Implications of applicators orientations on bladder, and rectal doses in gynecological mHDR brachytherapy: A retrospective analysis of reproducibility in multi-fractionated regimen

ABSTRACT

Introduction: Variation of doses due to positional uncertainties of applicators based on orthogonal radiograph has been evaluated by several researchers. The present study has analyzed the various possibilities of special alterations of applicator placements in intracavitary brachytherapy (ICBT) and its impact on the dose to target volume, rectum, and bladder. An innovative approach has been suggested to quantify and utilize the special coordinates for reproducibility of applicator based on the pelvic bone landmark. Materials and Methods: A total of 27 fractions of 9 (n = 9) cancer cervix patients treated with external beam radiotherapy followed by ICBT have been evaluated retrospectively. The first fraction of each patient was planned as per the International Commission on Radiation Units and Measurements report 38 guidelines and was considered reference for consecutive fractions regarding dwell positions, dose to target volume, bladder, and rectum points. For each fraction, positioning of applicators regarding their spatial orientations with respect to pelvic bone landmarks and their correlation with dose to bladder and rectum was recorded and analyzed. Results: It was found that mean angulations between (1) the two applicator points with respect to tip of central tandem (α), (2) the two applicator points with junction point situated on the sacrum bone (β) and (3) the angle between the line joining applicator points and a point defined on pelvic bone (γ) is 62.20° ± 5.74°, 37.13° ± 5.64° and 105.51° ± 6.58°, respectively. Bladder dose increased with increment in α but decreased with increase of β and γ. Dose to rectum remained unaffected for γ. Mean distance from couch top to tip of central tandem and two ovoids is, respectively, 11.17 cm and 8.70 cm. Conclusion: Optimal plans even with orthogonal radiographs may be generated by verifying the application based on the parameters mentioned in the present study and computed tomography-based plans could be utilized more effectively instead of doing automatic or manual optimization. Whereever orthogonal radiograph based or template based ICBT practices is performed this study may create a dataset to have an optimal treatment plan even without three-dimensional images