2 Principles of radiofrequency and microwave tumor ablation

Добавил:

Sekretar kiopkiopkiop18@yandex.ru t.me/Prokururor I Вовсе не секретарь, но почту проверяю Опубликованный материал нарушает ваши авторские права? Сообщите нам.

Вуз:

Ростовский Государственный Медицинский Университет

Предмет:

Медицина общая

Файл:

Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3658_Библиотеки_им_академика_М_И_Перельмана.pdf

Скачиваний:

Добавлен:

30.08.2026

Размер:

49 Мб

Скачать

☆

<<< < Предыдущая 12 / 352 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 > Следующая >>>

newgenprepdf

Contributors

Riad SalemMD

Department of Radiology, Section of Interventional Oncology, Northwestern University, Robert H.Lurie Comprehensive Cancer Center, Chicago, IL,USA

Lynn JeanetteSavic

Russell H.Morgan Department of Radiology and Radiological Science, Division of Cardiovascular and Interventional Radiology, Johns Hopkins Hospital, Baltimore, MD,USA

Constantinos T.Sofocleous

Department of Radiology, Section of Interventional Radiology, Memorial Sloan Kettering Cancer Center, NewYork, NY,USA

Stephen B.SolomonMD

Interventional Radiology Service, Department of Radiology, Memorial Sloan Kettering Cancer Center, NewYork, NY,USA

Michael C.Soulen MD FSIRFCIRSE

Department of Radiology, Abramson Cancer Center, University of Pennsylvania, Philadelphia, PA,USA

Govindarajan SrimathveeravalliPhD

Interventional Radiology Service, Department of Radiology, Memorial Sloan Kettering Cancer Center, NewYork, NY,USA

Ursina TeitelbaumMD

University of Pennsylvania, Philadelphia, PA,USA

Ashraf ThabetMD

Department of Radiology, Massachusetts General Hospital, Boston, MA,USA

Sean Tutton MDFSIR

Department of Radiology, Medical College of Wisconsin, Milwaukee, WI,USA

Vlastimil Valek MD CScMBA

Department of Radiology, University Hospital Brno and Medical Faculty, Masaryk University, Brno, Czech Republic

Aradhana M.VenkatesanMD

Radiology and Imaging Sciences and Center for Interventional Oncology, NIH Clinical Center, Bethesda, MD,USA

Bradford J.WoodMD

Department of Radiology, Center of Interventional Oncology, NIH Clinical Center, Bethesda, MD,USA

Hooman YarmohammadiMD

Department of Radiology, Memorial Sloan Kettering Cancer Center, NewYork, NY,USA

Section I

Chapter

Principles of oncology

Interventional oncology:The fourth pillar of cancercare

In the years since the rst edition of this book, interventional oncology (IO) has continued to establish itself as an essential pillar within the rmament of multidisciplinary oncologic care, alongside medical, surgical, and radiation oncology. e perception of IO has evolved from ignorance or skepticism into something that is taken for granted by knowledgeable oncologists, who now refer patients with the expectation that we will counsel appropriate image-guided therapies to oer optimal benet, and integrate that therapy into an oncologic care plan with our sister disciplines. is gratifying status is a result of years of eort to establish the credibility of our discipline. e path is now open to any practitioner of minimally invasive, image-guided therapy to build an IO practice.

starts with common services for cancer patients such as venous and enteral access, biopsies, and palliative procedures. ese basic services, although poorly remunerated and demanding little or no longitudinal care outside of the interventional radiology (IR) suite, are the initial point of contact with cancer patients and their referring oncologist. Viewed as a free IO consultation, every biopsy, chest port, or paracentesis provides an opportunity to oer more sophisticated services to manage the patient’s cancer, and to educate patients and fellow oncologists about the valueofIO.

mon to all clinical practices– an oce with examination rooms, staed by receptionists, secretaries, medical assistants, nurses, nurse practitioners, physician assistants; an electronic medical record; and billing and pre-certication services. e number of support sta will equal or exceed the number of physicians. Yet this investment is essential to a robust and protable practice. IO services are more remunerative than other IR services over the long run and will evolve to dominate group income. Downstream professional revenue per new IO consultation, including imaging as well as therapeutic services, is extremely high. Cancer has surpassed cardiovascular diseases and is now the leading cause of death worldwide. Given the increasing role of IO, growth in the eld is guaranteed for the foreseeable future.

free-standing centers capturing global charges are an attractive

Michael C. Soulen and Jean-François H. Geschwind

Entry into an IO practice for an interventional radiologist

Establishing an IO practice requires an infrastructure com-

With hospital-based professional reimbursement declining,

alternative for many IO services. ese require substantial investment to create and sta the facility, and volumes must be maintained near capacity, but they oer the satisfaction of providing high-quality services outside of a hospital environment.

e grandest infrastructure will fail without an interventional oncologist who has the skill and the will to grow a practice. To be the fourth pillar of clinical oncology requires much more than the technical skills learned in fellowship. To earn the condence of referring physicians, interventional oncologists need to speak the language of oncology, and be able to care for cancer patients longitudinally. is requires an intimate knowledge of the science and practice of IO, expressed in language common to other cancer specialists. It also requires education in the fundamentals of the other oncologic disciplines, so that interventional oncologists convey understanding and respect for what they have to oer, and can communicate all options to their patients. Another requisite is familiarity with the widely accepted treatment guidelines for various cancers which, while oen not based on strong evidence, nonetheless form the basis for most treatment plans. is new edition of

Interventional Oncology:Principles and Practice of Image-Guided Cancer erapy incorporates the standard oncologic guidelines

for each cancer type to help prepare you for your role in this excitingeld.

