title: Sampling considerations for intensity modulated radiotherapy verification using electronic portal imaging.
abstract: A model has been developed to describe the sampling process that occurs when intensity modulated radiotherapy treatments (delivered with a multileaf collimator) are imaged with an electronic portal imaging device that acquires a set of frames with a finite dead-time between them. The effects of the imaging duty cycle and frame rate on the accuracy of dosimetric verification have been studied. A frame interval of 1 s with 25%, 50% and 75% duty cycle, and a 50% duty cycle with frame intervals of 1, 2, 4, 8, and 16 s have been studied for a smoothly varying hemispherical intensity profile, and a 50% duty cycle with frame intervals of 1, 2, 4, and 8 s for a pixellated distribution. In addition an intensity modulated beam for breast radiotherapy has been modeled and imaged for 0.33 s frame time and 1, 2, and 3 s frame separation. The results show that under sparse temporal sampling conditions, errors of the order of 10% may ensue and occur with an oscillatory pattern. For the beams studied, imaging with a 1 or 2 s frame interval resulted in small errors at the 1%-2% level, for all duty cycles shown.
authors.affiliation: Joint Physics Department, Institute of Cancer Research and Royal Marsden NHS Trust, Sutton, Surrey, United Kingdom. p.evans@icr.ac.uk
authors.forename: P M
authors.initials: PM
authors.lastname: Evans
authors.forename: M
authors.initials: M
authors.lastname: Partridge
authors.forename: J R
authors.initials: JR
authors.lastname: Symonds-Tayler
mesh_terms: D001940:Breast / Q000528:radiation effects; D001943:Breast Neoplasms / Q000532:radiotherapy; D005260:Female; D006801:Humans; D015233:Models, Statistical; D011874:Radiometry; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods*; D013997:Time Factors