Objective To establish a statistical model that can quantitatively analyze the dosimetric sparing of the bladder based on individual patient’ s anatomy in the static intensity-modulated radiotherapy (IMRT) plans for rectal cancer.Methods Static IMRT plans (7 AP fields) for 100 rectal cancer patients were used to train the model from 2012 to 2013.The anatomical features were quantitatively analyzed by the sizes of overlap regions of bladder-planning target volume (PTV) and bladder-PTV+0.5(0.5 cm margin around the PTV) .The mathematic relationship between anatomical features and dosimetric sparing of the bladder was evaluated after the bladder sparing dose was analyzed using dose-volume histogram.The established model was verified in the IMRT plans for additional 20 rectal cancer patients.Results Bladder V50 was linearly correlated with the ratio of bladder-PTV overlap size to bladder volume ( denoted as x%) , with an equation of V50=0.89x-0.99.Bladder V40 showed an approximately linear correlation with the ratio of bladder-PTV+0.5 overlap size to bladder volume (denoted as y%).The mean dose depended on both x%and y%.For the additional 20 plans, the absolute deviation between predicted and actual values for V50 and V40 were (-3.13%-3.78%) and (-5.30%-5.66%) , respectively, and the relative deviation for the mean dose was (-3.94%-3.76%) .Conclusions The model obtained in this work provides an effective method for quantitatively estimating the bladder sparing dose in IMRT plans for rectal cancer.

this paper discussed that the concept of conformal model,the method,the request and the meaning of conformal model for radiotherapy.It is measured that the thickness of low-melting point lead is 6 cm for 60Coγ-ray,is 8 cm for 6 MV and 15 MV X-ray when transmision rate is lower 5%.