Tumor Boards are the gateway to an IO practice. Market research has revealed that major barriers to referral for IO services are the lack of familiarity among medical oncologists about IO services and the people who oer them. Presence at Tumor Board is critical to overcome these barriers. ere are always patients for whom no good alternatives remain except IO therapies. Establishing credibility as a member of the team by oering care to these patients eventually leads to acceptance of earlier integration of IO services.

While skill and will are essential, data are the coin of the realm for oncologists. Unfortunately, image-guided therapies do not lend themselves well to trial designs familiar to and accepted by the medical oncologists who write the guidelines. Interventional oncologists have accepted these limitations inherent in the eld and have evolved away from single-institution, retrospective, underpowered reports that have no inuence on clinical practice. Along-term challenge

Interventional Oncology, Second Edition, ed. Jean-François H.Geschwind and Michael C.Soulen. Published by Cambridge University Press. © Cambridge University Press2016

Section I: Principles of oncology

for IO is to design and execute prospective, multicenter clinical trials that oer compelling outcomes that will change therapeutic paradigms. On that front, although IO still has a long way to go, tremendous progress has been made over the last decade. e bar has been raised by interventional oncologists to the point where procedures have been incorporated into accepted clinical guidelines through data provided by care-changing clinical trials. As a result, the credibility of IO has risen. One can take the example of primary liver cancer (hepatocellular carcinoma) where two therapies performed by interventional oncologists, ablation and chemoembolization, are part of the treatment guidelines worldwide. Not only are they included in

the guidelines, but the evidence to get these procedures to that level was the highest possible, i.e., level 1a. at is a remarkable achievement given that the eld of interventional oncology is stillnew.

Since then, and as a result of having raised the bar so high, many clinical trials have been conducted designed to answer extremely important and relevant clinical questions. e new norm consists of prospective multicenter phase II or III studies, either single-arm or randomized. Growing acceptance by the three other branches of oncology can only lead to improvement in patient care. Aer all, this is what all interventional oncologists desiremost.

Section II

Chapter

Principles of image-guided therapies

Principles of radiofrequency and microwave tumor ablation

Anthony M. Esparaz, S. Nahum Goldberg, and MuneebAhmed

Keypoints

• Radiofrequency(RF)ablationand/ormicrowaveablation areviablealternativesforthetreatmentofmanysolidfocal malignancies,especiallyinthenon-surgicalcandidate.

• Benetsofimage-guided,minimallyinvasiveenergy ablationincludelowmorbidityandmortality,lowcost, inclusionofnon-surgicalpatients,andsame-daydischarge.

• ebasicprinciplesofRFandmicrowaveablationcanbe dividedintothreemainelements,including:(1)biology ofheating;(2)RFandmicrowavetechnology;and (3)operatorexpertiseandtechnique.

• Successfultreatmentisobtainedwhenbalancingbetween completetumordestructionandminimizingdamageto surroundingnormalparenchymaandadjacentstructures.

• WidespreadadoptionofRFandmicrowaveablation willrelyuponimprovingandincreasingtumorablation volume.Adjuvanttherapiessuchaschemotherapeutics, antiangiogenics,embolization,andradiationholdgreat promiseinthiscapacity.

Introduction

Image-guided, minimally invasive tumor ablation aims to eradicateorsubstantiallydestroyfocaltumorsbyinducingirreversiblecellularinjurythroughtheapplicationofthermaland non-thermalenergyorchemicalinjection.1Ablativemodalities canbedivided intoenergy-basedablationandchemicalablation.While chemical ablation uses agentssuchas ethanol or aceticacidtoinducecoagulationnecrosisandtumorablation, energy-basedablativetechniquesdestroyatumorviathermal (heat or cold) or non-thermal techniques.1 Two examples of energy-basedmodalities,RFablationandmicrowaveablation, aretheprincipaltopicofthischapter.esetherapies,initially RFablationandnow,toanever-increasingextent,microwave, havegained widespread attentionin the medical community and have become broadly accepted as methods for treating focalmalignanciesinawiderangeoftumortypesandtissues. ese include primary and secondary malignancies of the liver,kidney,lung,andbone.

2–7

Giventhemultiplicityoftreatmenttypes,complexityofparadigmsinoncology,anddiverse application of energy-based ablation techniques, a thorough

understandingofthe basic principles and recent advances in RFandmicrowaveablationisanecessaryprerequisitefortheir eectiveclinicaluse(Figure2.1).

e ultimate goal of minimally invasive, energy-based tumorablationforfocalmalignanciesistocompletelyeradicate allviablemalignantcellswithinthetargettumor.Based upon tumorrecurrencepatternsandpathologicanalysesinlong-term studiesinpatientswho have undergonesurgical resection or ablation,there are oenviable persistent microscopictumor fociinarimofapparentlynormal surrounding parenchymal tissue beyond the visible tumor margin. erefore, tumor ablationtherapies also attemptto include a 5–10-mm “ablative”marginofnormalsurroundingtissueinthetargetzone, particularlyintheliverand lung.

