Geometric and dosimetric variations of ICRU bladder and rectum reference points in vaginal cuff brachytherapy using ovoids

Purpose: To investigate the interfractional geometric and dose variations of rectal reference point (R) and bladder reference point (BL) doses in patients receiving vaginal cuff irradiation using high-dose-rate brachytherapy with 2 ovoids and the change in calculated radiobiologic-equivalent dose of R and BL. Methods: ICRU-38 R and BL reference doses were calculated. The variation in positions of ovoids, R, and BL were determined. Whether the magnitude of displacement shows a time trend and has an effect on calculated R and BL doses was evaluated. The relation between the ovoid diameter and both the magnitude of displacement and changes in R and BL doses was studied. Changes in radiobiologic-equivalent dose of rectum and bladder were determined. Results: The average magnitude of displacements was 3.2 and 12.1 mm, showed no time trend, and no tendency to displacement in a certain direction. Mean changes in BL and R doses were 64-75 cGy and 47-58 cGy, respectively. There was a relation between neither ovoid size and displacement nor R and BL dose change. Dose and geometric variation showed no correlation (p > 0.05). The differences in radiobiologic-equivalent dose of R and B were not significant. Conclusion: Although there were significant differences in R and B position and doses among the fractions, the magnitudes of dose changes were relatively small, and total calculated radiobiologic-equivalent doses of R and B did not change significantly. According to the results of this study, the benefit of treatment planning is limited to supporting treatment planning in each fraction. (C) 2004 Elsevier Inc.