Should the organs at risk be contoured in vaginal cuff brachytherapy?

To assess the dose to the organs at risk (OARs) and utility of repeated OAR dose-volume histogram calculations in multifraction high-dose-rate vaginal cylinder brachytherapy using 3-dimensional imaging.

Thirty-eight patients (125 fractions) received high-dose-rate brachytherapy to the vaginal vault between January 2005 and October 2005. All patients emptied their bladders before insertion. After each insertion, a CT scan with 2.5-mm slices and contours of the bladder, rectum, and sigmoid was performed. Dose-volume histograms were generated for the D(0.1cc) and D(2cc) for the OAR using a software program created at our institution. Variance component models estimated the within-patient variance of the dose to the OAR between fractions. Predictors of dose to the OAR were identified using linear mixed models.

The within-patient coefficients of variation of total D(0.1cc) dose were bladder 14.0%, rectum 7.9%, and sigmoid 27.6%; for D(2cc), these were 8.1%, 5.9%, and 20.3%, respectively. Intraclass correlations ranged from 0.27 to 0.79. Larger OAR predicted greater total D(0.1cc) and D(2cc). Other predictors of total D(0.1cc) and D(2cc) dose included the size of the cylinder and the length of the treatment field for rectum.

CT simulation provides a noninvasive assessment of the dose to the bladder, rectum, and sigmoid. The small within-patient variation in doses to the bladder and rectum do not support reporting doses to the OARs beyond the initial fraction.