Tumor RFA has a reported complication rate of less than 3% in many series (12
). However, much higher complication rates have recently been reported in abstract form (14
). This includes not only needle-related complications, but also thermal damage to non-target structures. The transition from dead to uninjured tissue is usually thin in most cases of liver RFA (15
). While this narrow transition zone facilitates fairly precise treatment, the exact thermal lesion margins remain somewhat unpredictable. Reported instances of thermal damage to normal structures include, injury to the gall-bladder, bile ducts, and bowel (16
). A number of strategies to protect non-target tissue have been described, including prophylactic measures as well as feedback methods. Prophylactic methods include careful preprocedural planning, ice bags, pro-peristaltic medications, chilled enemas, bladder emptying, and paracentesis. Monitoring or feedback methods include intraprocedural neurological exam and remote thermometry (18
). Prophylactic measures are useful, but do not provide direct quantified feedback to decrease risk. Feedback methods such as remote thermometry allow change of treatment during the procedure. Remote thermistors may be placed independently of the RFA probe, and can measure intraprocedural non-target tissue temperatures to protect tissue and improve the safety of RFA. While several previous RF studies have employed temperature sensors to analyze the distribution of RF induced heating in experimental models (4
), the use of such sensors as a safety mechanism to avoid complications of thermal damage to non-target tissue in humans is a natural next step. Goldberg et al employed remote sensors in ex vivo animal experiments to determine coagulation sphere diameters (17
). Dupuy et al used remote thermistors in animal experiments to analyze temperature distribution as RF energy was applied to pig vertebral bodies in vivo (4
) In the four patients described in this study, remote thermometry allowed monitoring temperature changes near various vital structures. According to the remote temperature information, the RFA treatment was modified in energy delivery or in treatment duration to limit risk to collateral structures. This temperature information may guide thermal lesioning based on the thermal thresholds of damage to that specific tissue (eg, lower for nerves). Proper positioning of the thermistor between the RFA probe and the collateral, at-risk structure makes overestimation of temperature more likely than underestimation, if the thermistor is closer to the heat than the collateral tissue. This could result in undertreatment of tumors with resulting regrowth as may have happened in three of four patients in this study. However, the alternative to undertreatment could be permanent or catastrophic thermal damage of non-target organs. Development of a tumor specific heating adjuvant would have great benefit in such cases.
The maximum equilibrium temperature after shutting off the current (with cool tip RFA probe) corresponded to ablation volume and low temperatures may suggest persistent perfused and viable tissue at the needle tip (20
Thermometry may be a more reliable surrogate marker of tissue damage than currently employed imaging parameters, which may be unreliable (especially US without contrast material). Ongoing evaluation with US during RFA may be difficult because of echogenic organic microbubbles released from cooking tissue. An additional 22-gauge needle thermistor or endorectal probe has minimal risk, given the added benefits, and gives continuous information, unlike other imaging modalities.
Intraprocedural temperature monitoring may play an increasingly important role as RFA technology evolves. For tumors near collateral structures, there is a trade-off between aggressive treatment, which risks complications, and undertreatment, which risks residual tumor. Any thermometry method may help the operator walk this tightwire. A clear limitation of the monitoring technique described here is that temperature information is provided for discrete points only. The ideal method would provide three-dimensional, volumetric data, thus more reliably indicating the temperature changes in the entire tissue of interest. Eventually, magnetic resonance (MR) imaging thermometry may provide such real-time volumetric information. Along with the technique described here, MR imaging thermometry may have added importance with the development of larger treatment spheres with multiple array electrodes, saline-enhanced RFA, energy pulsing, and internally-cooled needles. Independent thermometry remote from the treatment probe can provide information on the extent of tissue necrosis in the target lesion and also may help preserve vital structures in the area surrounding the lesion.