Automated megavoltage imager positioning model

Abstract

AbstractPurpose: Electronic portal imaging is widely employed to confirm positioning of a patient receiving radiation therapy. In most cases an orthogonal pair of images is sufficient to provide millimetre accuracy to most treatment sites in three dimensions when coupled with an online correction protocol. Accurate positioning of the imaging panel is crucial in sparing radiosensitive electronics surrounding the amorphous silicon (ASi) detector plate, and avoiding collisions between the equipment with the treatment couch. This paper presents a model that can be used to accurately calculate ideal imager panel positions for Varian® imaging panels without the need for time‐consuming and potentially inaccurate judgment. Methods: The position of the imaging panel is expressed using a coordinate system in three dimensions: longitudinal, lateral, and vertical. Calculation of ideal coordinates requires the input of radiation field dimensions, collimator rotation, and treatment couch parameters. A program using Microsoft® Excel (Microsoft, Seattle, WA, USA) was created to obtain the proper EPID position during the imaging process. Results: The model was designed using Microsoft® Excel and provides suggested offsets once field and patient set‐up parameters have been entered. Instantaneous assessment of any offset is provided and prompts are provided if the panel is irradiated beyond the ASi plate or if collision is likely. After testing for accuracy in predicting the imager position, the algorithm has been used clinically in over 700 patients with no reports of errors. Accurate positioning of the panel has been found to reduce time spent on the positioning of the panel to ensure it is accurately positioned. Conclusion: Through the use of the model, a single imager position can be calculated for multiple fields and positioned pressing a single button in the treatment room. Irradiation of electronics and collisions are no longer subject to human judgment, the model thereby delivering on its goals.