# The Fusion of MRI and CT in the Planning of Bracotherapy for Cancer of the Uterine Cervix
## 2 Materials and Methods  
The treatment files and imaging of all 53 patients who had been irradiated by image-guided adaptive brachytherapy (IGABT) [11] between January 2019 and August 2021 in the Department of Radiotherapy of the Hannover Medical School were evaluated retrospectively. Before a decision about treatment, MRI 1, of the pelvis, was acquired in the patients' hometowns using a broad variety of 1.5 T or 3 T MRI sequences, depending on the local radiologists' preferences [12]. The use of a gadolinium-based contrast agent was optional for pelvic imaging. All MRI 1 images were reviewed centrally by the same board-certified in-house-radiologist. For the staging of distant metastases, all patients received a chest CT. The upper abdomen was imaged with either contrast-enhanced CT or MRI. Patients were classified according to the TNM system, 8 th edition [13; 14]. A biopsy for histological proof of malignancy and to check HPV status was obtained from the tumor in all cases. According to gynecological guidelines [15], surgical staging of the pelvic lymphatic nodes was performed. All cases were reviewed by a board of in-house gynecologists, a pathologist, a radiologist, a radiation oncologist, and a medical oncologist to make treatment decisions and to allocate the patients to operations, radiotherapy, or chemotherapy, depending on TNM staging [14; 15]. Informed consent of each patient was obtained before treatment and for the later retrospective analysis.  
Patients allocated to radiotherapy were treated first with an external beam radiotherapy (EBRT) of 50 Gy in 25 fractions, 5 fractions per week, combined with a simultaneous chemotherapy consisting of Cisplatin 40 mg/m\({}^{2}\) weekly [16]. EBRT was 3D-planned on a pre-treatment CT-Scan using Monaco planning software (Elekta, Stockholm, Sweden) with Monte Carlo dose planning algorithm and was applied by a linear accelerator (Versa HD, Elekta, Stockholm, Sweden) with image guidance by cone beam CT (IGRT) and the volume modulated arc technique (VMAT). In the course of the EBRT series, preparation for IGABT [17] began by implanting a Smit sleeve into the cervical channel [18]. The clinical target volume (CTV) [19] for brachytherapy was contoured first in an MRI acquired before starting EBRT (MRI 1) and once more in a second MRI just before starting IGABT (MRI 2). Gross target volume (GTV) in MRI was contoured enclosing all macroscopic residuals of the tumor known from clinical examination and all suspicious T2-hyperintense areas within the uterus. CTV was contoured enclosing GTV and all residual T2-grey zones in parametric, uterine corpus or vagina, and the entire cervix. All contouring was performed in compliance with the recommendations of the Gyn-GEC-ESTRO Working group [20].Then, after inserting the intravaginal applicator (Vienna Tandem-Ring-Applicator or Geneva Tandem-Ovoid-Applicator, Elekta, Stockholm, Sweden), a helical CT-scan (Siemens Somatomatom 16 slices) was acquired, and the CTV was contoured in the CT. Contouring in CT was performed in knowledge of all MRI and clinical information, again in compliance with the recommendations of the Gyn-GEC-ESTRO working group [20]. Finally, the recordings of MRI 1, MRI 2, and CT were merged, and the congruence of CTVs was quantitatively evaluated. Calculating was performed within an Excel file (Microsoft, Redmond, WA, USA), which is available from the link below (Supplementary Materials). Merging was performed using Monaco software for treatment planning first (Elekta) and careful correction by hand until best possible concordance was obtained. To minimize inter-observer variability, all CTVs were contoured by the same board-certified radiologist [21] by mutual agreement with the executing brachytherapist. IGABT started an average of 4.2 weeks after the beginning of EBRT. MRI 2 was acquired in different sequences according to the choice of the local radiologists. Gadolinium contrast was optional in MRI 2.