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Background and Rationale: MRI based 3D-IGABT has become an advanced standard for cervical cancer brachytherapy (BT) and has shown improved clinical outocmes (local/pelvic control, survival, morbidity). Although MRI is regarded as « gold standard » for IGABT, its wide applicability is limited by its availability, logistics and financial implications. Hence, use of CT and UltraSound (US) has been explored. 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. They are based on the concepts and terms as published in the ICRU report 89, defining the advanced standard approach with repetitive clinical examination at diagnosis (DG) and at BT with 3D documentation and with MRI at DG (MR_DG) and at BT (MR_BT) with the applicator in place. The following recommendations represent a first draft designed by the two first authors.

Development of CT based Recommendations: The minimum requirements for CT based contouring are clinical examination_DG,BT with 3D documentation and CT_DG  and CT with  applicator in place (CT_BT). The recommendations are based on GTV and CTV assessment  (clin exam, US, MRI), on classification of clinical remission patterns within various clinico-radiological scenarios.

1. Assessment of GTV and CTV_HR: The cornerstone for CT based target contouring is the repetitive clinical examination with a revised scaled diagram for documentation. The CTV_HR definition focusses on dimensions related to width, height and thickness. For width the new “Near Maximum Distance” (NMD) is introduced which is related to the cervical canal (os) and specified for each parametrium (left, right). The different volumetric imaging methods (MRI, CT, US, TRUS) are outlined with emphasis on strengths & limitations. Protocols for CT and US (TRUS) are suggested to define appropriately anatomical structures for contouring in the various imaging environments.

Uncertainties are associated with assessment of GTV at diagnosis (major for CT) and of GTV response (least with MRI). These uncertainties can be reduced by repetitive clinical examination and TRUS, beside MRI.

2. Classification of Clinical Remission: A classification of common clinical remission patterns is introduced (« restaging ») related to anatomical structures which are reproducible both on CT and on other assessment methods (clinical, MRI, US). For the CTV_HR definition 4 categories are defined (I_BT-II_BT-III_BT-IV_BT) for the cervix, parametrium, vagina and uterine corpus (Table 1).

3. Definition of different clinico-radiological environments: Based on the availaibility of imaging modalities at DG and at BT these environments are classified into 3 major categories: CT_DG – CT_BT ; MR_DG – CT_BT ; MRI_DG – Pre BT MRI/CT_BT. Each environmenrt is divided into 2 sub-categories - with or without real time TRUS - depending on the use of real time trans-rectal ultrasonography during BT application (6 categories).

4. CT contouring recommendations for definiton and delineation of CTV_HR and OAR: CT based contouring recommendations were formulated in general for width, height and thickness of CTV_HR and elaborated in detail for the 4 categories of remission pattern classification related to cervix, parametrium, vagina, and uterine corpus for the 3x2 clinico-radiological environments (Figure. 1).

For CT_DG – CT_BT , GTV_CT contouring at BT imaging is not recommended, but is supported for the other environments. The definition of width, height and thickness of CTV_HR on CT imaging is mandatory for all environments, but represents a challenge and accounts for major uncertainties and inter-observer variations, in particular in CT_DG – CT_BT . These shortcomings can be minimized through repetitive clin exam with clinical documentation and more valid and reliable volumetric imaging_DG/BT (TRUS, MRI), all classifying systematically the clinical remission patterns.

For CT based OAR contouring, a reproducible organ filling status, preferably empty, and defined protocols of contrast within the organs are vital, especially for bladder and recto-sigmoid. The major OAR’s are rectum, bladder, sigmoid and bowel.

Discussion: For each clinico-radiological environment there is an attempt to minimize the specific uncertainties in order to arrive at the best possible contouring accuracy. CT based target (OAR) contouring recommendations based on 4 remission categories within 6 defined environments aim at improving the contouring accuracy for IGABT using CT, US MRI as available. They will be further discussed in international expert rounds during the next months and then decided through IBS, GEC ESTRO (ACROP), ABS before publication.

Evaluating feasibility and reproducibility of these recommendations and further clinical research on clinical outcome for CT Based IGABT following these recommendations will become the next steps.