Mri-only treatment planning for anal and rectal cancer radiotherapy (MANTA-RAY)

Introduction: A barrier to the widespread clinical implementation of magnetic resonance imaging (MRI)-only radiotherapy planning for pelvic sites is the limited assessment in the literature demonstrating technical achievability and benefit. The MRI-only treatment planning for anal and rectal cancer radiotherapy (MANTA-RAY) study aimed to validate synthetic-CT (computed tomography) model generation accuracy, show the viability of cone beam CT (CBCT) patient positioning using synthetic-CT and MRI as a reference image, assess the patient experience of radiotherapy MRI scans and quantify potential clinical benefits of MRI-only patient treatments for anal and rectal cancer sites. Methods: The MANTA-RAY study recruited 46 patients with anal and rectal cancers who received radiotherapy CT and MRI scans after informed consent. A deep learning synthetic-CT model was trained and validated in terms of dosimetric calculation accuracy. Differences in CBCT patient position registration accuracy were assessed when using synthetic-CT and MRI as the reference images. The patient experience of the MRI simulation was assessed. A planning study quantified the differences between MRI- and CT-only planned treatments. Results: The synthetic-CT model had excellent dosimetric accuracy (planning target volume (PTV) D95% dose difference to CT = 0.1%). MRI and synthetic-CT reference images had systematic differences to CT in all translational and rotational dimensions of <1 mm and <0.5 °. The radiotherapy MRI patient experience was found to be better or similar to the CT experience for the majority of respondents. MRI-only target volume delineations resulted in statistically significant GTV (gross tumour volume) and primary PTV volume reductions and treatment plan dose reductions to healthy organs compared with CT-only plans. Conclusions: These findings provide evidence that MRI-only treatment planning for anal and rectal cancers is technically achievable and accurate, can be clinically implemented without detrimentally affecting the patient pathway and could lead to improved patient outcomes if organ dose reductions translate into less treatment related toxicity.