Interfraction variations in doses at OARs during three-dimensional high dose rate brachytherapy planning of cervix cancer

Objective: The American Brachytherapy (BT) Society recommends that BT must be included as a component of the definitive radiation therapy for cervical carcinoma because recurrences and complications are decreased when BT is used in addition to external beam radiotherapy. The aim of this study is to quantify the interfraction dose variations (VARacts) during high dose rate (HDR) BT, the effect of variation in dose in terms of excess "unrecognized" dose to OAR and to conclude the reason of the variation in reference of applicator position/geometry versus deformation of the organ at risk (OAR) concerned. Materials and Methods: Total 30 patients of carcinoma cervix, biopsy proven, between June 2018 and May 2019, were taken for the study. All patients were treated with external beam radiation therapy to a dose of 50 Gy in 25 fractions over 5 weeks, followed by three fractions of HDR intracavitary brachytherapy (ICBT) (7.5 Gy to point A in each fraction) by two-dimensional (2D) X-ray-based planning. Before treatment in the first and last fraction of BT, computed tomography (CT) scan was done for every patient. Then, a 3D-based planning was performed with CT images on our HDR Plus software with image sequence option. VARact was calculated. Rigid image registration of consecutive fraction images was used for quantification of the hypothetical variation in dose (VARhypo) arising exclusively due to changes in applicator placement and geometry. Results: The mean contoured rectal volumes for the first and third fractions were 41.49 cc and 44.72 cc, respectively, while the respective volumes for bladder were 9.33 cc and 9.35 cc cm. These differences were statistically insignificant (P value: 0.263 and 0.919 for rectum and bladder, respectively). The mean equivalent dose in 2 Gy fraction (EQD2) bladder D2cc was 5.68 Gy and 5.79 Gy in the first and third fraction ICBT, respectively. The mean EQD2 for the rectal D2cc was 11.63 Gy and 12.85 Gy in the first and third fraction ICBT, respectively. None of the patients had an actual cumulative EQD2 more than 90 Gy for bladder, but 36.66% of the patients had a rectal dose exceeding the tolerance (75 Gy). Regression plots showed that VARhypo alone could predict about 42.2% of the VARact in the rectum and 19.2% of the VARact in the bladder. Thus, the remaining variation was due to the organ deformation-related dose variations between the two fractions. Conclusions: There were no statistically significant variations in the volumes or doses of OAR between the two fractions. However, a significant proportion of patients may have a higher dose to the OAR in the third fraction in the absence of individualized planning. This increase is likely to be more detrimental where higher doses per fraction are used. Variations in OAR doses may be caused by organ deformation and/or changes in applicator placement/geometry.