8,9

erequiredthickness of thismarginisvariablebasedupontumorandorgantype,asa smallermarginmaybeneededforsometumorsinthekidney.

Additionally, given that completetumordestruction only occurswhentheentiretargettumorisexposedtoappropriate temperatures,asingleablationtreatmentislikelynotsucient toentirelyencompassthetargetvolumeofalargertumor(usuallydenedasgreaterthan3–5cmindiameter).10epattern oftissue heatinginthe targettumormustalsobeconsidered. us, multiple overlapping ablations or simultaneous use of multipleapplicatorsmayberequiredto successfullytreatthe entiretumorandachieveanablativemargin.

11,12

Finally,whilecompletetreatmentofthetargettumorisof primary importance, specicity and accuracy are also highly preferred,withasecondarygoalofincurringaslittleinjuryas possibletosurroundingnon-targetnormaltissue.isability tominimizedamagetonormalorganparenchymaisoneofthe signicantadvantagesofRFandmicrowaveablation,andcan becriticalinpatientswhohavefocaltumorsinthesettingof limitedfunctionalorganreserve.Examplesofrelevantclinical situationsincludefocalhepatictumorsinpatientswithunderlyingcirrhosisandlimitedhepaticreserve,patientswithfamilialmultiplerenal cellcarcinomassuchas vonHippel–Lindau syndromewhohavelimited renal function andrequiretreatmentof multiplerenal tumors, and those with primary lung tumorswithextensiveunderlyingemphysemaandlimitedlung function.

13,14

Manyofthesepatientsarenotsurgicalcandidates duetolimitednativeorganfunctionalreserve,placingthemat ahigherriskforpostoperativecomplicationsororganfailure.

Interventional Oncology, Second Edition, ed. Jean-François H. Geschwind and Michael C. Soulen. Published by Cambridge University Press. ©Cambridge University Press2016

Section II:Principles of image-guided therapies

Optimizing Tumor Ablation:

typesdieinafewminuteswhenheatedat50°C.17us,optimal temperaturestoensureablationlikelyexceed 50°C. However, tissuevaporizationoccursattemperatures>110°C.isinturn

Technique

Operator

limitsfurthercurrentdepositioninRF-basedsystems(ascomparedtomicrowavesystemsthatdonothavethislimitation).

Immediate cellular damage is principally due to protein

Technology

Adjuvants

Figure 2.1 Conceptualization of the key components necessary to achieve

optimal ablation. The three key components for achieving successful radiofrequency (RF) ablation include technology (i.e., the RF generator and electrodes selected); the biology of the tumor and background tissue; and

operator factors. Interfaces between technology and biology include adjuvant therapies that modulate these two factors. Technique deﬁnes the interface

between technology and operator, whereas patient selection represents the operator interacting with tumor biology.

Optimal

Ablation

Patient

Selection

Biology

coagulation of cytosolic and mitochondrial enzymes and nucleic acid–histone protein complexes, which triggers cellulardeathoverthecourseofseveraldays.18“Coagulation” is thestandardizedterminologyusedtodescribethegrosspathological appearance of treated tissue aer thermal damage. Nonetheless,theultimatemanifestationsofcelldeathmaynot meet strict histopathologic criteria of coagulative necrosis.19 ishasnotableimplicationsin clinicalpractice,aspercutaneousbiopsyandstandardhistopathologicinterpretation,particularlybystandardhematoxylinandeosinstaining,maynot beareliablemeasureofadequatetumorablation.

eexacttemperatureatwhichcelldeathoccursismultifactorialandtissue-specic.Studieshaveshownthat,dependingonheatingtime,rateofheatincrease,andthetissuebeing heated,maximumtemperaturesattheedgeofablationarevariable.Forexample,maximumtemperaturesatthe edge ofthe ablationzone,knownasthe“criticaltemperature,”havebeen

Indeed,theabilitytominimizedamagetosurroundingnormal tissueshasfueledtherapidacceptanceofRF andmicrowave ablation.

showntorangefrom30°Cto77°Cfornormaltissuesandfrom 41°Cto64°Cfortumormodels(a23°Cdierence).20Likewise, thetotalamountofheatadministeredforagiventime(thermal dose) varies signicantlybetween dierent tissues. us, the

Biology of heating

Radiofrequency ablation

RF ablation produces irreversible cellular injury by focal high-temperaturetissueheatinggeneratedaroundanRFelectrode.CurrentlyavailabledevicestraditionallyutilizedforRF ablationfunctioninthe375–500-kHzelectromagneticenergy range.1e mechanism of RF ablation is likened to a simple electrical circuit. Electrical current fromthegeneratoroscillatesbetweenelectrodesthroughionchannelspresentinmost biologicaltissues.etissueitselfservesastheresistiveelement ofthecircuit.Focalheatisgeneratedintheimmediatearea.As RFcurrenttravelstotheremotegroundingpadfromtheapplicatorand because tissues areimperfectelectricalconductors, local tissue resistance to current ow causes frictional ionic agitationandheat generation,knownastheJouleeect.e second mechanism of tissue heatingreliesuponthermal tissueconduction.15Heatgeneratedaroundtheelectrodediuses throughthetumorandresultsinadditionalhigh-temperature heatingthatisseparatefromthedirectenergy–tissueinteractionsthatoccuraroundtheelectrode.

e endpoint of RF ablation is tissue heating adequate to induce coagulation throughout the dened target area. Relatively mild increases in tissue temperature above baseline (40°C) can be tolerated by normal cellular homeostatic mechanisms, and low-temperature hyperthermia (42–45°C) resultsinreversiblecellular injury,whichcanincrease cellularsusceptibilitytoadditionaltherapiessuchaschemotherapy andradiation.16Irreversiblecellularinjury,ontheotherhand, occurswhencellsareheatedto46°Cfor60minutes.Mostcell

thresholdtargettemperatureof50°Cshouldbeusedonlyasa generalguideline.

