Assessment of uterus position as a surrogate for high-risk clinical target volume with respect to the applicator position for multiple fractions of brachytherapy in cervical cancer

Hybrid magnetic resonance imaging/computerized tomography (MRI/CT) planning for high-dose-rate (HDR) brachytherapy in cervical cancer with MR/CT fusion for the first fraction followed by CT for fraction 2 and 3 is used at our center. The aim of this study is to evaluate the position of applicator intrauterine tube (IU) in relation to uterine serosa with each fraction of intracavitary high-dose-rate brachytherapy. Position of the applicator relative to uterus was measured from tip of the applicator (IU) to the top of uterus in the plane of IU and perpendicular to IU in anterior, posterior, left and right directions at the tip of IU, mid-point of the IU and 1 cm from the surface of vaginal ring. The mean absolute difference (+/- 95 % confidence interval) between these positions at fraction 2 and 3 was calculated with fraction one as reference. The mean absolute difference (+/- 95 %) of the applicator relative to uterus was 2.7 +/- A 0.5 mm at the tip, 1.5 +/- A 4 mm at mid-point and 1.1 +/- A 0.3 mm at 1 cm from the surface of the ring. This study shows that there is consistency in inter-fraction applicator position relative to uterus apart from at the tip and, therefore, in situations where high-risk clinical target volume (HRCTV) extends towards uterine fundus, MRI should be used for each fraction of brachytherapy planning to accurately define HRCTV.