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| REVIEW ARTICLE |
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| Year : 2022 | Volume
: 2
| Issue : 1 | Page : 6-10 |
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Stereotactic Arrhythmia Radioablation (STAR)
Nanditha Sesikeran Boindala
Department of Radiation Oncology, AIG Hospitals, Hyderabad, India
| Date of Submission | 25-Nov-2022 |
| Date of Acceptance | 02-Dec-2022 |
| Date of Web Publication | 31-Mar-2023 |
Correspondence Address:
Nanditha Sesikeran Boindala
Department of Radiation Oncology, AIG Hospitals, Gachibowli, Hyderabad- 500032
India
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/bjoc.bjoc_12_22
Ventricular tachycardia(VT) is a wide complex tachyarrhythmia which could be potentially life-threatening. Standard treatment options include catheter ablation, anti-arrhythmic drugs and ICDs (Implantable cardioverter defibrillator). In patients refractory to the above treatments, limited options are available. Stereotactic arrhythmia radio-ablation (STAR) is a novel non-invasive method of ablating the arrhythmogenic substrate using high dose of highly focused radiotherapy. This technique requires a multi-disciplinary team consisting of Cardiologist, Electrophysiologist, Radiation oncologist and Radiologist to integrate pre-treatment imaging and electro-physiological data to generate a target that can be radiated. Studies show excellent control of VT, 6–12 weeks post-treatment with no major toxicities. This article attempts to provide a practical step by step approach to stereotactic arrhythmia radioablation. Keywords: Stereotactic radioablation, substrate, electrocardiographic imaging
How to cite this article:
Boindala NS. Stereotactic Arrhythmia Radioablation (STAR). Bengal J Cancer 2022;2:6-10 |
| Background | |  |
Ventricular tachycardia(VT) is a characteristic wide complex tachyarrhythmia, with QRS duration more than 120miliseconds and heart rate more than 100 beats per minute [Figure 1]. Rapid stimulation results in ineffective pumping thereby causing symptoms of syncope and palpitations. It can sometimes result in cardiac arrest and death and hence warrants urgent treatment. Treatment options include electrical defibrillation in case of arrest and for long term control, options of Radiofrequency Ablation, Anti-arrhythmic drugs and ICD (Implantable Cardioverter Defibrillator). | Figure 1: ECG of ventricular tachycardia[1]
Source: https://www.aclsmedicaltraining.com/rhythm-recognition
Click here to view |
| Aetiology | | |
VT can occur as a result of
Ischemic heart disease (Acute, Chronic infarct)
Valvular heart disease
Cardiomyopathy
Myocarditis
Systemic diseases- ex Sarcoidosis
Dyselectrolytemia
Drugs
Congenital
In structural heart diseases, VT originates from myocardial scar tissue which is termed as “substrate”. When scar tissue is interspersed with some normal myocardial fibres, it is called a border zone. Slow and circuitous electrical conduction through the border zone, results in a re-entrant circuit causing a VT. Substrates located in the sub-endocardial region, can be ablated with a catheter. However, those substrates located deeper in the mid-myocardium and epicardium are not amenable to catheter ablation.
| Role and Rationale of Radiotherapy in VT | | |
Stereotactic Ablative Radiotherapy is a novel method of non-invasively ablating deeper substrates by inducing dense fibrosis in the border zones and thereby blocking the re-entrant circuit. Animal and human studies have demonstrated that radiotherapy indeed reduces VT. However, there has been some debate on the exact mechanism. Another hypothesis proven in animal models is that RT causes electrical conduction reprogramming by upregulation of cardiac sodium channels in the absence of trans-mural fibrosis.[2]
| Patient Selection | | |
The following criteria have been proposed to identify patients suitable for STAR[3]
Patients with structural heart disease:
≥3 episodes of sustained monomorphic VT
≥1 antiarrhythmic medication
≥1 catheter ablation (or has a contraindication to catheter ablation)
| Pre-treatment Work-flow | | |
The STAR work-flow includes pre-treatment imaging for anatomical scar as well as electrophysiological mapping to localize the substrate. Imaging includes some or all of the following:
ECG gated cardiac CT
MRI
Nuclear medicine imaging (SPECT/PET CT)
Electrophysiological mapping
ECG Gated CT: Acquisition of images in a specified phase of cardiac cycle where the motion is minimal helps in reducing blurring of images due to artifacts created by cardiac motion. Features such as reduced wall thickness (<5 mm), presence of hypo-attenuation and delayed contrast enhancement are characteristic of scar.[4]
MRI: Late Gadolinium Enhanced (LGE) MRI is the gold standard for identifying myocardial scar. It is found to have good correlation with electrophysiological imaging and histopathology. However, in the presence of ICDs, significant artefacts and possible device malfunction, precludes the use of MRI in most cases.
