Speaker
Description
MRLinacs provide the means of high resolution soft-tissue image contrast at the time of treatment to visualize the target and the surrounding normal tissue for image guided radiotherapy (RT).
The MRLinac is currently used for adaptation of the treatment based on the daily anatomical changes. This may lead to smaller margins and increased use of hypofractionation. The potential is to adapt the treatment not only based on anatomical changes but also biological response, e.g. observed from Diffusion-Weighted Imaging (DWI).
At Odense University Hospital (OUH), the MRLinac, UNITY, Elekta, was installed during 2018. The implementation of the MRLinac was performed in close collaboration with an international group of institutions, all having the UNITY system installed.
The local implementation is based on a multidisciplinary team of clinicians, physicists, and RTTs. All had experience in conventional RT in a department, where both CT and MR are used for RT planning. Most treatments are performed using VMAT and daily Cone Beam CT verification. All patients treated at the MRLinac are treated within clinical protocols focusing on treatment outcome and side-effects.
The MRLinac workflow differs from conventional RT, as it offers the opportunity of daily adaptation of the treatment. The workflow comprises of a pre-treatment workflow and an online workflow. During pre-treatment, a CT and an MR scan are acquired. All contouring is based on the MR scan, while the electron density information is linked from the CT to the MR scan using deformable image registration. In addition to the standard target and organ at risk (OAR) volumes used for the treatment optimization, a set of MR-Linac specific volumes are auto-generated:
• DelineationVolume: the volume close to the target, where OAR re-delineation is required on the daily MR scan to include the anatomical changes during the online plan adaptation.
• ActionVolume: the volume which determines if second adaptation is required.
• TrackingVolume: the volume that the CTV should stay within during irradiation.
A reference treatment plan is optimised in the treatment planning system, Monaco, based on the planning MR scan. This plan is independently checked and verified using the ArcCheck phantom. The reference-treatment plan is never used, it only serves as a starting point for the optimisation of the online created adaptive treatment plans.
The Online workflow is used after the patient is set-up in the MRLinac including the MR-coil positioning. A 3D MR is acquired, which is automatically fused to the planning MR scan. Based on the daily anatomy of the patient, the target structures from the initial pre-treatment planning scan may be shifted and rotated or possibly re-delineated while the OAR structure may require daily adjustment within the DelineationVolume. Based on the adapted contours, a new plan and dose distribution is optimised, and this new treatment plan is checked independently prior to delivery. While the delineation and re-planning are being performed, a DWI image sequence is acquired in the background for research purposes. Immediate prior to treatment, a fast 3D MR validation scan is acquired to evaluate whether the patient’s anatomy has changed during re-delineation and re-planning. If part of the target is outside the ActionVolume, the treatment beams are repositioned, and a fast re-calculation of the delivered dose is performed. Finally, during patient irradiation, a 2D MR scan is run in cine mode to validate the target position within the TrackingVolume. At the very last part of the treatment delivery, an additional fast 3D MR scan is acquired for evaluation purposes.
The total treatment time from the patient is entering the room to the patient is leaving the treatment room is in the range of 21-60 min.
Treatments on the MRLinac at OUH were initially only related to the pelvic region. However, currently, patients with abdominal cancers are also treated at the Unity system. Until 1st of July 2020 129 patients have been treated on the Unity system – most of the treatments are based on either a hypofractionation or SBRT scheme. Figure 1 shows the different sites treated and related fractionations; only prostate cancer patients are treated with 20 fractions.
Implementing the MRLinac in a clinical setting requires education of a multidisciplinary team. The institutions of the MRLinacs are accumulating clinical data in a joint registration protocol, that may form the basis for selecting future patients for MRLinac treatment based on the possible gains relative to standard linac treatment. The MRLinac gives the RT community a new opportunity to “see what is being treated while treating”. This can be used to evaluate, not only what is treated on the MRLinacs, but the knowledge can feed into margins and patterns of known anatomical movements during RT. The DWI research could provide biological response information, such that the treatments can be adapted based on the likely tumor and toxicity outcome of the individual patient.
| Country or Int. Organization | Denmark |
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| Affiliation | Laboratory of Radiation Physics, Odense University Hospital, Odense Denmark |
