Education: Minim

ally Invasive Cardiac A

rrhythmia A

blation: Use of R

adiosurgery

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Education:Minimally Invasive Cardiac Arrhythmia

Ablation: Use of Radiosurgery

PatrickMaguire,MD,PhDandThomasFogarty,MDCyberHeart Incorporated, Portola Valley, California

Stanford University, Stanford, California. USA

Ablationofcardiacarrhythmiashasprogressedoverthelastdecadeandthetechnologyisnowacceptedandutilizedinternationally. Avarietyofenergysourcesareusedincludingradiofrequency,cryothermy,laserenergy,etc.,allwithvaryingdegreesofsuccess. Mostcurrenttherapiesarecatheterbasedandinvolvetheuseofsophisticatedtechniques, includingintra-cardiacmanipulationofcathetersbyexperiencedclinicians.Improvementsinoutcomesarenoted,yet longtermreliefofarrhythmiainthecaseofatrialfibrillationandventriculartachycardiaiselusive.

Newablationtechnology(radiosurgery),usedinitiallyinthefieldofoncology,isminimallyinvasive,andcanprovidesaccurateplacementofbeamenergythatisguidedbyimaging. Ablationlesionscanbecreatedeveninconsiderationoftargetmotion.1,2Secondly,theenergydeliveryishighlyfocusedsoastominimizeanypossiblelongtermradiationeffectonotherstructuresnearby.

Radiosurgeryhasbeenpreviouslyused to treat theheart,albeit inoncologyapplications (tumorsmetastatictotheheart).3,4CyberHeartInc.(PortolaValley,CA)hasdeveloped3-Dtreatmentplanningsoftwareandtechnologytoallowtheprecisetargetinganddeliveryofablationtoanatomictargetsassociatedwitharrhythmia.5 Theconceptof thisnon- tominimally invasiveenergydelivery isintriguing,andhasthepotential toreducepatientproceduralrisk,reduceperi-proceduraldiscomfortandachieveequivalentefficacy.

Thetechnologyinvolvesuseofaradiosurgerysystem,suchastheCyberknife©(Accuray Inc.,Sunnyvale,CA)thathastheabilitytoplanaccurateradiationdosedelivery(<1.5mmaccu-racywithmotion)andsubsequentenergydelivery.Theadditionof three-dimensionalcardiacplanningsoftwareenables this technology toablate themyocardium in a precisefashion.Figure1showstheinstrumentandpatientset-up. Theenablingsoft-ware(CardioPlan®,CyberHeart,PortolaValley,CA)givestheplanningcliniciantheabilityto:1-directablationvolumeplacement; 2-examinetheplacementofdose relative tonearby structurestopreservesafety; and3-visualizethisin3-dimensionsbeforeever treatingthepatient. (Figure2). Oncea targetvolumehasbeenconstructed,otheror-ganscanbe‘protected’bynotallowingenergybeamstotraversewhilebeingdirectedat theclinical targetvolume.(Figure3).

Figure 1. Typical room set-up of the cyberknife radiosurgery

Figure 2. Panel to the right demonstrates the target (blue line) of the proposed ablation plan to create pulmonary vein isolation in 3-D. One can see the relationship of dose (red) to the other critical structures, such as esophagus (green) and bronchi (light blue). Panel on the left shows this treatment plan translated to the 2-D CT scan.

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Education: Minim

ally Invasive Cardiac A

rrhythmia A

blation: Use of R

adiosurgery

The requirements of this, or any newtechnologyareefficacyandasafetyprofile thatfits thebenefit-riskprofileof the individualandthedisease condition. Inorder toaccomplishthis, evidence of electrical conduction blockwith correlating histologic change had to bedocumented.

Inpre-clinical studies, lesioncreationhadtobeshowntocorrelatewithknownanatomiclesion sets that areused for the treatmentofarrhythmia. Electricalconductionblockhasbeendocumentedinthecavotricuspid isthmus,theAVnodeandthepulmonaryvein-leftatrial junctionwithcorresponding tissuechangeofcontiguousandtransmuralfibrosis.(Figure4andFigure5).

Inaddition, this technologymayhaveparticularapplicationindifficultorrefractorysituations, suchas the treatmentofdrugandcatheter-refractoryventriculartachycardia. Thisnewtechnologycanbeusedtotreatwhenendo-andepicardialtechniqueshave failed,and thecausative substrate isinaccessible. Cardiacradiosurgicalablationallowsforthecreationofaplanandenergydeliverytoadefinedareathatcanencompassdatafromfunctionalimagingtests(PET,‘positronemissiontomography’).(Figure6)

Theabilitytodelivernon-minimally invasivecardiacablationmayhave inherentadvantagesover catheterenergydelivery,whichrelyonperfecttissuecontact forcontiguity.Theabilitytoreliablycontrolandpredictlesionsizeforanindividualpatient’sanatomycanoffercliniciansalessinvasiveoptiontotreatthisgrowingpatientpopulationwithitshealthcarecostburden.

References

Figure 6. Superimposed PET scan image on CardioPlan © contouring software. View is from above looking into the apex of the left ventricle. Blue line documents location of planned ablation for ventricular tachycardia.

Figure 3. Typical Treatment Plan of the left atrium to create pulmonary vein isolation. Screen shot demonstrates an axial slice view showing ablation volumes.

Figure 4. Hematoxylin-eosin stain of the left atrium following radiosurgical delievery of 25 Gy.

Figure 5. Decapolar electrode recordings from a Lasso catheter in the Right Superior Pulmonary Vein showing dissociation (block) of conduction from a 10 mA electrical pulse to any intrinsic cardiac electrogram post ablation.

1.VanderVoortvanZypNC,Prevost JB,HoogemanMS,etal:Stereotactic radiotherapywithreal-timetumortracking fornon-smallcell lungcancer;clinicaloutcome.RadiotherOncol2009;91(3):296-300.

2.DieterichS,ClearyK,D’SouzaW,etal:Locatingandtargetingmoving tumorswith radiation beams. International J ofRadiationOncologyBiologyPhysics.2010;78:122

3.MartinAGR,ColtartDJ,PlowmanPN: Images inCyberKnife

radiosurgeryforanintracardiacmetastasis. BMJCaseReports2011;doi;10.1136/bcr.072010.3197

4.Soltys SG, KalaniMY, Cheshier SH, et al: StereotacticRadiosurgeryforaCardiacSarcoma: ACaseReport. TechnolCancerResTreat2008;7(5).

5.SharmaA,WongD,WeidlichG,etal:Non-invasivestereotacticradiosurgery(CyberHeart™)forthecreationofablationlesionintheatrium.HeartRhythmJ2010,7:802-810.