Poster — Thur Eve — 63: Prostate IMRT: Product‐Mixture model of a two‐dimensional probability density function integrating the variability of the motion of the rectum and the rectal wall thickness

This study investigated the dependence of the probability density function ( pdf ) describing rectal wall geometry on the rectum position (R m ) and the rectal wall thickness (t w ). Probability density functions describing the organ motion uncertainties of the rectum ( pdf M ) have been reported by many authors. In this study, we further proposed a pdf describing the changes in rectal wall thickness ( pdf TW ) and hence introduced a two‐dimensional function pdf M&TW , incorporating the variability of R M and t W using their pdf M and pdf TW , respectively. Our study is based on the average,t ¯ W , of 587 prostate patients. The new pdf TW was established as a mixture of a three‐mode distribution with specific mean value (μ), standard deviation (σ) and weight ( w ), namely, (μ F = 3.31, σ F = 1.82 and w F = 77.1%), (μ PF = 7.7, σ PF = 0.809 and w PF = 15.2%) and (μ E = 10.27, σ = 0.906 and w E = 9.4%) for the full, partially full and empty state of the rectum, respectively. The pdf M&TW function was introduced as a product‐mixture model of the two functions pdf M and pdf TW and it has been graphically and mathematically reviewed. Our pdf M&TW model is a more realistic representation of the probability of the geometric rectal configuration that can occur during prostate IMRT than the model using only the motional pdf M and assuming a constant rectal wall thickness in the plan optimization.