Impact of rectal distension on prostate CBCT-based positioning assessed with 6 degrees of freedom couch

Publication date: Available online 9 March 2018
Source:Practical Radiation Oncology
Author(s): Julien Charret, Julia Salleron, Magali Quivrin, Frédéric Mazoyer, Paul Lesueur, Etienne Martin, Didier Peiffert, Gilles Créhange
IntroductionProstate requires a daily correction of its 3D position in relation with rectal distension. In this study, we sought to determine whether rectal distension with respect to the rectal behavior might have an impact on prostate translations and/or rotations during prostate IGRT using a 6 degrees of freedom (DOF) couch.MethodsWe reviewed the data from 39 patients with localized prostate cancer patients treated with protracted external radiotherapy using a 6 DOF couch. Before each fraction, a kV-CBCT was performed. The automatic fusion algorithm was set to fuse on soft tissue and it allowed correction for translations in three dimensions and rotations in the longitudinal axis ("roll") and lateral axis ("pitch"). After contouring the rectum on each kV-CBCT, we determined the Cross Sectional Area (CSA) and relative CSA (CSArel) by dividing with the CSA of planning CT. The standard deviation of CSArel per patient was used to classify the patients in two groups: patients with a stable rectum and patients with an unstable rectum. The CSArel was compared between these two groups with a linear mixed model with group as fixed effect and patient as random effect.ResultsSix hundred and sixteen kV-CBCT were analyzed and 2 subgroups of patients could be defined a posteriori: 19 patients had a stable rectum: mean CSArel (1.06±0.08). The other 20 patients had an unstable rectum: mean CSArel (1.43±0.08). The average pitch in the group with a stable rectum was 0.36° (+/-0.21) versus 0.40° (+/-0.20) (p=0.898). The pitch was not correlated with the CSA rel (p=-0.065, r=0.119). The average roll in the group with a stable rectum was 0.27° (+/-0.16) versus 0.05° (+/-0.16) (p=0.137). The roll was not correlated with the CSA (p=0.094, r=0.068). The average CSArel was higher (p=0.0013) and more variable (p=0.035) in the unstable group.ConclusionRectal distension had neither impact on the pitch nor on the roll, which suggest that a 6 DOF couch has little interest in daily practice for prostate IGRT.



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