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

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 2-9 mm in a single plane. Vector analysis revealed the magnitude of the average shift ranged from 10-13 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 patient's 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 <10%. Sixty-two and one half percent of patients (10/16) who did not undergo a reduction of avoid size during the entire course of the treatment had volume change <10%. Since there is a change in the anatomy of the cervix and upper vagina during the course of a treatment along with the irreproducibility of the packing, there is movement of the absolute position of the prescription points between fractions, thus emphasizing the importance of individual dosimetry. Moreover, due to the same reasons, there are significant changes in the treatment volume among implants for the same patient. Volume reduction caused by reduction in ovoid size alone could not be extracted from this study.