Cardiac arrhythmia: treatment in the linear accelerator

Cardiac arrhythmia is a group of conditions where nerve cells trigger uncontrolled contractions of the heart muscle. They are treated with either medicine or catheter ablation of the tissue. In a recent interdisciplinary collaboration, cardiologists and radiotherapists took a different approach and used high-precision radiation therapy to treat a patient for whom the other options proved unfeasible.

In this interview with MEDICA-tradefair.com, Prof. Hendrik Bonnemeier and Prof. Jürgen Dunst talk about treating a patient with severe, treatment-refractory cardiac arrhythmia using high-precision radiation of the myocardium, they explain the backdrop and describe the new perspectives that open up as a result of this approach.

Prof. Bonnemeier, can you describe the clinical picture of the patient you recently treated for his heart arrhythmia?

Prof. Hendrik Bonnemeier: In the past, the patient had suffered from ischemic cardiomyopathy, a condition where portions of a ventricle experience a lack of blood supply due to narrowed arteries. This caused heterogeneous scarring in the anterior left ventricle. Due to the low cardiac output, the patient received an implantable defibrillator and had to take a maximum dose of antiarrhythmic drugs. Yet despite this treatment, the patient suffered shocks from the defibrillator every day, which drastically impacted his quality of life and that of his family members.

Why did you choose this particular treatment?

Bonnemeier: Cardiac arrhythmia is caused by impulses that uncontrollably loop around scar tissue in the ventricle, a so-called reentry (see devices for heart circulation diagnostics in the catalogue of MEDICA 2018). This palpitation causes circulatory instability. The typical weapon of choice is catheter ablation of the myocardial tissue in the ventricle to stop this from happening. In this case, however, the site of the reentry could not be accessed for catheter ablation due to its position. It is impossible to ablate cardiac muscle cells that are located this deep inside the tissue (find more information about vascular surgical instruments in the catalogue of MEDICA 2018).

After a positive vote by the ethics committee, we chose high-precision radiation as a personalized treatment attempt. My team and I collaborated with the team of Prof. Jürgen Dunst to perform the treatment.

What exactly happens when you subject the tissue to radiation?

Prof. Jürgen Dunst: When cells are subjected to high-dose radiation, they gradually die. The amount of delivered radiation is small to where it does not cause overheating or other immediate effects in the tissue, but the radiation damages the DNA. Radiation therapy for cancer - which involves multiple exposures to large doses - damages the DNA severely, making it impossible for cancer cells to repair the damage and they die if they want to divide.

For cancer cells, a one-time radiation dose of about 25 Gray is sufficient to safely kill all cells of a tumor. Meanwhile, cardiac muscle cells seem to survive this dose of radiation. It is likely that the radiation alters the function of the cells and affects the impulse formation and conduction that cause cardiac arrhythmia – but we are still unable to say how this process works exactly. So far, we can only observe phenomenologically that the patient is feeling better after a few short days.

In contrast, catheter ablation is a heat-based destruction method that scars or destroys the tissue. Radiation must have other effects, because if it were about the destruction of cells, the impact of the treatment would be a very gradual process that takes weeks and months to show.

How did you prepare for the treatment?

Bonnemeier: On the cardiological side, we created a high-resolution voltage map of the anterior left ventricle. That means we measured the conductivity of the cardiac tissue at 10,000 different points in the electrophysiology laboratory. This process allows us to differentiate between scarred and healthy tissue zones. The radiology department subsequently generated CT scans of the beating heart, which we combined with our voltage map to plan the radiation treatment (see information about nuclear diagnostics in the catalogue of MEDICA 2018).

Dunst: Two movements are usually relevant when you plan radiation therapy treatment: the respiratory and the heart motion of the patient. We know from animal testing that the respiratory motion is relevant and must be compensated. By comparison, heartbeats are small and minor motions. The respiratory motions can be recorded and compensated with modern radiation therapy equipment.

What device did you use for this treatment?

Dunst: We have an electron linear accelerator that is specially equipped for high-precision radiation therapy. It is the most modern and expensive device of its kind in Schleswig-Holstein. This is a standard radiotherapy device commonly used at academic medical centers and other major hospital facilities.

This was a personalized treatment approach. What are your next plans?

Bonnemeier: We are now conducting a multicenter, randomized, prospective trial. We want to engage a patient group with a similar profile as our first patient. We already submitted the study protocol to the ethics committee. We are still waiting for approval from the German Federal Office for Radiation Protection. I expect to include the first patients in the study in March or April of this year.