Treating patients with a cardiac implant on a 1.5T MR-linac: a multi-centre experience
Rick Keesman,
The Netherlands
PD-0900
Abstract
Treating patients with a cardiac implant on a 1.5T MR-linac: a multi-centre experience
Authors: Rick Keesman1, Erik van der Bijl1, Linda Kerkmeijer1, Neelam Tyagi2, Osman Akdag3, Jochem Wolthaus3, Sandrine van de Pol3, Juus Noteboom3, Martijn Intven3, Martin Fast3, Astrid van Lier3
1Radboud University Medical Center, Department of Radiation Oncology, Nijmegen, The Netherlands; 2Memorial Sloan-Kettering Cancer Center, Department of Medical Physics, New York, USA; 3University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands
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Purpose or Objective
Patients with a cardiac implantable electronic device (CIED) might need MR-guided radiotherapy (MRgRT). Guidelines exist to mitigate CIED-associated risks for MRI examinations and radiotherapy, separately. MRgRT introduces two additional risks, particularly for thoracic cancer patients, namely, distortion of the electrocardiogram (ECG) signal due to MRI and degradation of image fidelity due to presence of metals. We present a clinical workflow that includes methods to assess these risks and share our clinical experience treating 7 CIED patients using MRgRT.
Material and Methods
Existing CIED protocols for MRI and conventional radiotherapy were adapted locally to create an MRgRT protocol for CIED patients at three different institutions (Fig. 1). Treatment slots were extended with 15 minutes to facilitate cardiac monitoring tasks. Eligible patients were included in this study.
An ECG was acquired with an Invivo Expression MR400 (Philips Healthcare) for a healthy volunteer during baseline and while acquiring motion monitoring MRI for pelvis, pancreas, and lung on a Unity 1.5T MR-linac (Elekta AB).
The efficacy of a B0 field-mapping procedure, to assess geometric fidelity of the MRI, was verified for moving objects. To this end, phantom (Quasar MRI4D, ModusQA Inc.) measurements simulating a moving lung target near a static CIED or CIED leads in (cardio)respiratory movement were used to compare field maps acquired in various static positions with the in situ dynamic situation.
Results
Three patients with pancreatic cancer and four patients with prostate cancer were included in this study and treated with MRgRT without adverse events. During MRgRT, the CIED was in an MRI-compatible mode which necessitated cardiac monitoring of the patient. Two institutions use a pulse oximeter for heart rhythm monitoring while one institution also acquired ECG.
At baseline, and when imaging the pelvic area, ECG was largely undisturbed. In contrast, ECG signal was unusable for pancreas and lung imaging because the electrodes were closer to the isocentre. Heartrate readouts were 70 bpm, 65 bpm, 142 bpm, and undetermined, respectively.
Field-mapping in presence of CIEDs showed that median absolute frequency differences between static and dynamic situations, directly related to geometric offsets, were all smaller than the absolute offsets, except when the CIED was closer than 8 cm to the target in which case signal voids would have prevented target visualization (Fig. 2). For the leads, none of the offsets outside the signal voids were clinically relevant.
Conclusion
MRgRT treatment of CIED patients using a 1.5T MR-linac is feasible and was applied successfully in seven patients.
Both pulse oximeter and ECG can be used to monitor patients with CIED during MRgRT, although imaging can distort the ECG signal substantially.
Field-mapping procedures can be used in moving targets nearby leads and CIEDs for geometric accuracy assessment of MRI during MRgRT, paving the way for treatment of thoracic cancer patients with a CIED.