Microwave ablation

eterm“microwave”describeselectromagneticenergyinthe 300-MHzto300-GHzrange,and becauseofthis,microwave ablationfunctionswithinthespectrumof,and isasubsetof, RF ablation.1 However, the mechanism of microwave tissue heatingand destruction isdistinct from that ofRF ablation. Microwaveheating is produced as a result of dielectric hysteresis,orrotatingdipoles.21Whenelectromagneticenergyis appliedtotissue,someofitisusedtoforcemoleculeswithan intrinsic dipole moment(i.e., water) to continuouslyrealign with the applied eld. is rotation of molecules represents anincreaseinkineticenergyandaresultantelevationinlocal tissuetemperatures.erefore,tissueswitha higherpercentageofwater,suchasmostsolidorgans,aremoreconduciveto microwaveheating. Conversely,less heatingoccurs in tissues withlowerwatercontent,suchasfat.22Forpracticalandregulatoryreasons,microwaveablationdevicesaretypicallyoperatedateither915MHzor2.45GHz.

Several componentscomprisea basicmicrowaveablation system:a generator,a power distribution system, and antennasthatmostoencontainacrucialcoolingsystemtoprotect against antenna sha heating.21 e energy from the microwavegeneratorisdeliveredviaacoaxialtransmissionline to aninterstitialantenna,whereitisthendeliveredintoavolume oftarget.Mostantennasutilizeastraightneedle-like design, although deployable loops have also been reported.23 ose with smaller diameters may have diculty handling higher

Chapter2:Radiofrequency and microwave tumor ablation

powers,with resultantthermal damage aroundtheproximal antennasha. erefore,coolingsystems, suchascirculation ofchilledwaterorsaline,orcoolingjacketsarecrucialtoprotect against skin burns. Moreover, active cooling allows the deliveryofhigherpowersforlongerperiodsoftime,resulting inlargerablationzones.

Tissue factors aecting RF ablation

Anunderstandingoftheeectofthetumorandorganbiophysiologicenvironmentontissueheatingiscriticaltoperforming successfultumorablations.Moststudiestodatehavefocused on the eects of tissue characteristics on RF ablation. For example,tissue-heating patterns vary based upon the tumor andtissuecharacteristicsthatmayaectthermalconduction.

e foremost factor limiting thermal ablation of tumors continues to be tissue blood ow, for which the eects are twofold:

1. Large-vesselheat-sinkeect.Larger-diameterbloodvessels

withhigherowactasheatsinks,drawingawayeitherheat

(orcold)fromtheablativearea.Forexample,inastudyin

anin-vivoporcinemodel,Luetal.examinedtheeect of

hepatic vessel diameter on RF ablationoutcome.24 Using

computedtomography(CT)and histopathologicanalysis,

more complete thermal heating and a reduced heat-sink

eectwereidentiedwhenhepaticvesselswithintheheat-

ing zone were < 3 mm in diameter. In contrast, vessels

>3mmindiameterhadhigherpatencyrates,lessendothe-

lialinjury,andgreaterviabilityofsurroundinghepatocytes

aerRFablation.

2. Microvascularperfusion.Anothereectoftissuevascula-

tureisaresultofperfusion-mediatedtissuecooling(capil-

laryvascularow),whichalsofunctionsasaheatsink.By

drawingheatfromthe treatmentzone,thiseectreduces

the volume of tissue that receives the required minimal

thermal dose for coagulation. Several studies have used

pharmacologicalterationoftissueperfusiontoreducethese

eects.Goldbergetal.modulatedhepaticbloodowusing

intra-arterialvasopressinandhigh-dosehalothaneincon-

junctionwithRFablationinin-vivoporcineliver.25Arsenic

trioxide has recently received increasing attention as a

novelantineoplasticagentthathasbeenshowntopreferen-

tiallydecreasetumorbloodowandsignicantlyincrease

RF-inducedcoagulationin a renaltumormodel.26Recent

dependent on local electrical conductivity. To this end, the eectoflocalelectricalconductivitycanoccurinseveralways:

1. Eect of diering tumor and surrounding organ electrical conductivity.Dierencesinelectricalconductivitybetweenthe tumorand surrounding background organ can aect tissueheatingatthetumormargin.Severalstudieshavedemonstrated increases in tissue heating at the tumor–organ interfacewhen the surroundingmedium is characterized byreducedlowerelectricalconductivity.29Incertainclinicalsettings,suchastreatingfocaltumorsineitherlungor bone, marked dierences in electrical conductivity may resultinvariableheatingatthetumor–organinterface,and indeed,limitheatinginthesurroundingorganandmake anappropriateablativemargindiculttoobtain.

