Anatomic variation of gynecologic brachytherapy prescription points

Purpose: The purpose of this report is to evaluate the geometric movement (relative to the bony pelvis) of fixed brachytherapy reference points during the time interval of the first and second gynecologic intracavitary implant. Methods and Materials: The radiation therapy records of 40 consecutive patients with all stages of carcinoma of the cervix treated at the Radiation Oncology Center, Mallinckrodt Institute of Radiology, from January 1991 through December 1991 were reviewed. All patients received external beam irradiation and two intracavitary implants with Fletcher-Suit tandem and ovoids. Prescription points, per ICRU #38, were used: point A, B, and P, bladder, and rectum. Comparison of the location of points for the first and second implants (2 week time interval) was performed using the bony pelvis as reference anatomy and calculating the 3-dimensional spherical coordinates from a common origin for both implants. Results: Analysis of the movement of the second implant relative to the first revealed that the x, y, and z coordinates for the various points shifted from 0 to 6 mm in a single plain. Vector analysis showed that the magnitude and direction of the average shift was on the order of 1.0 to 1.5 cm with displacement posteriorly and inferiorly. The effect of this movement on dose rates to the various points can result in dose rate differences up to 35%. A regression analysis was performed to identify factors affecting this movement. The time interval from the first to the second implant was the factor which correlated best with the movement. Conclusion: There is movement of the absolute position of ICRU #38 reference points between the first and second intracavitary implants. This movement results in significant differences in absolute dose rates to these reference points in the two implants. The clinical significance of this movement relative to dose to the tumor is unknown.