The accuracy of Magnetic Resonance – Cone Beam Computed Tomography soft-tissue matching for prostate radiotherapy

Abstract

A B S T R A C TBackground and purpose: Magnetic Resonance (MR)-Only radiotherapy requires a method for matching image with on-treatment Cone Beam Computed Tomography (CBCT). This study aimed to investigate the accuracy of MR-CBCT soft-tissue matching for prostate MR-only radiotherapy. Materials and methods: Three patient cohorts were used, with all patients receiving MR and CT scans. For the first cohort (10 patients) the first fraction CBCT was automatically rigidly registered to the CT and MR scans and the MR-CT registration predicted using the MR-CBCT and CT-CBCT registrations. This was compared to the automatic MR-CT registration. For the second and third cohorts (five patients each) the first fraction CBCT was independently matched to the CT and MR by four radiographers, the MR-CBCT and CT-CBCT matches compared and the inter-observer variability assessed. The second cohort used a CT-based structure set and the third a MRbased structure set with the MR relabelled as a 'CT'. Results: The mean difference between predicted and actual MR-CT registrations was = ± R 0.1 0.2 mm All (s.e.m.). Radiographer MR-CBCT registrations were not significantly different to CT-CBCT, with mean differences in soft-tissue match 0.2 mm and all except one difference . This was less than the MR-CBCT interobserver limits of agreement [3.5, 2.4, 0.9] mm (vertical, longitudinal, lateral), which were similar ( 0.5 mm) to CT-CBCT. Conclusions: MR-CBCT soft-tissue matching is not significantly different to CT-CBCT. Relabelling the MR as a 'CT' does not appear to change the automatic registration. This suggests that MR-CBCT soft-tissue matching is feasible and accurate.

Patient position verification in magnetic-resonance imaging only radiotherapy of anal and rectal cancers

“…Our results are in line with those in the literature, whether comparing against the baseline findings from prostate studies [5][6][7][8][9][10] or the more relevant rectum study findings of Maspero [13] and Tyyger [14]. We found the systematic impact of sCT and MR on translations and rotations were <± 1 mm and <±0.5 • similarly to Maspero for sCT reference images.…”

“…Therefore MR-only treatment pathways must use either the MR-simulation or sCT instead. However, there is limited assessment within the current literature of CBCT registration accuracy [4] when using sCT or MR as the reference image, with the majority of those assessing prostate [5][6][7][8][9][10] CBCT patients, with Kemppainen [6] also assessing gynaecological patients. These assessments can be used as a bench mark level of acceptability for other pelvic sites with mean 3D translational differences between MR/sCT and CT of <±2 mm.…”

Technical Note: Efficient and accurate MRI‐only based treatment planning of the prostate using bulk density assignment through atlas‐based segmentation

“…The MRI image must be DICOM relabeled to be interpreted as a CT in the treatment delivery systems. It has been shown by Wyatt et al 21 that MRI‐CBCT soft tissue matching is not significantly different from CT‐CBCT matching which means that patient alignment using a relabeled MRI is accurate.…”

Cone beam computed tomography for dose calculation quality assurance for magnetic resonance-only radiotherapy

“…All patients received daily kilovoltage CBCT imaging using a TrueBeam STx (version 2.7 MR3, Varian Medical Systems, Palo Alto, USA), with a voxel size of , a tube voltage of and a field of view of . CBCT images were soft-tissue matched to the planning MR image by treatment radiographers for on-treatment image guidance [18] . This involved an automatic rigid registration between the CBCT and MR images, followed by a manual adjustment to ensure the prostate and seminal vesicles target as visually assessed from the CBCT were included within the PTV delineated on the MR. All patients were then shifted to the soft-tissue match position and treated.…”