Nuclear medicine imaging: Technetium99 Sestamibi or F18-FDG PET can be used to identify non-viable myocardium.[5]
Electrophysiological mapping: This involves utilisation of electrocardiographic data obtained either from the surface or through invasive catheters, to localize the substrate. The techniques commonly used for STAR include:
12 Lead ECG
Invasive Electro-anatomic Mapping (EAM)
Non-invasive Electro-cardiographic Imaging (ECGI)
12 Lead ECG: Analysis of 12-Lead ECG of clinically documented VT, can help identify the site of origin based on QRS morphology in different leads.[6]
Invasive electro-anatomic mapping: EAM utilizes a catheter mounted electrode to record intra-cardiac electrical activity in relation to anatomic location in a cardiac chamber of interest.[7]CARTO mapping, EnSite Precision, Rhythmia HDx are some of the commercially available EAM systems. Although they are primary used to aid ablation procedures, the EAM data can be used in target localization for STAR.
Non-invasive Electro-cardiographic Imaging (ECGI): ECGI represents an inverse technique to determine non-invasively and with high resolution, the electrical activity of the heart from electrical data recorded on the body surface together with cardiac CT images [Figure 2].[8] | Figure 2: ECGI Procedure. Body surface potentials are recorded from 256 electrodes. Patient specific heart torso geometry is obtained from thoracic CT or MRI scan. The data are combined using mathematical algorithms to reconstruct pericardial potentials and unipolar electrograms on the heart surface. Maps of epicardial activation and recovery can be further derived from the electrograms[8]
Source: https://doi.org/10.1016/j.ijcard.2017.02.104
Click here to view |
| Radiotherapy Planning Work-flow | | |
Simulation
Patient is immobilized in supine position with abdominal compression using a wing board and vacuum assisted cushion. 4D Respiratory cycle CT is done with IV contrast. Patient is required to fast for 2hrs prior to the scan and oral contrast administered, if the target is close to the stomach.
Target volume delineation
Pre-treatment imaging and electrophysiological mapping information is integrated in order to generate the target for STAR. This can be done manually, by side to side comparison and clinician consensus. The 17-segment AHA heart model [9] is a useful reference to identify the location of the substrate using the various imaging modalities. Cardiac imaging integration platforms are a very useful tool to merge electrophysiological and anatomical data to enable accurate target delineation. MUSIC (Multimodality Platform for Specific Imaging in Cardiology) and Mimics are commercially available platforms that allow analysis of multi-parametric datasets and to interface such data with simulation platforms. They generate a three dimensional target, that is defined directly on the myocardial surface and a target volume is generated that is exported to create a DICOM (Digital Imaging and Communication in Medicine) RT (Radiotherapy) structure directly in the planning system [Figure 3].[10] | Figure 3: Integration of imaging and electrophysiological data to generate a 3 dimensional target that is imported directly into the planning system as a DICOM RT structure[10]
Source: https:// doi: 10.1136/openhrt-2021-001770
Click here to view |
The target is labelled as GTV and an ITV is generated using 4D CT images. A 5 mm margin is added to ITV to generate the PTV. The 4D average is used as the reference image for planning. OARs – spinal cord, oesophagus, stomach, trachea/main bronchi, chest wall, lungs and bowel are delineated. Specific Cardiac OARs – Great vessels, chambers, coronary arteries, valves and ICD are contoured to aid optimization.[11]
Planning and dose prescription
A plan is generated using 2 to 6 coplanar arcs to deliver a dose of 25Gy in a single fraction to PTV. OAR constraints are not well defined for central thoracic structures for such high doses, however based on Lung SABR experience and achievable constraints, the ENCORE-VT trial group have formulated OAR constraints that can be followed [Figure 4].
Treatment delivery
A dummy run is generally done a day before the planned treatment in order to assess the extent of motion and CBCT matching. Gating can be an option if the target is close to the stomach or respiratory motion is excessive. Gordon Ho et al. suggests a useful algorithm to decide the need for gating to optimize delivery of STAR.[12]
On the day of treatment, patient is required to fast for 2hrs. Cardiac electrophysiologist is required to be present at treatment delivery, to ensure adequate functioning of the ICD pre and post-treatment. A crash trolley should be available in the vicinity.
