A retrospective inter-observer variability study was carried out to quantify the GTV delineation uncertainty from patients with RGC in the central pelvis. This study was approved by the appropriate Institutional Review Board (SE433) and written informed consent was obtained from all subjects.

Twenty patients with RGC in the central pelvis, having previously had a hysterectomy, were retrospectively selected for this study on the basis of having a treatment GTV >10 cm3, and with both CT and MR images available.

CT scans were carried out on a 16-slice scanner, with a 2.5 mm slice separation for 17 of the patients, 1 mm for two patients and 1.3 mm for one patient. MRI scans were carried out on 1.5 Tesla scanners acquired at a range of institutions. All patients had a T2-weighted turbo spin echo sequence acquired axially, except for patient 13 who had a T2-weighted three-dimensional space sequence. MRI slice separations depended on the clinical need at the time of scanning, ranging from 3.0 to 6.0 mm (mean ± standard deviation = 4.5 ± 1.1 mm).

Seven clinicians from a single institution participated in the study: two radiologists and two radiation oncologists specialising in pelvic radiotherapy with at least 4 years' experience, and three radiation oncologists training in pelvic radiotherapy with 1–3 years' experience. Each clinician delineated the GTV on each of the patient scans using local clinical protocols.

To avoid the CT information biasing the MRI-only outlines, delineations were first performed on MRI alone, followed by CT-MRI registered using rigid registration applied to the local soft-tissue anatomy around the GTV. Registrations were carried out by an experienced medical physicist, and then reviewed by a consultant radiation oncologist. Each of the observers was blinded to all other delineations by having their own dedicated image dataset and password to the contouring software. Delineations were carried out on the Eclipse Treatment Planning System, version 13.6 (Varian Medical Systems, Palo Alto, CA, USA).

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