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Principles of Tumour Ablation


Radiofrequency Ablation (RFA)

  • Monopolar RFA involves the application of high frequency (460-500 kHz) alternating current to the target tissue using a needle like applicator (dispersive grounding pads are attached to the patient's trunk / thigh) ► the resultant alternating electric field around the uninsulated probe tip causes ‘radiofrequency’ agitation of water molecules (inherently polarized) and local frictional heating within a few mm of the probe tip

  • Coagulative necrosis results if the target tissue is maintained at temperatures > 45°C (RFA can induce temperatures of 100–110°C within a few millimetres of the probe) – beyond this it relies on conductive heating

  • Reproducible 3-5 cm spheres of tissue destruction can be achieved within 15-20 minutes ► larger ablation volumes can be achieved by ‘clustering’ needles on a single-hand piece, expandable multi-tined devices or via multipolar arrays ► rapidly switching multiple electrode solutions simultaneous ablation zones

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      The temperature of the tissue at the lesion edge needs to be high enough to avoid marginal recurrences – the aim is to ablate an adequate margin of adjacent normal tissue

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      The heating effect on a tumour can be inadvertently reduced by the ‘heat-sink’ effect of blood flow in adjacent vessels >3 mm in diameter ► adjacent temporary vessel occlusion or embolization may help

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      Over aggressive RFA can lead to dessication and charring which increases tissue impedance, and prevents the application of additional current and temperature ► internal electrode cooling to limit charring may help

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      RFA ablation zones can vary according to the local tissue environment (e.g. aerated lung tissue is associated with high impedance and therefore poor heat transfer)

Microwave Ablation (MWA)

  • Needle like probes harbouring a microwave broadcast antenna towards the needle tip (900-2400 MHz) ► antenna design will affect the size and shape of the ablation zone (e.g. elongated or rounded) ► multiple antennae can create zones of constructive or destructive wave interaction

  • Water molecules oscillate when subjected to microwave radiation with significant local tissue heating ► in contrast to electric currents with RFA, microwaves radiate through all tissues including those with high electrical impedance – MWA can produce faster (approx. 5 mins) and larger ablation zones in multiple tissue types compared with RFA

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      Advantages over RFA: faster ablations ► higher temperatures without tissue impedance limitations ► reduced sensitivity to tissue types and more consistent results ► relative insensitivity to ‘heat-sinks’ ► the ability to create larger ablation zones

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      Disadvantages over RFA: potential for increased normal tissue damage due to potentially larger ablation zones generated

Cryoablation

  • Uses narrow gauge (17G) argon cryoprobes – the phase change of liquid to gaseous argon can induce temperatures as low as -150°C to -170°C within the immediate vicinity

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      Faster freezing leads to intracellular ice formation which disrupts cellular organelles ► slower freezing leads to extracellular ice formation and osmotic dehydration which causes cellular disruption, also compounded by microvascular endothelial injury

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      The cell lethal isotherm lies at -20°C to -30°C and is ensured by the use of a double freeze-thaw cycle

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      Cell death may only occur 8 mm deep to the edge of the visualized ice ball

  • In practice several probes (3-4) are placed into the tumour (approx. 10 mm from the edge and 15-20 mm apart)

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      Advantage: the main advantage is the production of a predictable physical iceball that can be monitored with USS, CT or MRI (cf. RFA)

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      Disadvantages: as the ablation zone is reperfused after the ice ball melts, the rapid release of cellular debris may explain the increased systemic complications (cryoshock) that can be seen compared with coagulative techniques ► as there is no diathermy effect, bleeding complications are more common ► longer ablation times required (25-30mins than with RFA or MWA)

Focused Ultrasound

  • Small focal areas of tissue destruction are achieved by focusing sound energy in the 1 MHz range using an extracoporeal acoustic lens – although avoiding breaching the body wall sound energy can be severely attenuated by intervening tissues

  • The focused energy results in small ovoids of tissue destruction usually of rice grain size – these areas are stacked together to create larger ablation zones

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