Evaluation of Rectal Balloon Reproducibility for Prostate Radiation Therapy using Cone Beam CT

Abstract

To evaluate the rectal balloon reproducibility for prostate treatment in terms of the relative location and the volume of balloon, retrospective analysis of our daily cone beam CT (CBCT) data for prostate patients was carried out. The results would reveal not only the reproducibility of the rectal balloon but also the stability of prostate position with respect to bony anatomy. Five prostate patients underwent image-guided radiation therapy with daily CBCT were selected for this study. The patients were treated with 78 Gy total dose (2 Gy x 39) using IMRT. All patients were immobilized using a custom-fit Blue Bag vacuum mold (Medical Intelligence, Inc.) in a supine position with a rectal balloon inflated with 30 cc of air. A CBCT (XVI, Elekta Oncology Systems) was used to correct for the translational setup errors based on three intra-prostatic seeds. The CBCT images were exported to a commercial TPS (Pinnacle3, Philips Medical Systems) for analysis. The rectal balloon, the seeds, and the fibrocartilage of pubic symphysis were contoured. The centroid of the fibrocartilage, three seeds in prostate, and rectal balloon were used as the reference point of the bony anatomy, prostate, and rectal balloon, respectively. The relative positions of the rectal balloon to the prostate, and that of the prostate to the bony anatomy were analyzed to obtain the weekly-based reproducibility or inter-fractional uncertainties of the rectal balloon and the prostate. The group mean of SD of the volume of the balloon (root mean square of the SD's of all patients) was 2.0 cc. The group mean of SD of the rectal balloon position relative to prostate was 1.8 mm, 1.7 mm, and 3.1 mm, in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The group mean of SD for the prostate position relative to bony anatomy was 0.8 mm, 1.8 mm, and 2.3 mm in the RL, AP, and SI directions, respectively. The largest deviation of the rectal balloon position relative to the prostate was seen in the SI direction. This trend was found for the position of the prostate, too. The evaluation of weekly-based positioning reproducibility of the rectal balloon and the prostate based on CBCT images was performed. Teh et al., reported the overall mean prostate displacement is less than 1mm for all directions with rectal balloon1. The inter-fractional uncertainty of the prostate obtained by this study was considerably larger than the internal motion as reported by van Lin et al.,2. In this study, it was found that the inter-fractional movement of rectal balloon was larger than that of the prostate.