Verification of dynamic multileaf collimation using an electronic portal imaging device

High standards of treatment verification are necessary where complex new delivery techniques, such as intensity modulated radiation therapy using dynamic multileaf collimation, are being developed. This paper describes the use of a fluoroscopic electronic portal imaging device (EPID) to provide real-time qualitative verification of leaf position during delivery of a dynamic MLC prescription in addition to off-line quantitative verification. A custom-built circuit triggers the EPID to capture a series of snap-shot images at equally spaced dose points during a dynamic MLC prescription. Real-time verification is achieved by overlaying a template of expected leaf positions onto the images as they are acquired. Quantitative off-line verification is achieved using a maximum gradient edge detection algorithm to measure individual leaf positions for comparison with required leaf positions. Investigations have been undertaken to optimize image acquisition and assess the edge detection algorithm for variations in machine dose rate, leaf velocity and beam attenuation. On-line verification enables the operator to monitor the progress of a dynamic delivery and has been used for independent confirmation of accurate dynamic delivery during intensity modulated treatments. Off-line verification allows measurement of leaf position with a precision of 1 mm although image acquisition times must be less than or equal to 140 ms to ensure coincidence of the maximum gradient in the image with the 50% dose level.