Anatomic variation of prescription points and treatment volume with fractionated high-dose rate gynecological brachytherapy

Abstract

The purpose of this report is to evaluate the geometric movement (relative to the bony pelvis) and dose variation of brachytherapy reference points in the same patient at repeated high-dose rate (HDR) intracavitary implants. A study was also concluded to find the variation in treatment volume from repeated fractions. Twenty-five consecutive cervical cancer patients (all stages) treated with external beam and fractionated HDR intracavitary implants at the University of Wisconsin were reviewed. Each brachytherapy insertion had a different plan generated prior to treatment delivery. ICRU #38 prescription points (A, B, P, bladder, and rectum) were used. Dose volume histogram was generated and treated volume to the prescription dose was recorded for each fraction. Motion analysis of the various points (from a common origin) in subsequent fractions relative to the first fraction revealed a shift of 29 mm in a single plane. Vector analysis revealed the magnitude of the average shift ranged from 1013 mm. These shifts resulted in a dose difference of >20 for the bladder and rectum points, but < than 8 for the other points. Dose volume histograms revealed that with the change in the anatomy of the cervix and upper vagina during a patients course of treatment, the treatment volume changes considerably. Thirty-six percent of all patients (9/16) had a reduction in the size of the ovoid during the treatment course. Sixty percent of all patients (15/25) had volume changes