A predictive method of calculating the dosimetric effect of 1-D motion on narrow multileaf collimated segments

This study aims to investigate the utility of 1-D convolution as a predictive method to quantify the effect of patient movement as a result of respiration on segmented dose distributions typically found in IMRT. This is for the restricted case of 1-D motion perpendicular to the beam direction. A modified respiratory motion phantom was coupled to a solid water phantom and used to measure the dose distribution of several narrow MLC segments. The measured data was compared to dose distribution calculated by the mathematical convolution of static profiles with a probability distribution function (PDF) generated for sinusoidal motion and power cosine motion. The MLC segments for an IMRT field were also studied. Additionally, the dosimetric effect of intra-fraction patient motion combined with that of inter-fraction setup error, modelled using a Gaussian PDF, was calculated. Set-up variation and respiratory motion can significantly degrade the dose distribution of narrow segments. Our results emphasise the need to maintain the smallest feasible amplitude of respiratory motion and minimise set-up uncertainties, particularly when narrow segments are used.