Multi-institutional experience treating patients with cardiac devices on a 1.5 Tesla magnetic resonance-linear accelerator and workflow development for thoracic treatments

Abstract

Background and purpose: Patients with cardiac implantable electronic devices (CIED patients) are often ineligible for online magnetic resonance-guided radiotherapy (MRgRT), most likely due to the absence of established guidelines. Existing radiotherapy (RT) and magnetic resonance imaging (MRI) guidelines offer an opportunity to construct MRgRT protocols, promoting equitable access. Our objective was to present such a workflow, share multi-institutional experiences treating CIED patients with MRgRT on a 1.5 T magnetic resonance-linear accelerator (MR-linac), and investigate geometric accuracy and electrocardiogram (ECG) monitoring for thoracic treatment.

Materials and methods: A risk analysis identified strategies for safe MRgRT for CIED patients. At three institutions, 21 pelvic and abdominal patients were treated. Patient records were analyzed for adverse events. Geometric accuracy was investigated using B₀-mapping with a phantom simulating moving lung and cardiac lesions near a CIED. Volunteer measurements evaluated the effects of patient positioning and MRI sequences on ECG signal distortion.

Results: MRI and RT workflows were adaptable to MRgRT. No adverse events were recorded. B₀-maps showed a maximum mean difference between static and dynamic phantom configurations of 0.1 mm, increasing to 0.4 mm distortion in the presence of a CIED. ECG readings exhibited severe distortions during scanning, hampering heart rhythm detection for most MRI sequences.

Conclusions: CIED patients can safely undergo treatment on a 1.5 T MR-linac following RT and MRI guidelines. For targets near CIEDs, a B₀-mapping procedure was considered accurate enough to determine MRgRT eligibility. Pulse oximetry is recommended for cardiac monitoring during MRI scanning due to ECG signal distortion.

Keywords: CIED; MR-linac; MRgRT.

Fig. 1

All acquired B0-maps contained phase-wraps that were unwrapped. For the CIED measurements, difference maps were calculated between the dynamic and each individual static B0-map. For the lead measurements, the static images were averaged prior to calculating a difference map with the dynamic image. For the CIED, images contain orange contours of the central sphere filled with liquid, simulating a lung tumor. For the leads, four purple contours are shown around the signal voids of the lead wrapped around an empty central cylinder. These contours were used for further analysis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Bland-Altman plots showing 50 % (left) and 95 % (right) of the data for the four CIED configurations (no CIED in setup 1 and a CIED 175, 105, and 85 mm from isocenter, respectively, for setup 2 to 4). Each plot contains data comparing B0-maps acquired for the static case (sphere of interest at −10, −5, 0, 5, and 10 mm from isocenter) to the dynamic (moving phantom) case with a 20 mm peak-to-peak amplitude.

Fig. 3

Mean difference (a) and the standard deviation of the difference (b) between the averaged B0-map for the static case and the dynamic (moving phantom) case for a range of distances from the signal void created by the lead.