IBS-GEC ESTRO-ABS Recommendations for CT Based Contouring in Image Guided Adaptive Brachytherapy for Cervical cancer.

Abstract MR Imaging is regarded as the gold standard for Image Gudied Adaptive Brachytherapy (IGABT) for cervical cancer. However, its wide applicability is limited by its availability, logistics and financial implications. Use of alternative imaging like CTand Ultrasound (US) for IGABT has been attempted. In order to arrive at a systematic, uniform and international approach for CT based definition and contouring of target structures, GEC ESTRO, IBS and ABS agreed to jointly develop such recommendations based on the concepts and terms as published in the ICRU Report 89. The minimum requirements are clinical examination & documentation, CT or MR imaging at diagnosis and at a minimum, CT imaging with the applicator in place. The recommendations are based on (i) assessment of the GTV at diagnosis and at brachytherapy, (ii) categorizing the response to external radiation into different clinical remission patterns, (iii) defining various clinico-radiological environments and (iv) definition & delineation of a target on CT imaging at the time of brachytherapy with the applicator in situ. CT based target contouring recommendations based on 4 remission categories within 8 defined environments, aim at improving the contouring accuracy for IGABT using CT, US and MRI as available. For each clinico-radiological environment, there is an attempt to minimize the specific uncertainties in order to arrive at the best possible contouring accuracy. Evaluating feasibility & reproducibility, to achieve a benchmark towards a gold standard MR IGABT and further clinical research including outcomes with CT Based IGABT will become the next steps. Background and Rationale Cervical cancer is the fourth most common cancer among women globally, with an estimated 579,200 new cases and 316,800 deaths worldwide in 2018 (1). The incidence is higher in low-middle income (LMIC) or low income (LIC) countries settings accounting for 85% of locally advanced cervical cancer worldwide, in which sufficient resources for management, especially radiotherapy facilities, represents a major challenge. (2)