2. Altering electrical conductivity of the target zone. Altering theelectricalenvironmentimmediatelyaroundtheRFelectrodewithionicagentscanincreaseelectricalconductivity priortoorduringRFablation.eincreaseinconductivity allowsgreaterenergy deposition and,therefore,increased coagulationvolume.30Saline mayalso be of benet when attempting to ablatecavitary tumors that might not otherwise contain a sucientcurrentpath.Ingeneral,small volumesof highly concentratedsodium ions are injected inandaroundtheablationsite tomaximizelocalheating eects.31However,itshouldbenotedthatsalineinfusion isnotalwaysapredictableprocess,asuidcanmigrateto unintendedlocationsand causecomplicationsif notused properly.Additionally,too muchsaline can increase conductivitytothepointthatlessheatingisachieved.

Anothercharacteristictoconsiderisatissue’sthermal conductivity. Initial clinicalstudiesusingRFablationforhepatocellularcarcinomainthesettingofunderlyingcirrhosisnotedan “oven” eect (i.e., increased heating ecacy for tumors surroundedbycirrhoticliver or fat, suchasexophyticrenalcell carcinomas),oralteredthermaltransmissionatthejunctionof tumortissueandsurroundingtissue.10Verypoortumorthermal conductivity limitsheat transmission centrifugallyaway fromtheelectrodewithmarkedheatinginthecentralportion ofthetumor,withthepotentialforlimitedheatinginthetumor periphery.Incontrast,increasedthermalconductivity(suchas in cysticlesions or tumorssurrounded by ascites) results in fastheattransmission (i.e.,heatdissipation),with potentially

incompleteandheterogeneoustumorheating. studies have demonstrated modication of tumor vesseldensityusing antiangiogenicagents,suchassorafenib, toincreaseRF coagulation. In one study,the administrationofsorafenibpriortoRFablationmarkedlydecreased microvasculardensityandledtosignicantlylargerzones ofRF-inducedcoagulationnecrosis.27Finally,pre-ablation intra-arterial microembolization (using 100–500-μm particles),either alone or as part ofperformingtransarterial chemoembolization(TACE),hasalsobeenusedtoincrease thesizeoftheablationzone.

Inadditiontotissueperfusionproperties,local electrical conductivity is a tissue characteristic that specically inuences energy deposition in RF-based systems, which are strongly

Tissue factors aecting microwave ablation

Relativepermittivity,eectiveconductivity,andbulkconduct-

ivityarethemostimportantpropertiesthatdeterminehowthe

biophysiologicenvironmentaectsmicrowaveenergypropa-

gationandtissueheating.Relativepermittivity,orthe“dielec-

tricconstant,”measureshowwellatissuewillacceptanelectric

eld compared to the relative permittivity of a vacuum.

isimpactsenergypropagationthroughatissue,withhigher

permittivities leading to shorter wavelengths. On the other

hand, eective conductivity refers to the rotationof dipoles

andmeasureshowwellaspecictissuewillabsorbmicrowave

energy. is is in contrast to electrical conductivity for RF

21,22



Section II:Principles of image-guided therapies

ablation,whichcharacterizesanalternatingowofelectrons.22 Describedearlier,tissuescomprisedofahigher percentageof waterreadilyabsorbmicrowavesandthushavehigh eective conductivities.A tissue’s bulk conductivity,whichreects the amountofenergy loss inside a material,mustbeconsidered formicrowave-inducedheating.SimilarlytoRF ablation,the rateof blood perfusion inatissueaects microwaveablation zonesize as heat isdrawnawayfromthe ablation zone periphery.Yet,somehavereported thatthis perfusion-mediated coolingislessthanthatobservedforRFgivenhigherstarting temperatures.

21,32,33

powersincreaseablationzonesize,butexcessivepowerinthe antennashacanleadtounintendedinjuriestoothertissues, suchastheskin.34Addingacoolingjacketaroundtheantenna canreducecableheatingandeliminateskinburnswhileeectivelyincreasingtheamountofpowerthatcansafelybedeliveredtothetumor.

RFablation,ontheotherhand, hassteadilygainedbroad acceptanceasatreatmentmodalityfor small (< 3 cm) hepatocellular carcinomas and colorectal metastases to the liver, althoughmanyofthe currentlyutilized devicesrequiremultipleinterstitialelectrodeplacements,whichinturnincreases the invasiveness of the procedure. While RF ablation is also

Radiofrequency ablation vs. microwave ablation– benets and trade-os

Whileeachenergy-basedablationtechniqueisdistinct,thegoal ofeachistoelevatetissuetemperaturesenoughtocreatezones ofirreversiblecellulardamage.RFenergyisrelativelyinexpensiveandeasy to generate,butis limited bytheneedforelectricalcurrentow.Forthisreason,RFsuersinareasofhigh bloodoworhightissueimpedance(e.g.,lung),andrequires electricalswitchingforeectivemultiple-applicatoruse.

Importantly, the chief distinction between RF andmicrowaveablationliesinthearea oftissueheatingproduced.22In RFablation,tissue heatingislimitedto areasofhighcurrent density,whileavolumearoundtheapplicatorantennaisheated in microwave ablation. erefore, an electrically conductive pathisnecessaryforRFheating,butnotformicrowaves,which can propagate through tissues with little or no conductivity. Essentially, low-conductivitytissues will allow better microwavepropagation,whilehinderingRFcurrent.

