First clinical experiences with a high field 1.5 T MR linac

Abstract

Purpose: A 1.5 T MR Linac (MRL) has recently become available. MRL treatment workflows (WF) include online plan adaptation based on daily MR images (MRI). This study reports initial clinical experiences after five months of use in terms of patient compliance, cases, WF timings, and dosimetric accuracy. Method and materials: Two different WF were used dependent on the clinical situation of the day; Adapt To Position WF (ATP) where the reference plan position is adjusted rigidly to match the position of the targets and the OARs, and Adapt To Shape WF (ATS), where a new plan is created to match the anatomy of the day, using deformable image registration. Both WFs included three 3D MRI scans for plan adaptation, verification before beam on, and validation during IMRT delivery. Patient compliance and WF timings were recorded. Accuracy in dose delivery was assessed using a cylindrical diode phantom. Results: 19 patients have completed their treatment receiving a total of 176 fractions. Cases vary from prostate treatments (60 Gy/20F) to SBRT treatments of lymph nodes (45 Gy/3F) and castration by ovarian irradiation (15 Gy/3F). The median session time (patient in to patient out) for 127 ATPs was 26[21-78] min, four fractions lasted more than 45 minutes due to additional plan adaptation. For the 49 ATSs a median time of 12[1-24] min was used for contouring resulting in a total median session time of 42[29-91] min. Three SBRT fractions lasted more than an hour. The time on the MRL couch was well tolerated by the patients. The median gamma pass rate (2mm,2% global max) for the adapted plans was 99.2[93.4-100]%, showing good agreement between planned and delivered dose. Conclusion: MRL treatments, including daily MRIs, plan adaptation and accurate dose delivery is possible within a clinically acceptable timeframe and is well tolerated by the patients.

Feasibility of magnetic resonance guided radiotherapy for the treatment of bladder cancer

“…The time taken to deliver this workflow is comparable to the reported workflow treatment times for other tumour types on this platform [2]. It was well tolerated by patients with no sessions requiring termination due to distress or discomfort.…”

“…The commercial availability of combined magnetic resonance imaging (MRI) and radiotherapy units has fuelled pursuit of the clinical applications of magnetic resonance image guided radiotherapy (MRgRT) [1][2][3][4][5][6]. MRI provides superior soft tissue contrast compared to standard onboard X-ray based imaging without the additional ionising radiation risk [7].…”

Problems and Promises of Introducing the Magnetic Resonance Imaging Linear Accelerator Into Routine Care: The Case of Prostate Cancer

“…Further, our respondents have identified concerns about software deficiencies and the relative longer treatment fractions. Technological development should focus on improving workflow and the automation of both imaging and treatment ( 12 , 36 , 47 ).…”

“…MR-Linac minimizes uncertainties about the actual tumor’s location, shape and the surrounding organs at risk, which may reduce adverse effects and in turn improve a patient’s quality of life ( 9 , 11 ). Second, clinical interest in MRI-guided radiotherapy in prostate cancer management has been increasing in recent years ( 10 , 17 , 36 ), and understanding the dynamics of its implementation is timely.…”

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“…The field is still in development as reviewed recently in [22]. As daily plan adaptation becomes the next step in development of high-precision radiotherapy, the speed of treatment planning becomes important for achieving a viable workflow thus emphasizing even more the relevance of automated planning [23]. This is already a reality in newer applications including both integrated MR treatment units (MRIdian, ViewRay Inc and Unity, Elekta AB) and cone-beam CT based online adaptive systems (ETHOS, Varian Medical Systems, Inc).…”