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Tumour Ablation (RFA / MWA) (Primary / Metastasis)

Percutaneous Ablation Techniques / Radiofrequency ablation (RFA)

Another route to treat primary and secondary solid organ tumours is via direct ablation (using thermal energy to destroy cells and cause tumour necrosis.)  This is an evolving technique and has been used in liver, renal and lung tumours with some success.

Liver ablation techniques

Percutaneous Ethanol Injection (PEI) was an early technique involving the injection of absolute ethanol (alcohol) directly into HCC lesions under ultrasound control and achieved satisfactory results in small tumours <3cm. [16]  Other techniques that have been used include cryoablation (freezing of tumours), microwave ablation, and laser techniques, but radiofrequency ablation (RFA) remains the predominant technique.  [17]  RFA has been approved by NICE (National Institute of Clinical Excellence) for the treatment of unresectable HCC and colorectal hepatic metastases. [18]

RFA – Mechanism of action

RFA produces movement of ions in the tissue which results in heating and cellular death.  Heating to a temperature of 60-100 oC results in almost immediate tissue damage.

RFA is based on producing tissue necrosis using a high-frequency alternating current that is delivered through an electrode placed in the centre of the tumour [19, 20].  Tissue necrosis begins as the temperature approaches 60°C, and RFA treatments often result in local tissue temperatures that approach or exceed 100°C, which result in tumour cell death.

It is possible to treat single tumours of up to 5 cm in diameter, and multiple tumours of <3cm diameter. 

Technique of RFA

RFA may be performed either under sedation or general anaesthesia.  The liver lesions will have been identified using either ultrasound (US) or computed tomography (CT), and the RFA procedure can be performed under either US or CT guidance, which is usually determined by the interventional radiologist prior to the procedure.  
The procedure would normally be performed in the CT scanner or the interventional radiology suite.  Once positioned upon the scanning table, the skin over the liver will be cleaned and sterilised and a sterile drape applied.   Local anaesthetic is infiltrated into the overlying tissues and either sedation or general anaesthesia is required for pain relief during the procedure.
An insulated needle with an electrode at the tip is used which transmits high-frequency alternating current to the tumour tissue.  The needle electrode is inserted into the tumour usually under ultrasound guidance with CT to confirm the final position.
Following ablation of the tumour, continued heating of the needle on withdrawal or “track ablation” avoids spreading of tumour cells.

Figure a – Small peripheral liver tumour in right lobe of liver (black arrow) pre-ablation
Figure b – Intraoperative CT image of RFA needle in position within lesion (white arrow)
Figure c – Post RFA CT image demonstrating tumour necrosis (white arrow) 

Restrictions to the use of RFA

The following factors may mean that RFA cannot be performed:
Significant evidence of cancer outside the liver, invasion of bile ducts or major vessels, liver cirrhosis or active infection.  Difficult to access lesions (may be sometimes necessary to perform RFA under open or laparoscopic (keyhole) surgery.)
Tumours that occupy >40% of the volume of the liver (the amount of liver left after RFA might not be sufficient to preserve liver function.)
Close distance to important structures like vessels and nearby organs (but open RFA may be possible.)
A relative contraindication may be if lesions are larger than 5 cm:
RFA should be used cautiously for lesions larger than 5 cm.  One study suggests the use of open / surgical RFA for lesions larger than 5 cm. [21, 22]

Complications of RFA

Complications of RFA include haemorrhage, liver abscess, and heat injury to adjacent structures e.g. bowel and gallbladder.  The use of “hydrodissection” (the injection of dextrose solution to push away other nearby organs) can be used to avoid local complications or injury to other structures.
Results of RFA in HCC, either alone or in combination with TACE have been encouraging.

Is RFA the preferred treatment?

Despite many published reports of RFA in HCC and liver metastases, large scale randomised trials comparing RFA, TACE and SIRT in the treatment of these tumours are still awaited.  It is not easy to clearly state which treatment is best for each tumour size and distribution, and multidisciplinary discussion between oncologists, surgeons and interventional radiologists is the best approach to determining the best course of treatment.
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