How much margin reduction is possible through gating or breath hold?

Abstract

We determined the relationship between intra-fractional breathing motion and safety margins, using daily real-time tumour tracking data of 40 patients (43 tumour locations), treated with radiosurgery at Hokkaido University. We limited our study to the dose-blurring effect of intra-fractional breathing motion, and did not consider differences in positioning accuracy or systematic errors. The additional shift in the prescribed isodose level (e.g. 95 %) was determined by convolving a one-dimensional dose profile, having a dose gradient representing an 8 MV beam through either lung or water, with the probability density function (PDF) of breathing. This additional shift is a measure for the additional margin that should be applied in order to maintain the same probability of tumour control as without intra-fractional breathing. We show that the required safety margin is a nonlinear function of the peak-to-peak breathing motion. Only a small reduction in the shift of isodose curves was observed for breathing motion up to 10 mm. For larger motion, 20 or 30 mm, control of patient breathing during irradiation, using either gating or breath hold, can allow a substantial reduction in safety margins of about 7 or 12 mm depending on the dose gradient prior to blurring. Clinically relevant random setup uncertainties, which also have a blurring effect on the dose distribution, have only a small effect on the margin needed for intra-fractional breathing motion. Because of the one-dimensional nature of our analysis, the resulting margins are mainly applicable in the superior-inferior direction. Most measured breathing PDFs were not consistent with the PDF of a simple parametric curve such as cos4, either because of irregular breathing or base-line shifts. Instead, our analysis shows that breathing motion can be modelled as Gaussian with a standard deviation of about 0.4 times the peak-to-peak breathing motion.