Differences Between MRI- and CT-Based Delineation of Target Volume and Organs at Risk in High-Dose-Rate Brachytherapy of Cervix

Aim With superior soft tissue imaging characteristics, MRI is better than CT in determining the local disease extent during intracavitary brachytherapy of carcinoma cervix. The aim of the study is to evaluate the differences in dimensions and volumes of the target and organs at risk and the subsequent changes in dosimetry between MRI- and CT-based plans. Methods and Materials MRI and CT datasets of 34 locally advanced cervical cancer patients taken up for intracavitary brachytherapy between January and September 2017 were analyzed. The target volumes and organs at risk, namely bladder, rectum and sigmoid, were contoured by the same radiation oncologist on both the MRI and CT images as per the GEC ESTRO guidelines. The dimensions of HRCTV, the dose volume parameters of the target and OAR were recorded for the CT and MRI plans. Results CT image significantly overestimated the width ( p = 0.000) and thickness ( p = 0.009) of HRCTV. The volumes of HRCTV ( p = 0.000) and IRCTV ( p = 0.041) were larger with CT image compared to MRI. There was no statistically significant difference between rectal ( p = 0.107) and sigmoid ( p = 0.365) volumes on CT and MRI. There was statistically significant difference (all p