Considerations in Selecting a Fitting a Function for Film Dosimetry in Radiation Therapy

Although many technologies, such as an electronic portal imaging device, a software-based quality assurance system, an ion chamber array,etc., have been developed for the purpose of quality assurance of radiotherapy planning, radiochromic film is still one of the most reliable technologies and is being widely used. In particular, compared with other technologies, EBT3 film has unique advantages, including its unlimited spatial resolution, relatively small volume and waterproof quality, which means it can be used inside the patient's body. The main obstacle to using EBT3 film is that various equations have to be used in the calibration process, which makes it very sensitive to the results. The manufacturer provides a fitting function, but many software programs and researchers use their own functions. In this study, some functions normally used for calibrating EBT3 films are compared by analyzing a couple of characteristics in the course of fitting the function to the measured values. Although our study shows the lack of a suitable fitting function that works well in every situation, it can provide valuable insight into selecting suitable functions for fitting in a clinical setting.