Evaluation of High Doses to Small Volumes of the Bladder and Rectum During HDR Endocavitary Vaginal Brachytherapy With Bladder Filling Techniques

Abstract

There are approximately 4800 cases of vaginal and 60,000 cases of endometrial cancer diagnosed each year in the United States. Treatment for these diseases often includes the use of HDR endocavitary vaginal brachytherapy for boosting the dose to the Vaginal apex. The intent of this work is to determine if bladder filling techniques influence the high dose to different volumes of the Bladder and Rectum. 10 sequential patients scheduled to receive HDR treatments for gynecologic cancer were selected for evaluation. A Foley catheter was placed into the bladder using sterile technique and then placed to straight drain.(“Empty bladder CT”) The patient was immobilized using standard techniques. A CT scan was performed of the patient from bottom of SI joints to the distal end of the vaginal cylinder with slice thickness of 1.25mm, to include the vagina, bladder and rectum to the level of recto-sigmoid junction. The CT scan was repeated after 120 cc’s of saline & contrast were instilled into the bladder with the Foley catheter clamped. (“Full bladder CT”). Both sets of CT data were exported to the treatment planning system. The Dosimetrist and Physician contoured the Rectum, Bladder, and Urethra on both image sets. The physician determined the treatment length and depth (standard 5mm) for the treatment delivery. Dose structures were constructed on both data sets and the dwell position and times were optimized. Comparison DVH’s and volume doses for the Rectum, Bladder, and Urethra were determined for each data set. The dose increased on average for all small volumes of the Bladder when the “full bladder” was used for treatment planning as compared to the “empty bladder” plans. This increase was on the order of 5% (±1.0%) for Bladder volumes of (0.03cc, 1cc’s, 2cc’s, 3cc’s, 10cc’s, and 20cc’s.). For the Rectum, the dose was increased by 1.5%, but the variability was higher on the order of (±1.2%) for Rectum volumes of (0.03cc, 1cc’s, 2cc’s, 3cc’s, 10cc’s, and 20cc’s). In contrast, the mean dose for the Bladder and Rectum decreased by 20%, and 2% respectively with the "full bladder”. With the “empty bladder” the dose to 1cc, and 2cc of the Urethra was reduced on average by 3%. Bladder filling for this cohort of patients increased the average high dose to small volumes of the Bladder and Rectum. The increase in dose to the Bladder was approximately 5% and was more consistent for all patients when compared to the average dose increase of 1.5% for the Rectum.