Background and purpose: First reports on clinical use of commercially automated systems for Electronic Portal Imaging Device (EPID)-based dosimetry in radiotherapy showed the capability to detect important changes in patient setup, anatomy and external device position. For this study, results for more than 3000 patients, for both pre-treatment verification and in-vivo transit dosimetry were analyzed. Materials and methods: For all Volumetric Modulated Arc Therapy (VMAT) plans, pre-treatment quality assurance (QA) with EPID images was performed. In-vivo dosimetry using transit EPID images was analyzed, including causes and actions for failed fractions for all patients receiving photon treatment (2018)(2019). In total 3136 and 32,632 fractions were analyzed with pre-treatment and transit images respectively. Parameters for gamma analysis were empirically determined, balancing the rate between detection of clinically relevant problems and the number of false positive results. Results: Pre-treatment and in-vivo results depended on machine type. Causes for failed in-vivo analysis included deviations in patient positioning (32%) and anatomy change (28%). In addition, errors in planning, imaging, treatment delivery, simulation, breath hold and with immobilization devices were detected. Actions for failed fractions were mostly to repeat the measurement while taking extra care in positioning (54%) and to intensify imaging procedures (14%). Four percent initiated plan adjustments, showing the potential of the system as a basis for adaptive planning. Conclusions: EPID-based pre-treatment and in-vivo transit dosimetry using a commercially available automated system efficiently revealed a wide variety of deviations and showed potential to serve as a basis for adaptive planning.

In all treatment sites of our radiotherapy network, in vivo dosimetry (PerFRACTION TM) was fully implemented in February 2018. We hypothesized that additional help with bladder and rectum preparation by home nursing would improve patients' preparation and investigated if this could be assessed using in vivo dosimetry (IVD). Materials/methods: A retrospective study was conducted with a test group who received additional help with bladder and rectum preparation by home nurses and a control group who only received information on bladder and rectum preparation according to the standard protocol. Patients were treated with a 6 MV Volumetric Modulated Arc Therapy (VMAT) technique. Electronic portal imaging device (EPID)-based integrated transit dose images were acquired on the first 3 days of treatment and weekly thereafter or more if failed fractions (FF) occurred. Results were analyzed using a global gamma analysis with a threshold of 20%, tolerance of 5% (dose difference) and 5 mm (distance to agreement), and a passing level of 95%. Results: Data of 462 prostate patients was analyzed: 39 and 423 in a test and control group respectively with a comparable number of measurements (on average 8.0 (r = 4.8) and 7.1 (r = 4.5) respectively per treatment course). Of the FF, 39% and 31% were related to variations in bladder and rectum filling for the test and control group respectively. Subgroups were created based on the number of FF, no statistically significant differences were observed. Conclusion: Two dimensional EPID-based IVD successfully detected deviations due to variations in bladder and rectum filling, however it could not confirm the hypothesis.

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