Ablation means intentional destruction of an abnormal tissue (usually tumor) using various methods such as burning, freezing and by applying electricity or chemical agents. In oncology, ablation of a tumor is done by placing special needles percutaneously into the tumor under imaging guidance. The goal of ablation is to kill the tumor with a narrow (0.5-1cm) area of surrounding healthy tissue (safety margin). If this goal is reached, ablation is accepted to be a curative treatment comparable to surgery.
How is it done?
For percutaneous ablation, first the doctor examines the patient’s films such as ultrasound, CT, PET-CT and MRI. On these films, the doctor evaluates the type, size, number and location of tumors. Based on these findings and other data, the doctor then decides which ablation method to choose, which imaging modality to use during ablation, what king of anesthesia to apply and where to put the needle on the skin. Most ablations can be performed under local anesthesia and sedation. Ultrasound is preferred as the imaging guidance because of its ease of use and ability to provide real-time monitoring. If the tumor can not be seen on ultrasound, ablations are mostly performed under CT guidance.
In some conditions, it may be necessary to use both CT and angiography for percutaneous ablation. For example, some hepatic tumors that are not visible on ultrasound or CT alone, may become visible on CT when a special dye (contrast) is injected into the hepatic artery through a catheter placed from the groin via angiography. These tumors can then be successfully ablated under CT guidance. Recently, “cone beam CT” equipment has been added to some angiography machines, and with such machines it has become possible to do most percutaneous ablations, especially lung and bone ablations more easily and safely.
What ablation techniques are used?
Today, three groups of ablations are commonly used in cancer patients. 1. Chemical ablation, 2. Thermal ablation, 3. Irreversible electroporation (nanoknife).
What kind of tumors are suitable for percutaneous ablation?
Chemical ablation is mostly used in the percutaneous ablation of hepatocellular carcinoma (HCC), which is the most common primary liver tumor. For this purpose, alcohol and acetic acid have been successfully used for decades. Chemical ablation is also used for the treatment of thyroid nodules and lymph node metastases. However, since it is less effective than thermal ablation methods, chemical ablation is mostly used in combination with thermal ablation or in tumors where thermal ablation is not suitable.
Termal ablation is the most commonly used ablation method in oncology. The most popular thermal ablation technique is radiofrequency, which has been extensively used in all over the world, particularly for liver tumors. Radiofrequency ablation has been also performed in the lung, kidney, bone, soft tisues, lymph nodes and spleen. The most important advantages of radiofrequency and other thermal ablation techniques are their ease of application, ability to produce a controlled and predictable ablation and few side-effects. The goal of thermal ablation is to destroy the tumor and a 1cm healthy tissue around it. It has been shown in many studies that thermal ablation is able to kill all the tumor cells in this ablation area. For this reason, it is today accepted to be an alternative to surgery in primary and secondary liver and lung tumor, whereas in the past, it was recommended only in cases where surgery was not suitable.
Despite these advantages, thermal ablation has also some weaknesses: 1. In tumors adjacent to large blood vessels, the heat is taken away by the moving blood, and thus, the tumor tissue around these vessels may not be sufficiently heated. This phenomenon, which is called “heat sink” protects the vessel wall from thermal ablation but may also prevent the complete destruction of the tumor tissue around the vessel. 2. In bile or urine channels where the flow is much slower than the blood, the heat sink is minimal and the surrounding tumor tissue is easily killed. But, since the protective effect of heat sink is absent, these channels may also be harmed during the tumor destruction. 3. Thermal ablation may be harmful also to the nerves. For this reason, it is avoided in organs like prostate which contains nerves for erection and urination. Otherwise, the patient may develop impotance and urinary incontinance. 4. Thermal ablation may be harmful to digestive organs such as stomach and bowels. Thus, it must be performed carefully in tumors close to such organs, preferably using special techniques that separate them from the ablation area.
In conclusion, thermal ablation is the most commonly used ablation method in oncology, but it may not be very safe and effective in tumors containing or adjacent to vessels, bile and urine channels, and organs like stomach and bowels.
Irreversible electroporation (IRE) may be a good alternative in such locations. One of the typical organs IRE may be preferred over thermal ablation methods is pancreas. The head of the pancreas is adjacent to the arteries and veins of the liver and intestines, and also to the small bowel. For this reason, thermal ablation is generally not performed for the pancreatic head tumors. IRE has the potential to kill the tumor tissue without harming the vessels and intestines in this region. The technique had been found to be safe and effective in recent studies.
Another potentially important application of IRE is the liver tumors located close to the large vessels and bile ducts. As mentioned above, thermal ablation techniques may harm the bile ducts and may not kill the tumor tissue around the vessels in such locations. IRE may be safer and more effective in these tumors. However, IRE has also some disadvantages: İt is technically more difficult, requires general anesthesia and is more expensive than thermal ablation techniques like radiofrequency and microwave.
In conclusion, each percutaneous ablation method has its spesific advantages and disadvantages. The decision as to which percutaneous ablation should be used in a given tumor must be made by an interventional radiologist, who has sufficient knowledge and experience in percutaneous ablation, interventional oncology and also general oncology.
What are the advantages and disadvantages of percutaneous ablation?
Like surgery and curative radiotherapy, percutaneous ablation is a locally effective treatment that can cure some cancers. Its advantages over surgery: İt is a minimally invasive percutaneous intervention, is done under local anesthesia has fewer risks and requires no or shorter stay in the hospital. Its advantages over radiotherapy: It can be repeated as needed, is not harmful to nearby organs and its results does not depend on the tumor type. In many patients however, percutaneous ablation is used as an adjunct to standart therapies such as surgery, radiotherapy and chemotherapy, rather than an alternative to them.
Percutaneous ablation has also some disadvantages: First, local recurrence rate is slightly higher after percutaneous ablation compared to surgery and radiotherapy. But in case of recurrence, ablation can be repeated as many as needed. Second, the results of percutaneous ablation, as in surgery, depend largely on the knowledge and experience of the doctor who performs it. And third, the technical equipment and the trained staff are not widely available. Thus, the number of centers where percutaneous ablation is performed is rather limited.
What kind of tumors are most suitable for the percutaneous ablation?