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mode. The EPID-derived fluence maps were used to reconstruct the 3D dose to the planning CT data set. The 3D dose for each fraction was compared to the planned 3D dose distribution (Eclipse treatment planning system). Our measured 3D doses were calculated using an in- house, MATLAB-coded, superposition-convolution collapsed cone convolution (CCC) algorithm which has been previously verified for IMRT and VMAT. Frame average optimization was also performed partway through this study as it was found to substantially increase accuracy in the 3D dose reconstruction. Dose distributions were compared using a 3D γ-test, with a 3%/3mm criteria, on the planning target volume (PTV). We chose pass-rate action levels of 85% for lung and 80% for spine as a starting point for this work. Fractions that failed this action level were analyzed offline by initially comparing the CBCT vs CT as well as treatment parameters and patient setup. Errors were categorized into: EPID-specific, patient setup, anatomical, dose model differences, or unknown. Results For the 537 lung fractions, 126 (23.5%) flagged our action level pass-rate. Of those, the error categories were identified as 82 (65.1%) EPID-specific, 25 (19.8%) patient setup, 10 (7.9%) anatomical, 8 (6.3%) model difference, and 1 (0.8%) unknown. Specifically, of the 82 EPID- specific errors, 53 were acquisition failures (i.e. forgot to schedule imaging, deploy the EPID, computer system crashes, etc.) and 29 were due to frame-averaging artefacts. After optimizing the frame averaging, and assuming we remove human error contributions through increased education, the total flagged fractions would then be 44 (126-82), and the leading category flagging our alert level would be patient-specific, i.e., 35 of 44 fractions or 79.5%. Model differences then make up most of the remaining issues (18.2%). This is an important finding because patient-specific errors are not detectable using standard pre-treatment quality assurance approaches. For the 55 acquired spine fractions, 27 (49%) flagged our action level pass-rate. Of those, 20 were categorized as EPID-specific and 7 were anatomical. Like the above argument, 100% of the errors are patient- specific once the EPID-specific errors are removed.

dosimetric discrepancies. To be sustainable without a significant increase in resources, large-scale clinic-wide implementation of an in vivo patient dosimetry program requires a high level of automation for data acquisition, data transfer, analysis, and communication/notification of results. In our work this is facilitated by intimate knowledge of the vendor’s software and hardware, but could also be achieved by appropriate vendor development of a commercial solution. The EPID can be used for transmission imaging during treatment delivery to verify planned radiotherapy treatments, thus providing improved patient safety and treatment efficacy. It is a cost effective, automated process adding negligible extra work in the clinic. Reports can provide 3D dose data and DVHs for all structures in the patient plan (i.e. targets and OARs) providing an important tool to monitor the quality of complex radiation treatments. Ref 1-McCowan, PM et al. , Clinical Implementation of a Model-Based In Vivo Dose Verification System for Stereotactic Body Radiation Therapy-Volumetric Modulated Arc Therapy Treatments Using the Electronic Portal Imaging Device, Int J Radiat Oncol Biol Phys. 2017 Apr 1; 97(5):1077-1084. SP-0665 Dose painting B. Speleers 1 , W. De Neve 2 , I. Madani 3 1 Speleers Bruno, Department of Radiotherapy, Ghent, Belgium 2 Ghent University Hospital, Department of Radiotherapy, Ghent, Belgium 3 University Hospital of Zurich, Department of Radiotherapy, Zurich, Switzerland Abstract text The term “dose painting” was launched by Ling et al. in 2000 [1]. Given advances in tumor biology and imaging technology, non-invasive 3D-visualization of different biological abnormalities within the tumor becomes possible. Those abnormalities – tumor hypoxia, metabolic and proliferative activity, abnormal blood flow among others – may drive the tumor aggressive and resistant to treatment. Mapping abnormal tumor subvolumes detected by biologic imaging (biologic MR imaging [DCE, DWI], MRS, PET, MR/PET, …) with higher radiation dose presents a promising treatment strategy. The rest of the tumor may not need higher radiation dose but conventional radiation dose or even dose de-escalation in its radiosensitive subvolumes. Generally, dose painting is a concept of intentionally non-uniform radiation dose prescription and delivery based on (multimodality) biologic imaging. There are three principle components in dose painting: 1) identifying and validating a target volume for dose painting; 2) 3D-imaging of the target volume; 3) treatment planning and delivery. Each component has uncertainties and limitations, understanding of which is important for a successful implementation of dose painting. Proof-of-principle studies demonstrated technical and clinical feasibility of dose escalating dose-painting in solid tumors [2-5] and a number of randomized phase II-III clinical trials are on their way [6-9] aimed at improving disease control and survival. Although the tumor is the primary target volume for dose painting, unavoidably irradiated normal organs and tissues may present another target volumes. Healthy tissue subvolumes of higher importance for their function or of higher radiosensitivity (detected by biologic imaging) may need dose de-escalating dose-painting. Planning studies demonstrated possibility of this Symposium: Advanced planning

Fig. 1. Result examples: Planning CT contours overlaid with transverse CBCT images (left column – a, c, e) and percentage dose difference images (right column – b, d, f) for three lung patient fractions: Lung #72 – passing, Lung #53 – near pass-fail, and Lung #51, failing. The patient setup discrepancy noticed in the CBCT in Lung #51 is evident in the percentage dose difference. Discussion-Conclusion Patient anatomy changes while on-treatment will be the main cause of observed differences between the dose delivered to the patient and that planned. The use of CBCT is essential in assessing the causes of in vivo