations, particularly for laterally incident fields. Conclusions: EPID-based IVD was a promising quality assurance approach for verifying oART delivery, achieving higher GPRs under tighter evaluation criteria than typically reported for standard image-guided radiotherapy. These findings indicate that workflow-specific tolerance criteria should be defined to reflect the reduced anatomical variation in adaptive treatments, requiring more accurate beam modeling. Automating EPID-based IVD integration into clinical protocols will be essential for safe and efficient implementation.
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