Impact of Uterine Deformation Induced by Applicator on Brachytherapy Dosimetric Value: an Uterine-Mimicking-phantom Study

Brachytherapy (BT) is subjected to uncertainties from imaging, treatment planning, and anatomical variations. It is necessary to identify the uncertainties, their magnitude, and their impact on the overall uncertainty of dose delivery. The aim of the present phantom-based study is to investigate the deformation of the uterus anatomy induced by BT applicator insertion and the relationship between dose and location of the inserted applicator. A novel flexible phantom was designed based on female pelvic anatomy by the 3D printing process to simulate the deformation and movement of the uterine during the BT process. The applicator was inserted into the uterus and in order to simulate the uterus motion, the phantom was moved in four directions, right, left, down, and up. The CT and ultrasound images were obtained. A two-stage registration algorithm was designed to register the ultrasound and CT images in order to calculate the applicator displacements map. This map was used to calculate delivered dose error to uterus and rectum and to correct the planned dose values in treatment planning procedure. The results showed that in a planned D90 (dose) of 9.4 Gy for the uterus and maximum dose of 1.8 Gy to the rectum, movement of applicator leads to a big deviation in final delivered doses. After the applicator moved to the right and left directions, the D90 delivered to the uterus was 6.2 Gy and 5.6 Gy, respectively. Moreover, by moving downwards and upwards, the uterus received 0.4 and 1.2 Gy lower than 9.4 Gy. Similarly, the maximum dose to the rectum in right, left, down, and up directions after applicator movement were 3.4 Gy, 3.3 Gy, 3.7 Gy and 3.2 Gy, respectively which highlighted the higher delivered dose to the rectum. The results of the study demonstrate that applicator displacement can lead to uterine underdosing and also increase the risk of rectum irradiation. Ultimately, image-guided brachytherapy and using image registration can increase the accuracy of treatment and reduce the errors of delivered dose to target volume and organs at risk, especially in uterus cancer.