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

Abstract

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. 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. 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   0.05) in the dose received by rectum and sigmoid on CT and MR plans. MRI-based brachytherapy planning has shown considerable improvements in tumor control and reductions in normal tissue toxicity. However, the high cost of MRI and non-availability of MRI preclude its use in many centers. CT, on the other hand, is widely available, but it can lead to overestimation of the target, at the time of brachytherapy. Hence, it is important to identify the subset of patients who will benefit from MRI-based planning at the time of brachytherapy.