Session Item

The role of MRI in radiotherapy is increasing due to the superior soft-tissue contrast offered by MRI compared to X-ray imaging. When developing an MR-Only radiotherapy pathway with conventional treatment machine delivery, the validation of a synthetic CT (sCT) is only the start of the process. This paper presents the routine clinical implementation of MR-Only radiotherapy for prostate cancer patients, believed to be the first using MR-kV CBCT soft tissue matching on a conventional treatment machine.

Patients received a planning MRI scan (1.5T Somatom Espree, Siemens, Germany) using a flat couch top, lasers and a coil bridge. In-house manufactured silicone dots were used as skin reference marks on the patient. Two MRI sequences were acquired: a 2D T2* weighted small field-of-view (FOV) image for prostate delineation and a 3D T2 weighted large FOV scan for sCT generation and OAR delineation. A sCT was produced using MriPlanner (Spectronic Medical, Sweden). A VMAT treatment plan was produced on the sCT using RayStation (RaySearch Laboratories, Sweden) according to standard local practice. RadCalc (Lifeline Software Inc. USA) was used to provide and independent MU check. The treatment plan and structure set were copied from the sCT onto the large FOV MRI which had been DICOM relabelled as a CT to permit transfer to the treatment machine. Treatment delivery was performed using an on-line soft-tissue match comparing the relabelled MRI to a kV-CBCT on a TrueBeam linear accelerator (Varian, USA).

The patient workflow is shown in Fig 1.

Early experiences allowed refinement of the procedures. Greater care was required when setting up the patient on the MRI scanner to ensure that the entire skin contour was visualised. Bladder preparation time was reduced by 5 minutes to ensure that the bladder was not over-filled during MRI acquisition. Delineation training and guidelines were revised to improve MRI delineation accuracy and reproducibility. Treatment radiographer training provided prior to implementation included anatomy training from a physician, benchmark MR-CBCT matching and off-line matching to a library of example datasets. This allowed efficient and effective on-line soft tissue matching using MRI anatomy as the reference image set, illustrated in Fig 2.

Routine implementation of an MR-Only pathway using a conventional treatment delivery machine with X-ray IGRT has been successfully demonstrated. Building on the foundations of rigorous technical validation of the sCT calculation and MR based treatment verification, a crucial key to smooth implementation was multi-disciplinary engagement and expertise-sharing across the whole multi-disciplinary team. Despite treatment radiographers having no prior MRI experience, MR-CBCT matching was found to be achievable and repeatable with a minimum of training. The workflow was enthusiastically received by all staff groups and was perceived to be straightforward and efficient.