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.