Verification of dynamic multileaf collimation using an electronic portal imaging device

High standards of treatment verification are necessary where complex new de livery techniques, such as intensity modulated radiation therapy using dyna mic multileaf collimation, are being developed. This paper describes the us e of a fluoroscopic electronic portal imaging device (EPID) to provide real -time qualitative verification of leaf position during delivery of a dynami c MLC prescription in addition to off-line quantitative verification. A cus tom-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 positio ns onto the images as they are acquired. Quantitative off-line verification is achieved using a maximum gradient edge detection algorithm to measure i ndividual leaf positions for comparison with required leaf positions. Inves tigations have been undertaken to optimize image acquisition and assess the edge detection algorithm for variations in machine dose rate, leaf velocit y and beam attenuation. On-line verification enables the operator to monito r the progress of a dynamic delivery and has been used for independent conf irmation of accurate dynamic delivery during intensity modulated treatments . Off-line verification allows measurement of leaf position with a precisio n of 1 mm although image acquisition times must be less than or equal to 14 0 ms to ensure coincidence of the maximum gradient in the image with the 50 % dose level.