CBCT is done before each arc. Bony structures, ICD leads and left ventricular outline serve as landmarks to ascertain positional accuracy. Treatment is delivered in a single session.
| Safety and Efficacy | | |
Common acute side effects include fatigue, hypotension. Pericarditis, pneumonitis and gastric fistula have been reported as occasional toxicities.
Clinical evidence of STAR is limited mostly to small case series. However, results are promising with >85% reduction in VT after a 6–12 week blanking-out period.[13] The ENCORE-VT study showed that 94% of patients had reduction in VT, 6months post STAR with a 12 month overall survival (OS) of 72%.[14]
| Conclusion | | |
STAR is a novel non-invasive approach to treat patients who have failed multiple modalities of anti-arrhythmic treatment. A randomised study is unlikely to be done in view of the nature and selection criteria of patients. Based on safety and efficacy results so far, it is reasonable to implement a STAR programme under supervision of experienced centres or as part of a consortium. It is a truly multi-disciplinary undertaking involving radiology, cardiology, electrophysiology and radiation oncology teams.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
| References | | |
| 1. | https://www.aclsmedicaltraining.com/rhythm-recognition  |
| 2. | Zhang DM, Navara R, Yin T, Szymanski J, Goldsztejn U, Kenkel C, et al. Cardiac radiotherapy induces electrical conduction reprogramming in the absence of transmural fibrosis. Nat Commun 2021;12:5558. https://doi.org/10.1038/s41467-021-25730-0 |
| 3. | Cuculich PS, Schill MR, Kashani R, Mutic S, Lang A, Cooper D, et al. Noninvasive cardiac radiation for ablation of ventricular tachycardia. N Engl J Med 2017;377:2325-36. |
| 4. | Palmisano A, Vignale D, Benedetti G, Del Maschio A, De Cobelli F, Esposito A. Late iodine enhancement cardiac computed tomography for detection of myocardial scars: Impact of experience in the clinical practice. Radiol Med 2020;125:128-36. |
| 5. | Crean A, Khan SN, Davies LC, Coulden R, Dutka DP. Assessment of myocardial scar; comparison between F-FDG PET, CMR and tc-sestamibi. Clin Med Cardiol 2009;3:69-76. |
| 6. | de Riva M, Watanabe M, Zeppenfeld K.; Twelve-Lead ECG of Ventricular Tachycardia in Structural Heart Disease. Circ Arrhythm Electrophysiol. 2015;8:951-62. |
| 7. | Bhakta D, Miller JM. Principles of electroanatomic mapping. Indian Pacing and Electrophysiology Journal 2008;8:32-50. |
| 8. | Rudy Y. Noninvasive ECG imaging (ECGI): Mapping the arrhythmic substrate of the human heart. International Journal of Cardiology 2017;237:13-4, ISSN 0167-5273, https://doi.org/10.1016/j.ijcard.2017.02.104 |
| 9. | Cerqueira MD, Weissman NJ, Dilsizian V, Jacobs AK, Kaul S, Laskey WK, et al; American Heart Association Writing Group on Myocardial Segmentation and Registration for Cardiac Imaging. Standardized myocardial segmentation and nomenclature for tomographic imaging of the heart. A statement for healthcare professionals from the cardiac imaging committee of the council on clinical cardiology of the american heart association. Circulation 2002;105:539-42. |
| 10. | Lee J, Bates M, Shepherd E, Riley S, Henshaw M, Metherall P, et al. Cardiac stereotactic ablative radiotherapy for control of refractory ventricular tachycardia: Initial UK multicentre experience. Open Heart 2021;8:e001770. doi: 10.1136/openhrt-2021-001770. PMID: 34815300; PMCID: PMC8611439. |
| 11. | Duane F, Aznar MC, Bartlett F, Cutter DJ, Darby SC, Jagsi R, et al. A cardiac contouring atlas for radiotherapy. Radiother Oncol 2017;122:416-22. |
| 12. | Ho G, Attwood TF, Bruggeman AR, Moore KL, McVeigh E, Villongc CT, et al. Computational ECG Mapping and respiratory gating to optimize stereotactic ablative radiotherapy workflow for refractory ventricular tachycardia. Heart rhythm O2 2:511-20. |
| 13. | van der Ree MH, Blanck O, Limpens J, Lee CH, Balgobind BV, Dieleman EMT, et al. Cardiac radioablation-A systematic review. Heart Rhythm 2020;17:1381-92. |
| 14. | Robinson CG, Samson PP, Moore KMS, Hugo GD, Knutson N, Mutic S, et al. Phase I/II trial of electrophysiology-guided noninvasive cardiac radioablation for ventricular tachycardia. Circulation 2019;139:313-21. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
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