Validation of IBS-GEC ESTRO-ABS CT Based Contouring Recommendations using Pre-Brachytherapy MRI in Cancer Cervix Patients Treated with Definitive Radiotherapy

Purpose: Consensus guidelines from IBS-GEC ESTRO-ABS for CT-based contouring has recently introduced possibilities of CT-based adaptive brachytherapy (BT) for cancer cervix.The present study attempts to validate the guidelines by using MRI at diagnosis and in pre-BT environment. Methods and Materials:The study involved Stage IIB to IVA patients of carcinoma cervix with pre-treatment MRI, treated with definitive chemoradiation and hybrid-interstitial BT, during 01 April, 2022 to 01 October, 2023.The target volumes HRCTV, IRCTV and OARs that included bladder, rectum and sigmoid were prospectively delineated on BT planning CT using BT planning MRI for guidance, as per IBS-GEC ESTRO-ABS recommendations by a radiation oncologist.The dimensions, volumes, doses, DSC on CT were compared with that of MRI images.Results: Two hundred eight scans from 104 sets of 52 patients of CT and MR scans, were basis for this study.With respect to dimensions, CT overestimated HRCTV maximum width, IRCTV width at Point A and maximum thickness of HRCTV and IRCTV, compared to MRI p-value: <0.001, <0.001, 0.049, and 0.007, respectively.In analysis of OAR volumes, CT overestimated sigmoid and rectum volumes with median difference of paired values: 7.2(IQR3.1-13.6), and 7.8(IQR3.6-12.8),p-value: <0.001 and 0.003, respectively.Bladder2cc and Sigmoid2cc doses were underestimated with CT images with median difference of paired values: 0.5(IQR0.3-0.9) and 0.5(IQR0.3-0.8),p-value: <0.001 and 0.003, respectively.There was insignificant difference in HRCTV and IRCTV doses between CT and MR images.The median DSC of HRCTV, IRCTV, Bladder, Rectum and Sigmoid were 0.72, 0.78, 0.76, 0.68 and 0.46, respectively. Conclusion:Despite the dimentional differences there was no dosimetric difference in HRCTV and IRCTV.CT based contouring using IBS-GEC ESTRO-ABS recommendations with MRI at diagnosis and brachytherapy, similar to the MRDG-Pre BT MR/CTBT environment, is safe, convenient and addresses logistic issues in institutions, especially in low middle-income countries (LMICs).