Microwaveheating is fast and ecientand,thus,appears betterequippedtoovercomeheatsinksandtreatlargetumor volumes.Microwavesarealsorelativelytissue-insensitiveand oerimproved multiple-applicatorsupport,butcanbe more diculttodistributethanotherenergysources.

As such, microwave energy has demonstrated several advantages for tissue ablation.

34,35

 Microwaves readily penetratethroughbiologicalmaterials, includingthose with low electricalconductivitysuchaslungandbone,anddehydrated or charred tissue. Consequently, microwave power can be continuallyappliedtoproducevery hightemperatures(over 150°C),whichimprovesablationecacybyincreasingthermal conductionintothesurroundingtissue.32Microwavesalsoheat tissuemoreecientlythanRFenergyintissue;microwavesdo notrequiregroundpads;andmultipleantennascanbeoperated simultaneously.33 In fact, due to the improved passive heatingof the tumor margin byincreased thermal gradients frommicrowavesandsucheectivepropagationofmicrowave energythroughnormallung,microwaveablationmaybeideallyttingforthetreatmentofpulmonarytumors.

Ontheotherhand,microwaveenergyisinherentlymore diculttodistributethanRFenergy.Microwavesmustbecarriedinwaveguides,suchascoaxialcable,whichare typically morecumbersome than thesmall wires used to feedenergy to RF electrodes and prone to heating when carrying large amounts of power. It is well known that higher microwave

gainingpopularityforthetreatmentofpulmonarytumors,it has met limited success.is is largelydueto the decreased ability of RF to penetrate through aerated, low-conductivity lungtissueandlimitationsinthermalconductionrequiredto createanadequateablativemargin.Infact, low conductivity andpoorthermalconductionarethesamelimitingfactorsfor RFablationecacyinbone.Inthekidneys,RFhasbeenused to eectively ablate small renal cell carcinomas.22 However, high perfusion ratesnearthe calyces andrenal hila produce heatsinksthat areproblematicforanyenergy-basedthermal ablationmodalities.

Energy-deposited technology

Development of energy-based, thermal ablation technology has,notsurprisingly, focused on the abilitytosafelyincrease andimproveenergy depositioninto target tissue and to reliably achieve larger ablation zones. Energy algorithms, electrodesandapplicators,andgeneratorscanbealteredtodeliver acomplexalgorithmofdieringmagnitudesandspectrumof RFandmicrowaveenergy.Overall,thesestrategieshavebeen balanced with the need for smaller-caliber devices, for their continueduseinaminimallyinvasivemanner.

Multiapplicatorarrays

e easiest way to increase the volume of coagulation is to lengthentheexposureoftheRFapplicatortip.However,this resultsina cylindricallesionshape thatdoesnotcorrespond wellwiththesphericalgeometryofmosttumors.Onemethod tocreatemoresphericalablationistomanuallyinsertasingle applicatormultipletimesinasingleprocedure. thisistime-consumingandcomplicated,makingitimpractical forroutineuseinaclinicalsetting.

us, using multiple conventional monopolar RF applicatorssimultaneouslyin a pre-setcongurationcanincrease ablationsize withoutprolongingtreatment time. no greater than 1.5 cm between individual applicators can produce uniform and reproducible tissue coagulation, with simultaneousapplicationofRFenergyproducingmorenecrosisthan sequential application.Infact, this arrangementcan increasecoagulationvolumebyover800%comparedtoasingleelectrode.

In similar fashion, multiple microwave antennas can be used simultaneously to increase ablationsize and utilize thermal synergy when placed inclose proximity to one another.21

37–39

However,

37,39

 Spacing

Chapter2:Radiofrequency and microwave tumor ablation

Alternatively,theantennasmaybewidelyspacedtoablatemore thanonetumorconcurrently.UnlikeRFablation,multiapplicator microwave ablation can be poweredcontinuously without switching between electrodes during activation. Additionally, antennascanbepositionedandphasedconstructivelytoexploit overlapoftheelectromagneticeld,whichallowsmoreecient heating and generation of higher temperatures than a single antenna.isfeatureisuniquetomicrowaveablation.Heating increasesproportionallytothesquareofthenumberofantennas. Moreover,thisincreaseinheatingissupplementaltothethermal synergyseenwithothermultiapplicatorablationtechnologies.

Multitine applicators

Workingtoovercomethe technicalchallengesof multiprobe application,whichrequiresmultiplepuncturesites,multitined expandableRFelectrodeshavebeendeveloped.esesystems involvethedeploymentofavaryingnumberofmultiplethin, curved tines in the shape of an umbrella or more complex geometriesfromacentralcannula. uteenergyspatiallytoimproveheatingeciencyandincrease total electrode surface area to ultimately create larger zones ofablationinashorterperiodoftime.issurmountsearlier diculties by allowingeasy placement of multiple probes to createlarge,reproduciblevolumesofnecrosis,suchas up to

3.5cmindiameterinin-vivoporcineliveroreven>5cmusing commerciallyavailableexpandableelectrodeswithoptimized stepped-extensionandpowerinputalgorithms. multitinedelectrodesaremoreinvasiveandmayincreasecomplication rates, especially in percutaneous settings, although relevantcomparisonsbetweendevicesarelacking.

