Radiotherapy Beyond Cancer - The MRI-Linac for cardiac targets

Atrial fibrillation - an epidemic in industrialized nations

The most common type of cardiac arrhythmia, atrial fibrillation (AF), is considered an increasing epidemic and a substantial economic burden in high-income countries. AF is caused by chaotic electrical signals inside the heart which interfere with the natural rhythm and lead to rapid and disorganized contractions.
Some facts on AF:

  • Prevalence: Estimates suggest that over 6 million Europeans, up to 7 million Americans and more than 400,000 Australians suffer from AF - numbers that are growing exponentially and likely underestimated because AF is often asymptomatic.
  • Cost of illness: In the United States, AF hospitalizations cost over 6.65 billion USD (2005), while the cost burden approached 13.5 billion Euro in Europe (2006) and 1.25 billion AUD in Australia (2009).
  • Consequences for the patient: AF rises the patient's stroke risk by up to five times, doubles their risk of death and significantly degrades their quality of life. Untreated AF worsens and eventually becomes chronic and far more complicated to treat.

The current gold standard to treat symptomatic AF patients is catheter ablation. During this complex procedure, several catheters are inserted into a vein in the leg or sometimes in the neck and guided into the heart. By thermally injuring - or ablating - small areas of the cardiac tissue it is possible to insulate the signals that cause AF and stop the arrhythmia.

One main disadvantage of catheter ablation is its invasive nature and concomitant risks like severe injuries to the heart or adjacent organs (e.g. the esophagus), blood clot formation that could lead to stroke and also major bleedings. Furthermore, the procedure is highly complex and requires several hours under anaesthesia. Patients too unwell to undergo such physical stress cannot receive this treatment.

Cardiac radiosurgery - A non-invasive treatment alternative

Cine TrueFISP sequence, coronal view of the left atrium

Real-time cine MRI scan of a healthy volunteer. The beating heart and breathing motion are clearly visible in the images.

Ablative doses of highly focussed ionizing radiation are already being used in Stereotactic Ablative Body Radiotherapy (SABR) for treatment of certain tumours, for example in the lung or liver. By adapting the targets from catheter ablation, we aim to provide an entirely non-invasive treatment alternative using radiotherapy combined with MRI guidance - the MRI-Linac.

Throughout the treatment, the heart of the patient will be visualized in real-time with MRI. The target is detected in the images in order to account for the occurring motion due to breathing and the heart beat. The measured target motion can then be compensated, for example by adapting the leaves of a multi-leaf collimator to adjust the radiation beam direction. The real-time MRI guidance facilitates accurate, non-invasive treatment delivery without exposing the patient to additional imaging dose.

We estimate the treatment time to be of much shorter duration than a typical catheter ablation session, therefore offering better patient comfort and potentially including a patient cohort that to date had to be excluded from ablative therapy.

Flowchart Catheter Ablation vs. MRI-Linac Ablation

Current procedural workflow of most catheter ablations (left) and the proposed workflow of an image-guided AF treatment with the MRI-Linac (right).

1. J. Ball, M. J. Carrington, J. J. V McMurray, and S. Stewart, “Atrial fibrillation: profile and burden of an evolving epidemic in the 21st century.,” Int. J. Cardiol., vol. 167, no. 5, pp. 1807–24, Sep. 2013.
2. H. Calkins, K. H. Kuck, R. Cappato, et al. “2012 HRS/EHRA/ECAS Expert Consensus Statement on Catheter and Surgical Ablation of Atrial Fibrillation (...)” Europace, vol. 14, no. 4, pp. 528–606, Apr. 2012.
3. A. J. Camm, P. Kirchhof, G. Y. H. Lip, et al. “Guidelines for the management of atrial fibrillation: the Task Force for the Management of Atrial Fibrillation of the European Society of Cardiology (ESC).,” Eur. Heart J., vol. 31, no. 19, pp. 2369–429, Oct. 2010.
4. O. Blanck, F. Bode, M. Gebhard, P. Hunold, S. Brandt, R. Bruder, M. Grossherr, R. Vonthein, D. Rades, and J. Dunst, “Dose-Escalation Study for Cardiac Radiosurgery in a Porcine Model.,” Int. J. Radiat. Oncol. Biol. Phys., in press, Apr. 2014.
5. Jun Dong and Hugh Calkins, "Technology Insight: catheter ablation of the pulmonary veins in the treatment of atrial fibrillation.," Nature Clinical Practice Cardiovascular Medicine, vol. 2, pp. 159-166, 2005.

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