Study of dosimetric and spatial variations due to applicator positioning during inter-fraction high-dose rate brachytherapy in the treatment of carcinoma of the cervix: A three dimensional dosimetric analysis

Background: This study evaluates dosimetric and spatial variations in inter–fraction applicator positioning in high dose rate (HDR) brachytherapy. Materials and Methods: This study includes 25 retrospective patients of carcinoma of the cervix. Each patient received 5 fractions of HDR intracavitary brachytherapy. High Risk-Clinical Target Volume (HR-CTV) were drawn on CT images. First implant was considered as a reference, and all subsequent CT data sets were rigidly registered on first implant’s CT data set. Another point A, called point Aabs, was defined on first plan and all subsequent plans. Registration properties were recorded for all 125 plans that include X, Y, Z DICOM offset, rotational, translational values and normalized D90 (ND90) doses were also recorded. Results: The mean angle of rotation on X, Y and Z axis are 0.63 ± 1.85 deg, -0.86 ± 1.30 deg, -1.14 ± 2.44 deg, respectively. While mean translational motion on X, Y and Z axes are –2.77 ± 10.32 mm, -6.12 ± 9.71 mm and 14.62 ± 23.83 mm, respectively. Mean ND90, and mean HR-CTV were found to be 1.18 ± 0.26 and 26.91 ± 17.70 cc, respectively. Conclusion: Results of the study reveals that translational motion is higher than the rotational motions, and inter – fraction applicator variation does not produce any significant change in Point A doses. The change in volume coverage is observed only due to applicator motion. HR-CTV coverage decreases with increasing HR-CTV volume. Hence, dose prescription should be based on 3D HR-CTV volume.