Electronic Portal Imaging Devices (EPIDs) are digital x-ray imaging systems that use the megavoltage treatment beam to acquire projection images of radiotherapy patients. For the past few decades EPIDs have been used to accurately position radiotherapy patients immediately prior to treatment, and to provide a record of treatment for physicians to review. EPID imaging systems are integrated into the treatment delivery hardware and software systems and are a standard feature of modern medical linear accelerators. EPIDs have also been demonstrated for dosimetry applications, with various models of calibrating the image signal to dose.

Figure 1.
EPIDs are uniquely placed as the only device that directly measures the position of the treatment beam with respect to patient anatomy in a single image. (Image accessed from ELEKTA – iViewGT)

Figure 2.
The evolution of our detector design development. From a standard EPID on the left to a segmented plastic scintillator on the right. An example of image quality for each detector is shown on the bottom row of images of a test object.

Figure 3.
A photo of a prototype segmented plastic scintillator developed at Liverpool Cancer Therapy Centre.

Figure 4.
The Modulation Transfer Function of a segmented plastic scintillator array EPID with different length plastic scintillator fibres.

This is a well established collaborative research project between SWSLHD, the University of Sydney and Perkin Elmer. The project has a strong track record in publications, grants and student projects. The areas of research and clinical applications involving EPIDs at Ingham and the Liverpool and Macarthur Cancer Therapy Centres are listed below:
 

Dual layer detector

Figure 5.
Standard EPID layers on the left, proposed dual detector layers on the right.

Clinical EPID dosimetry

Figure 6.
Time taken to perform patient QA measurements using the different techniques that have been used at Liverpool and Macarthur Cancer Therapy Centres.

Potential future student projects

Contact Dr Shrikant Deshpande for information on student projects. Projects suited to highly motivated students with experience or interest in modelling x-ray detectors are available.