Base to apex.
Top Line: Time to dust off that old copy of Pocket Medicine.
The Studies: Cardiac radioablation (CRA) is evolving to be a potential non-invasive treatment option for refractory ventricular tachycardia. As with any type of ablative radiation, target volume delineation is key. For CRA, the target is defined by myocardial electrical activity—not necessarily a visible lesion. Electrophysiologists identify electrical sources (i.e., arrhythmogenic substrates) of ventricular tachycardia (VT) using electroanatomic mapping and cardiac imaging. But to perform CRA, this topographical map of electrical activity must be translated to an anatomic target volume. This is usually done with an ECG-gated cardiac CT fused to a respiratory-gated radiation planning CT. And that’s where interdisciplinary communication is key. In fact, a benchmarking study for the ongoing RAVENTA trial found that, although VT substrate identification was highly consistent among centers, there was considerable variation in the CTVs generated from those substrate maps. So, here are two helpful papers describing 1) the CTV delineation process for the RAVENTA trial, and 2) a standardized approach to CRA CTV delineation that’s based on the American Heart Association 17-segment model of the left ventricle. Fortunately for us, the left ventricle is a conical structure with an inferior apex and superior base (sans seminal vesicles). The framework provides a step-by-step process for re-orienting the treatment planning CT and fused cardiac CT to a cardiac-specific coordinate system with its axis along that of the left ventricle. The axial plane becomes the short-axis view, and the left ventricle is then divided into 4 regions and 17 segments: true apex (segment 17), apical region (segments 13-16), middle region (segments 7-12), and basal region (segments 1-6). This tedious process allows the radiation oncologist and cardiologist to effectively communicate about the location and extent of the target volume.
TBL: Get acquainted with cardiology-specific cardiac anatomy to prepare for the future of cardiac radioablation. | Boda-Heggemann and Brownstein, Int J Radiat Oncol Biol Phys 2021