One type of multitined electrode utilizes three single 17-gauge electrodes, spaced 5 mm apart in a triangular congurationanddriveninparallelbythesamegeneratorsource toeectivelybehaveasasingle,largerelectrodebutwithalimitedpuncturearea.Itcancreatezonesofablationover3cmin diameterinnormalliverin12minuteswitha200-Wgenerator.44 Othermultitinedelectrodedesignsdeployseveralsmallerelectrodesfromasingleneedlesha.Twosuchdesignsareclinically availabletodaythatcreateeitherstar-shapedorumbrella-shaped arrays.Star-shapedelectrodes are deployable froma 14-gauge (2.1mmdiameter)needleusingarraysoffour,nine,or12tines. Umbrella-shaped electrodes, on the other hand, contain ten tinesandaredeployedfroma13-gaugeneedle.esetinesare electrically connected and operated in parallel, which means that current owing through each tine can vary depending uponlocaltissueproperties. Deployableelectrodesarecapable

40,41

eirgoalistodistrib-

40–42

Ingeneral,

with the conventional monopolar system. tems, applied RF current oscillates between an active electrodetoa second interstitial groundingelectrode in place of a grounding pad, theoretically utilizing twice as much heat for ablation. Current ow is theoretically restricted primarily to the area between the electrodes and protectsthis area from perfusion-mediated cooling, resulting in faster, more focalheatingbetweenthe electrodes. is also eliminatesthe needforsurfacegroundingpadsandtheriskofgroundingpad burns.Bipolaroperationmayrequiremorepreciseplacement oftheelectrodestocreateaconuentzoneofnecrosisandcan belimitedbylocalchangesinconductivityresultingfromthe ablation.48 For this reason, bipolar systems oen use saline infusiontoincreaseenergydeliverybetweentheelectrodes.

However,theheatgeneratedaroundbothelectrodescreates ellipticallesions,andwhilethisresultsinanoverallincreasein coagulationvolume,theshapeofnecrosisisgenerallyunsuitable for tumorsthat are usually spherical, making thegains in coagulation less clinically signicant. Some experimental applicatorshaveutilizedacombinationofcryoablationandRF inanattempttocreatemoresphericallesionswhilealsoutilizingtheproposedbenetsofabipolarsystem.

Internally cooled electrodes

OneofthelimitationstogreaterRFenergydepositionhasbeen overheatingsurroundingtheactiveelectrode,leadingtotissue charring, rising impedance, and RF circuit interruption. To addressthis,internallycooledelectrodeshavebeendeveloped thatarecapableofgreatercoagulationcomparedwithconventional monopolar RF electrodes. cooledelectrodesdierfromperfusionelectrodes in thatthe coolingagent(saline,water,orgas)doesnotcomeintodirect contactwithpatienttissues.

Internally cooled electrodes contain two hollow lumens thatpermitcontinuousinternalcoolingofthetipwithachilled perfusate,andtheremovalofwarmedeuentto acollection unitoutsideofthebody.isreducesheatingdirectlyaround theelectrode,tissuecharring,andrisingimpedance,allowing greaterRFenergydepositionandresultantbroaderdepthoftissueheatingfromthermalconduction.Withthistechnique,RF energydepositedintotissueandresultantcoagulationnecrosisweresignicantlygreater(P < 0.001) than those achieved withoutelectrodecooling. torshaveused alternativecoolingagents(forexample,argon ornitrogengas)toachieveevengreatercooling,andtherefore largerzonesofablation,aroundtheRFelectrodetip.

37,40,44,50,51

50,51

Furthermore,severalinvestiga-

46,47

 In these sys-

 Notably, internally

ofcreatingzonesofablationapproximately5–7cmindiameter in30–45minutes,althoughcareshouldbetakenwhenevaluatingdeviceperformance,sincedeployabledesignshavealsobeen associatedwithirregularheatingpatterns.

Multitined applicators are currently in development for

microwaveablationplatforms.

Bipolararrays

Severalgroupshaveworkedwithbipolararraystoincreasethe volume of coagulation created by RF application compared

Perfused electrodes

Incontrasttointernallycooledelectrodes,perfusionelectrodes havesmallaperturesattheactivetip,allowinguids(i.e.,normalorhypertonicsaline)tobeinfusedorinjecteddirectlyinto thetissuebefore,during,oraertheablationprocedure.is mayimproveheatdiusionandelectricalandthermalconduction of the tissue. this technique to improve the quality of ablation produced bytheir applicator.

41,52,53

 At least one system clinically utilizes

41,53

 Sodium chloride(NaCl)will alterthe

Section II:Principles of image-guided therapies

tissuecharacteristicsinamannermorefavorableforablation through multiple factors. First, NaCl increases the electrical conductivity.Also,NaClpotentiallyimprovesthethermalconductionif theowrateishigh byaidingthediusionofheat fromthecentralelectrode.53ereisalsoareductioninchar-

whichtimethesystemwouldtypicallybeapplyingnoenergy torecuperate.Oneproposedtheoryisthat,becauseofelectricalinterferencebetweenelectrodes,thesimultaneousmethod leadstoless heatingatthe centerthanrapidswitching,when onlyasingleelectrodeisactiveatasinglepoint.

ringaswellthe“ushing”ofbubblesfromtheelectrodetract, whichformduringRFablationandareknowntolimitelectricalconductance.Lastly,hypertonicNaClsolutionshaveamild toxictumoricidalqualitywheninjectedalone.

