The effect of bladder contrast on dose calculation in volumetric modulated arc therapy planning in patients treated for postoperative prostate cancer

The aim of this study was to evaluate the effect of contrast agent on dose calculation in volumetric modulated arc therapy (VMAT) in the post-prostatectomy setting. Ten patients were studied. Each patient received planning computed tomography (CT) images with contrast agent. All of the plans were done on virtually simulated contrast-free CT scans. The plan approved by the radiation oncologist was replicated to the contrast CT series. In both of the plans the same monitor unit was used. The doses calculated from the two plans were compared in regard to target volumes and organs at risk. A paired sample t-test was used to evaluate the differences in cumulative dose volume histogram between the two plans. We showed that the use of contrast agent may cause significant differences in dose distribution. There was a significant decrease in doses received by planning target volume (PTV70), rectum V65 Gy, rectum V40 Gy, bladder V65 Gy, penile bulb V40 Gy in plans with contrast-enhanced CT sets. The decrease in mean, maximum and minimum doses received by PTV70 also contributed to the significant decrease in conformity index. Using a contrast agent at the time of CT simulation may cause significant differences in dose distribution. For this reason, the plan should always be carried out on non-contrast CT data sets to avoid additional errors in the treatment planning process.