Imaging of moving fiducial markers during radiotherapy using an active pixel sensor based EPID

The purpose of this work was to investigate the use of an experimental CMOS (Complementary Metal-OxideSemiconductor) APS (Active Pixel Sensor) for tracking of moving fiducial markers during radiotherapy. The APS has an active area of 5.4 x 5.4 cm and maximum full frame read-out rate of 20 frame s−1, with the option to read out a Region-Of-Interest (ROI) at an increased rate. It was coupled to a 4 mm thick ZnWO4 scintillator which provided a quantum efficiency (QE) of 8% for a 6 MV x-ray treatment beam. The APS was compared with a standard iViewGT flat panel amorphous Silicon (a-Si) Electronic Portal Imaging Device (EPID), with a QE of 0.34% and a frame-rate of 2.5 frame s−1. To investigate the ability of the two systems to image markers, four gold cylinders of length 8 mm and diameter 0.8, 1.2, 1.6 and 2 mm were placed on a motion platform. Images of the stationary markers were acquired using the APS at a frame-rate of 20 frame s−1, and a dose-rate of 143 MU min−1 to avoid saturation. EPID images were acquired at the maximum frame-rate of 2.5 frame s−1, and a reduced dose-rate of 19 MU min−1to provide a similar dose per frame to the APS. Signal-to-Noise Ratio (SNR) of the background signal and Contrast-to-Noise Ratio (CNR) of the marker signal relative to the background were evaluated for both imagers at doses of 0.125 to 2 MU. Image quality and marker visibility was found to be greater in the APS with SNR ∼5 times greater than in the EPID and CNR up to an order of magnitude greater for all four markers. To investigate the ability to image and track moving markers the motion platform was moved to simulate a breathing cycle with period 6 s, amplitude 20 mm and maximum speed 13.2 mm s−1. At the minimum integration time of 50 ms a tracking algorithm applied to the APS data found all four markers with a success rate of ≥92% and positional error ≤90 μm. At an integration time of 400 ms the smallest marker became difficult to detect when moving. The detection of moving markers using the a-Si EPID was difficult even at the maximum dose-rate of 592 MU min−1 due to the lower QE and longer integration time of 400 ms. This work demonstrates that a fast read-out, high QE APS may be useful in the tracking of moving fiducial markers during radiotherapy. Further study is required to investigate the tracking of markers moving in 3-D in a treatment beam attenuated by moving patient anatomy. This will require a larger sensor with ROI read-out to maintain speed and a manageable data-rate.