Cooling in microwave ablation

Coolingsystemsthatsimultaneouslyincreasethepowerhandling of smaller-diameter antennas can reduce unwanted skinburnsandtissuedamagebymicrowaveantennasathigh powers.21 e most common cooling method is circulation of chilled wateror saline. is strategy has enabled delivery of higher powers for extended periods of time, with resultinglargerablationzones. One system (Certus 140, NeuWave Medical,Madison,WI)usestherapiddecompressionofcarbondioxidegas,causingtheJoule–ompsonphenomenonat theprobetipwithgasventingupthesha.High-powergenerators(140 W)andsmallsha diameterscanbe usedwiththis system.

ClusterRF

Baseduponsuccessininducinggreatervolumesofnecrosisby usingboth multiprobearraysandcooling,one standardsystemnowinvolvesthree2-cmtipinternallycooledapplicators spaced0.5cmapart,producingreproducibleablationgreater than3 cm in perfused liver.44is is one of themorepopularablationdevicescurrentlyusedintheclinicalsetting,with muchreportedliterature.

8,54,55

Operator and technique

erearedierentapproachestoablationtreatments,includingwhoperformsablation,howitisperformed,andtheimagingguidancedevices used, be it ultrasound (with or without contrast,whereavailable),computedtomography,and/ormagneticresonance.Allofthesewillhavesubstantialimpactonthe ultimateresults.

Ablativemargin

Toachievecompletetumordestruction,theablationofappropriatetissuemarginsbeyondthebordersofthetargettumoris essential.Formanyprocesses,particularlyintheliver,lung,and kidney,mostinvestigatorssettheablativemarginsat5–10mm. However, dataarecurrentlylackingtosupportdenitiverecommendationsforidealablativemarginsize.

Targettumors undergoing thermal ablation can be conceptuallydividedintothreezones:(1)acentralablationarea which undergoes heat-induced coagulation necrosis; (2) a peripheral rim which undergoes reversible changes from sublethal hyperthermia; and (3) surroundingtumor or normaltissuethatis unaected by focal ablation, although still exposed to concurrentand combinationtherapies.e area of maximum synergy between ablation and adjuvanttherapies is the peripheral rim immediately surrounding the high-temperatureablationzone.

eextentofinducedtumorcoagulationisideallyreported

in three dimensions, particularly including the short-axis

Pulsed RF application

Pulsingofenergy isanotherstrategy thathasbeenusedwith RFtoincreasethemeanintensityofenergydeposition.When pulsingisused,periodsofhigh-energydepositionarerapidly alternatedwithperiodsoflow-energy deposition. Ifaproper balancebetweenhigh-andlow-energydepositionisachieved, preferentialtissuecoolingoccursadjacenttotheelectrodeduringperiodsofminimalenergydepositionwithoutsignicantly decreasing heating deeper in the tissue. us, even greater energycanbeappliedduring periodsof high-energy deposition, thereby enabling deeper heat penetration and greater tissuecoagulation.56 Synergy between acombinationofboth internalcoolingandpulsinghasresultedingreatercoagulation necrosis and tumor destruction than either method alone.56 Pulsed-energytechniqueshavealsobeensuccessfullyusedfor microwaveandlaser-basedsystems.

diameter.Importantly,microwaveablationcancauseimmediatetissuecontractionduetocollagenandproteinremodeling, signicantwaterevaporation,anddehydrationwithintheablation zone.1us,ablationzonemeasurementsbyimagingor gross inspection could underestimate the pre-ablation tissue dimensions.

Choice of applicator

Animportantdecisionfortheoperatorinvolvesthechoiceof applicator. Proper geometric coverage that involvescomplete tumordestructionwhileminimizingnormaltissueispartially dependent upon choice of single or multiple applicators. Choiceofmultitineapplicatorinvolveswhen to use a cluster electrodeversusagivenmultitinedapplicator.us,thereare alsomanyvariationsintechnique described (orpracticed)as tohowtoadequately,buteciently,coagulateatumor.

40,55



Switching RF applicatorenergy

Several RF generator systems that enable switching among three electrically independent applicators have been created that generate signicantly larger ablation than simultaneous use.57 is relies upon switching energy to applicators duringtheimpedancespikesoftheremainingapplicators,during

Overlapping techniques

It is not uncommon that tumor burden exceeds the size of ablationthatcan be reproduciblycreatedbyasingleablative session,and,forthisreason,multipletreatmentsareoennecessary duringanRF ablationsessionforadequatetreatment. is requires readjustment of the applicators into untreated

<<< < Предыдущая 12 / 352 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 > Следующая >>>

Соседние файлы в папке Библиотека им академика М.И. Перельмана

#
30.08.202651 Мб0Книга_3653_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202650 Мб0Книга_3654_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202649 Мб0Книга_3655_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202649 Мб0Книга_3656_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202649 Мб0Книга_3657_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202649 Мб0Книга_3658_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202649 Мб0Книга_3659_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202649 Мб0Книга_3660_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202648 Мб0Книга_3661_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202648 Мб0Книга_3662_Библиотеки_им_академика_М_И_Перельмана.pdf
#
30.08.202648 Мб0Книга_3663_Библиотеки_им_академика_М_И_Перельмана.pdf