A dosimetric evaluation of using a single treatment plan for multiple treatment fractions within a given applicator insertion in gynecologic brachytherapy

PURPOSE: To evaluate the dosimetric impact of using one treatment plan for multiple fractions from a single tandem and ring applicator insertion of high-dose-rate brachytherapy for cervical cancer. METHODS AND MATERIALS: Thirteen cervical cancer patients undergoing high-dose-rate brachytherapy were followed. Patients received the total dose from a single applicator insertion in two fractions, given with at least 6 hours apart within 24 hours. The treatment plan was based on a CT scan taken before the first treatment fraction. A second CT was obtained before the second treatment fraction. The co-registered image series were used to evaluate the dosimetric impact of using a single treatment plan for both fractions. Applicator and catheters were measured to quantify interfraction displacement. RESULTS: When the Day 1 plan was applied to the Day 2 images, high-risk clinical target volume (HR-CTV) coverage was reduced by as much as 17.4 percentage points. The mean decrease was 9.4 5.0 percentage points (p < 0.0001). The rectum V-75 increase was significant (p = 0.03), whereas the bladder V. increase was not significant (p = 0.28). Volume changes in the BRCTV contour from Day 1 to Day 2 were also observed (p = 0.29). Maximum applicator and catheter displacements of 10-30 mm were seen, from Day 1 to Day 2. CONCLUSIONS: When the Day 1 plan was used on the Day 2, the HR-CTV coverage decreased significantly (p <0.0001). Our study establishes the need for institutions to evaluate the necessity for replanning based on imaging obtained before each treatment fraction for their gynecologic brachytherapy techniques. (C) 2013 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.