In-vivo dosimetry using electronic portal imaging in online-adaptive radiotherapy for rectal cancer

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.

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.

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.

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.