Stereotactic
Body Radiotherapy (SBRT) for cardiac arrythmias, also called cardiac
radioablation (CR), is a new and promising therapy for patients with refractory
ventricular tachycardia (VT) and potentially atrial fibrillation (AF). CR presents
unique clinical and technical challenges and close interdisciplinary
collaboration and interaction between cardiology and radiation oncology is paramount
to perform safe and effective treatments. Medical physics plays an important
role in tackling these new challenges and could function as a bridge for the
different physical, technical and biological perspectives in the respective specialties.
In the past
15 years, numerous pre-clinical studies have investigated the possible
application of focused radiation to the heart to treat arrythmias using photon
and particle beams and a large variety of technical equipment and treatment
methods. Worldwide, more than 100 VT and 5 AF patients have been treated with
CR with various target lesions, dose distributions and motion management
strategies mainly using photons with different robotic, c-arm, and MRI linear
accelerators. Recently, the first case using proton radiotherapy for CR was
reported. Today, more than 60 treatments have been presented in clinical-trial,
case-series and case-report publications and recently were summarized together
with comprehensive pre-clinical work in several systematic and narrative reviews
[1,2]. Besides different technical and clinical solutions for CR, we also find
large variations in patient selection, treatment outcome with respect to
efficacy and toxicity and reporting of the cases.
The
treatment workflow for CR is very comparable to standard SBRT. However, there
is an additional need to incorporate electrophysiology and/or scar anatomical
imaging for target delineation. The mapping systems used in electrophysiology
are not compatible with radiation oncology systems, yet. Some technical
solutions have been developed by various researcher and developers worldwide
[3,4], but standardization of these processes seems still far away.
Furthermore, both respiration-induced and cardiac-induced target motion needs
to be carefully assessed and compensated. Respiratory motion management
techniques such as the internal target volume (ITV) concept (with and without
abdominal compression), gated deliveries (with and without breath hold) and
tracking techniques (robotic, gimbaled, collimator, etc.) are well established
for SBRT and were also reported for CR [2]. Cardiac motion is of higher
frequency and smaller magnitude than respiratory motion and only limited data
exists on the requirements and suitability of active motion compensation and its
impact on the dose distribution. Direct tracking or gating or dose calculation
for cardiac or combined cardio-respiratory motion has been conceptually
proposed or utilized in phantom studies [5] but have not yet been used in
clinical treatments.
In summary, many open questions remain on the detailed mechanisms of
focused radiation in the diseased human heart, the minimal dose needed for each
of the different clinical case scenarios and the optimal planning and delivery
options for CR as well as harmonized reporting standards for target definition
and technical/dosimetric treatment parameters. These questions will be
addressed in ongoing clinical multi-center trials [6] and in a large EU
funded Horizon 2020 project called STOPStorm. Medical physics will play an
essential role in addressing these open questions.
[1] van der Ree MH, Blanck O, Limpens J, et al. Cardiac radioablation-A
systematic review. Heart Rhythm. 2020;17(8):1381-1392.
[2] Lydiard PGDip S, Blanck O, Hugo G, et al. A Review of Cardiac
Radioablation (CR) for Arrhythmias: Procedures, Technology, and Future
Opportunities. Int J Radiat Oncol Biol Phys. 2021;109(3):783-800.
[3] Hohmann S, Henkenberens C, Zormpas C, et al. A novel open-source
software-based high-precision workflow for target definition in cardiac
radioablation. J Cardiovasc Electrophysiol. 2020;31(10):2689-2695.
[4] Brownstein J, Afzal M, Okabe
T, et al. Method and atlas to
enable targeting for cardiac radioablation employing the American heart
association segmented model. Int J Radiat Oncol Biol Phys. 2021;S0360-3016(21)00313-8.
[5] Poon J, Kohli K, Deyell MW, et
al. Technical Note: Cardiac
synchronized volumetric modulated arc therapy for stereotactic arrhythmia
radioablation - Proof of principle. Med Phys. 2020;47(8):3567-3572.
[6] Blanck O, Buergy D, Vens M, et al. Radiosurgery for ventricular
tachycardia: preclinical and clinical evidence and study design for a German
multi-center multi-platform feasibility trial (RAVENTA). Clin Res Cardiol.
2020;109(11):1319-1332.