Radiology and Imaging Sciences

References for Radiofrequency Thermal Ablation as Tumor Therapy

An Overview for the Oncology Team

Bradford J. Wood, M.D., is clinical investigator and staff interventional radiologist at the National Institutes of Health Clinical Center, Diagnostic Radiology Department, Special Procedures Division, in Bethesda, Md. He is also clinical associate at Massachusetts General Hospital in Boston, Mass. Mary T. Winkler is research intern at the NIH Clinical Center, Diagnostic Radiology Department. She is also a third-year medical student at George Washington University Medical Center, Washington, D.C.

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The RFA Treatment Session 

Radiofrequency thermal ablation can usually be performed as an outpatient procedure under general anesthesia or conscious sedation. Alternatively, RFA may be performed laparoscopically or during open surgery.[36]

Under light sedation, lidocaine or bupivacaine is administered subcutaneously at the needle entry site and down to the liver capsule. A needle is placed through the skin and into the tumor with imaging guidance. Treatment sessions of percutaneous RFA are easily monitored using real-time ultrasound imaging, computed tomography, or magnetic resonance imaging. Most patients feel little pain during the procedure and go home the same day or the day after the procedure, usually with minimal to no pain or soreness, although there is a spectrum, and some patients will experience severe pain the day of the procedure.

During a 10- to 30-minute treatment session, nitrogen micro-bubbles gradually create a hyperechoic area on ultrasound that provides a rough estimation of the treated tissue, which is 2.5 to 5 cm per 10- to 30-minute treatment sphere. CT, MR imaging, or positron emission tomography (PET) imaging may provide more exquisite detail for follow-up verification of the treatment zone and for finding residual or recurrent neoplastic tissue. Although real-time MR imaging and CT are available, they are not in widespread use. Ultrasound is a safe, common, and easy guidance method, although it is somewhat operator dependent.

Once the needle has been properly positioned within the tumor, the tissue is heated. At temperatures exceeding 50o C, cells are destroyed. To treat tumors of different size and shape, the needle is available in different lengths and shapes of exposed tips.[37]

Energy is transferred from the uninsulated distal tip of the needle to the tissue as current rather than as direct heat. The circuit is completed with grounding pads placed on the patient's thighs. As the alternating current flows to the grounding pads, it agitates ions in the surrounding tissue, resulting in frictional heat. The tissue surrounding the needle is desiccated, creating an oval or spherical lesion of coagulation necrosis, typically 2.5 to 5 cm in diameter for each 10- to 30-minute treatment. These spheres are added together in three dimensions to overlap and completely envelop the tumor. Ideally, the treated tissue will contain the entire tumor plus a variable rim of healthy tissue as a safety margin.

Failure to ablate the entire tumor with clean edges results in regrowth of the tumor. Depending on the size and configuration of new growth, the patient may or may not be suited for another treatment session. Over months to years, as the dead necrotic cells are reabsorbed and replaced by scar tissue and fibrosis, the size of the thermal lesion shrinks, although the remaining cells are ideally dead. The possibility of successful surgical resection may be augmented by decreasing the number of tumors.[38] Treatment of a tumor in one lobe may broaden the surgical indications of a tumor in the other lobe. Due to the natural course of the disease, new or recurrent tumors may be suited for additional treatment sessions as well.

Various methods of increasing the volume of treated tissue have been explored. One type of ablation needle-electrode consists of a coaxial system, or an expandable needle within a needle. The inner hooks are deployed once properly situated within the tumor.[39] Different configurations allow for treatment of various shapes and locations of tumors. Another ablation system utilizes a triple parallel needle array, which synergistically increases the treated volume.[40]

At temperatures exceeding 100o to 110oC, the tissue surrounding the needle vaporizes. The gas from the vaporization insulates the area immediately around the needle, limiting energy deposition in the target zone and decreasing the volume of tissue treated. Overcooking or charring around the outside of the needle also insulates and causes incomplete destruction of target tissue remote from the needle, much like a hamburger cooked too fast on a grill, charred on the outside and raw in the middle.

The deleterious effects of charring and vaporization may be decreased by monitoring temperature and/or impedance during treatment, and adjusting the current accordingly. The generators have computer chips or treatment algorithms to assist in optimizing this process. One system perfuses chilled saline within a closed-tip needle in order to deposit more energy without increasing the temperature.[41] This system allows an increase in the lesion diameter, while keeping the temperatures below the vaporization point.

At the end of a treatment session, the active needle is slowly retracted to heat and cauterize the needle pathway. This action prevents bleeding and tumor seeding of the needle track by destroying any cell that becomes attached to the needle or dislodged in the needle tract.

Three companies (RITA Medical Systems, Radionics, and RadioTherapeutics) market RFA systems. They currently have FDA 510-K clearance for soft tissue ablation, and have or are pursuing FDA 510-K clearance for unresectable liver tumor ablation. Although it is in its infancy as a technique, RFA is no longer a completely experimental procedure.

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