Hepatic resection in combination with systemic chemotherapy is a standard treatment modality for colorectal liver metastases (CRLM). Recently, thermal ablation, including microwave coagulation therapy (MCT) and radiofrequency ablation (RFA), has been utilized in the treatment of unresectable and partially resectable CRLM. A review of the English language literature and a summary of our experiences in applying thermal ablation in the treatment of CRLM are described here. RFA is used worldwide, and MCT is primarily utilized in eastern countries. In using percutaneous and laparoscopic/open surgical RFA, local recurrence rates were16% and 4% for tumors < 3 cm, 26% and 22% for tumors between 3-5 cm, and 60% and 50% for tumors > 5 cm. In a large series of treatments which utilized RFA for liver tumors, the mortality and morbidity rates were only 0.3 and 7.2%, respectively. The incidence of tumor seeding after the use of RFA for the treatment of CRLM is as high as 1.4%. Cumulative 5-year survival rates were 29%-36% using MCT and 14%-35% using RFA for unresectable CRLM. Long-term survival data for resectable CRLM are unclear. In our experience, local recurrence rates were undetectable in following the treatment of 30 cases of CRLM (average tumor diameter : 1.7 cm, average observation period : 26 months) which were treated with surgical RFA combined with hepatic resection after efficacious systemic chemotherapy. In Conclusion : 1) thermal ablation can be applied to unresectable CRLM without perivascular invasion, and for tumors < 3 cm with a percutaneous or surgical approach, and for tumors < 5 cm with a surgical approach ; 2) RFA after effective chemotherapy can provide an extremely high local control rate ; and 3) the application of thermal ablation for resectable CRLM is still controversial due to the lack of sufficient evidence obtained from a randomized trial.

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Our reports have described robust noninvasive techniques for reconstructing the thermal properties of living tissues. The properties of interest are thermal conductivity, thermal capacity, and thermal diffusivity, and these reconstructions can aid in the diagnosis, monitoring, and planning of thermal treatments which utilize agents such as high-intensity focused ultrasound. Internal tissue temperature distributions can be measured by using ultrasound or magnetic resonance. Then, if reference thermal properties are provided as initial conditions in the region of interest, the thermal property distribution values can be obtained by solving bioheat transfer equations which are simultaneous first-order partial differential equations having temperature distributions as inhomogeneous coefficients. Although in using these techniques, perfusion caused by blood flow and the effects of a thermal source/sink can also be reconstructed, thermal property reconstructions were performed here after stopping heating and perfusion. The feasibility of using this method to reconstruct thermal property distributions was verified by three dimensional simulations. Because the design of a thermal source to be realized in a tissue and the prediction of generated series of temperature distributions are allowed, a minimally invasive thermal treatment will be realized.

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Interstitial hyperthermia using a single antenna would reduce invasiveness and expand indications. We developed an antenna with a nearly spherical temperature distribution, which is the ideal temperature distribution for single-antenna heating that matches the shape of common tumors. We also developed a means of inserting the antenna so that the center of the spherical temperature distribution matches that of the tumor. By increasing the antenna output, we considered the central region of the tumor could undergo thermal ablation while the peripheral region could be exposed to hyperthermia. Five patients were treated using microwave interstitial hyperthermia and radiation therapy with intent to cure. Four patients had supraclavicular or inguinal node metastasis, and one patient had a soft palate primary lesion. The mean follow-up time was 2 years and 2 months. The heated region was controlled in all patients during follow-up without evidence of tumor re-growth, in spite of the sizable tumors (3-7 cm, mean 4.5 cm).
Only one antenna was inserted into three patients. In all sessions with these three patients, the antenna output was set at 10-15 W (high) per antenna, which increased the temperature of the tissue near the antenna to 47-66°C. Coagulation necrosis should have occurred at this temperature range over a wide area around the tumor center in all three patients. The advantages of single-antenna interstitial hyperthermia are as follows : 1) it is less invasive than multiple antennas, and the indications are expanded. 2) from the viewpoint of combined radiotherapy, local control rates increase because the hypoxic radioresistant region at the tumor center can be ablated more effectively and 3) it can be safely applied when ablation is contra-indicated, such as when a tumor is adjacent to nerves, vessels or healthy skin, because the temperature in the tumor periphery is lower. Interstitial hyperthermia with a single antenna is a "thermal therapy" that acts as both hyperthermia and ablation due to a longer period of heating and higher output power.

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