Imaging linear accelerator output using a high-speed scintillation based electronic portal imaging device (Hi-EPID)

We have developed a high-speed scintillation based electronic portal imaging device (Hi-EPID) to image the fluence output of a linear accelerator as it exits the head. The Hi-EPID consists of a high speed digital camera coupled with a fast terbium-doped gadolinium-oxy-sulfide (Gd2O2S:Tb) scintillator. The camera can record every single radiation pulse out of the accelerator, at up to 500 frames per second. Since the Hi-EPID allows us to simultaneously obtain both the fluence and the field shape in a delivery, it serves as an independent means of verifying treatment delivery. In this study, we used the Hi-EPID to analyze the characteristics of the fluence delivered from a commercial clinical linear accelerator (Synergy (R) S, Elekta). Both open field and multileaf collimator (MLC) based intensity modulated radiation therapy (IMRT) fields were evaluated. Fluence delivery irregularities were observed with the Hi-EPID device. Fluence was not constant over the duration of a segment, resulting in a varying dose rate. For the open field delivery, the fluence increased with time until it reached a plateau (in approximately 100 ins), remaining constant over the remainder of the delivery. However, in the case of a six segment IMRT field delivery, the fluence variation differed between segments. For some segments, the fluence increased in the beginning and then reached a plateau. For others, the fluence rose and fell sharply, followed by a slower rise to the plateau. The rapid fluence variation before a plateau was as high as 50% of the maximum. Though the total fluence was correctly delivered, this result is intriguing and calls for further investigation. An analysis of field shapes showed no unplanned motion of MLC during step and shoot delivery, as has been reported for another clinical linear accelerator.