Part of a radiotherapy treatment plan to ablate a heart to treat ventricular tachycardia (TV) in the ENCORE-VT trial.
Radiation is usually something discussed in cardiology in regards to how it needs to be reduced in the cath lab and in cardiac computed tomography (CT) imaging. It is also discussed in regards to the long-term health problems it causes for interventionists, and how it damages the heart in cardio-oncology patients who received radiation therapy for targets in the chest near the heart. However, researchers have been busy in recent years to see if that cardiac damage from radiotherapy can now be used as a noninvasive cardiac ablation method in electrophysiology (EP).
I also cover radiation oncology for our sister publication Imaging Technology News (ITN) magazine and regularly attend the large American Society of Radiation Oncology (ASTRO) annual meetings. So, I found it extremely interesting when sessions and late-breaking studies on this research began to be presented on this concept over the past few years. It also has been discussed in sessions at the Heart Rhythm Society (HRS) and the Society of Cardiovascular Computed Tomography (SCCT).
Here is a quick overview of why I feel this technology is a very promising.
Advantages of Radiotherapy Ablation vs. EP Catheter Ablation Procedures
First, it holds the promise of completely noninvasive EP ablation procedures. It could eliminate the need for surgical or catheter based ablations by using external beam radiation oncology systems. These linear accelerators use high energy photon (X-ray) energy to damage cancer cells, mostly by breaking apart their DNA strands on a molecular level. The same thing can be done to cardiomyocytes that are the source of arrhythmias.
Current technology to kill these heart cells include radio frequency, cryo and laser ablation devices. But the procedures tend to be long because of the need to first electromap the heart’s electrical activity. This is needed to pinpoint the source of errant electrical signals causing an arrhythmia so the lesions created can be minimized to avoid more damage than necessary. Because the heart is always moving, the pressure used and the energy delivered to create good lesions sometimes varies in catheter ablation. Using point-to-point ablation techniques to connect a series of ablation dots also can be difficult since it is done without direct visualization. This means catheter ablations are not always an exact science, and gaps can remain between ablation points used to isolate an area of cardiac tissue, allowing bad electrical impulses to continue to cause problems. About one third of ablation patients need a second ablation procedure.
In terms of the long-term health problems X-ray radiation causes for EPs, this technology can remove them completely from the radiation field.
Radiotherapy might be able to greatly shorten procedures by using noninvasive CT combined with ECG mapping techniques. The actual radiotherapy ablation would only take seconds and would not require catheterizing or anesthetizing a patient.
From a patient prospective, the therapy would be a major improvement over current technology, being much faster and completely noninvasive. This would almost certainly result in higher patient satisfaction scores. But, from the care team prospective, it will actually be more involved. It will require a lot of behind the scenes preplanning to convert the electro mapping information and CT datasets into a treatment plan that can be used by the radiotherapy system.
The Basics of Radiation Therapy Treatment Planning
Radiation oncology has become very precise in the past decade or so, being able to target very small tumors of just a few millimeters. These treatments are also designed to avoid damaging healthy tissues through physics preplanning.
The technology uses CT scans to identify the area to be treated. A radiation physicist uses these scans in treatment planning software to develop an optimized plan for how to deliver a prescribed dose of radiation. This includes breaking the prescribed dose to needed kill a cancer cells into fractions of the total dose. These are delivered using a set of different radiation beam lines from different paths and directions in the body, so that only one location (the target cancer) receives the total radiation dose. The beam lines are positioned to avoid critical structures such as the spinal cord, or other radiosensitive organs.
Complex calculations need to be done on each beam line to account for the radiation absorption and beam blocking capacity of all the tissues the radiation beam passes through, such as bone, air in the lungs, muscle, fat, etc. For those familiar with CT scans, this is based on tissue Hounsfield unit measurements, which have a correlation to electron density mapping use in these radiotherapy treatment plans.
Patient positioning in radiotherapy is also extremely important, requiring patients to be in the exact position they were in for the CT scan as the day of their therapy deliver the treatment table. Most radiotherapy systems now have on-board imaging systems to create radiographic images so the patient can be precisely aligned to anatomical landmarks in the original CT planning images.
All this planning for EP ablations will require close cooperation in a EP-radiotherapy heart team, composed of the electrophysiologist, radiation oncologist, radiation physicist and the support team from both EP and radiation oncology.
EP Radiotherapy May Reprogram Heart Cells
The Washington University School of Medicine in St. Louis showed in 2017 that radiation therapy can be directed at the heart to treat ventricular tachycardia to reproduce the scar tissue usually created through catheter ablation. Surprisingly, the the researchers found patients experienced large improvements in their arrhythmias a few days to weeks after radiation therapy, much quicker than the months it can take scar tissue to form after radiation therapy, suggesting that a single dose of radiation reduces the arrhythmia without forming scar tissue. The data indicated that radiation treatment worked just as well, if not better, than catheter ablation for certain patients with ventricular tachycardia, but in a different and unknown way.
New research from Washington University published in September 2021 suggests that radiation therapy can reprogram heart muscle cells to what appears to be a younger state. Researchers say this may fix electrical problems that cause life-threatening arrhythmias like ventricular tachycardia without the need for invasive ablation procedures.
The researchers said the radiotherapy appears to reprogram the heart muscle cells to a younger and perhaps healthier state, fixing the electrical problem in the cells themselves without needing scar tissue to block the overactive circuits. The study also suggests that the same cellular reprogramming effect could be achieved with lower doses of radiation, opening the door to the possibility of wider uses for radiation therapy in different types of cardiac arrhythmias.
Read more details on this study, see the article “Radiation Therapy Reprograms Heart Cells to Younger State.”
Other academic hospitals are also researching the use use of radiotherapy for cardiac ablation, including Brigham and Women’s Hospital in Boston. The large radiation oncology treatment system vendor Varian is working with some centers to develop the technology needed to transition the oncology technology for use in electrophysiology. Varian was recently acquired by Siemens Healthineers.
Related Cardiac Radiotherapy Content:
VIDEO: Cardiac Radiotherapy Ablation to Treat Ventricular Tachycardia — Interview with Clifford Robinson, M.D.
VIDEO: Use of Radiotherapy to Noninvasively Ablate Ventricular Tachycardia — Interview with Pierre Qian, MBBS