Although the thermal therapy methods (interstitial laser therapy, radiofre-quency therapy, and focused ultrasound therapy) are still in the experimental stage, mainly because of difficulties in accurately monitoring tissue temperature changes, they may develop into a minimally invasive alternative to open skull surgery for some brain tumors in the foreseeable future (1,2).

There is compelling physical and biological evidence that localized high-temperature thermal therapy is effective. If any tissue is heated beyond 57-60°C, protein denaturation and subsequent coagulation necrosis occurs. The thermal treatment results in irreversible cell damage in both normal and neoplastic tissues. Thermal energy is not selective, and both normal and neoplastic tissue is coagulated; therefore thermal ablation above the critical temperature is analogous to surgery and not to hyperthermia, which is performed around 41-42°C (3,4).

The magnitude and spatial distribution of temperature changes in the tissue depends on both the delivery parameters (i.e., intensity, duration) and the tissue properties, such as absorption, perfusion, and flow.

Optical characteristics and heat conductivity are variable even among tumors of the same grading (5). Both vascular distribution pattern and tissue perfusion rate have an influence on the spatial distribution of energy delivery and consequently on the volume of tissue affected by the treatment (6). Because of variable tissue properties, the outcome of such a treatment is difficult to predict.

The use of optical fibers for interstitial laser therapy (ILT) and the medical application of radiofrequency (RF) and microwave devices significantly advanced the ablation field by allowing percutaneous treatment. These are probe-delivered minimally invasive targeted heat deposition methods. Currently, high-energy focused ultrasound (HIFU) is a noninvasive extracorporeal alternative (1).

Thermal ablation methods have been known for decades, but their broader acceptance was restricted because of the lack of real-time volumetric "closed-

From: Minimally Invasive Neurosurgery, edited by: M.R. Proctor and P.M. Black © Humana Press Inc., Totowa, NJ

loop" feedback control of energy deposition. Without control of thermal energy spread in the tissue, safe and efficient thermal therapy cannot be carried out. This is especially true in the brain where not only is it important to achieve complete tumor treatment but injury of critical normal structures should be avoided.

Clinically applicable thermal coagulation has to be limited to the target volume and should not damage the surrounding normal tissue. This can be achieved by defining the exact 3D extent of the targeted tissue volume and the extent of heating within and outside the targeted tissue volume. Image guidance and image-guided therapy delivery control is necessary for thermal ablation of tumors. Accurate spatial and temporal temperature control is essential in the brain, where thermal damage must be limited to the target, especially if the targeted volume is close to critical structures (7).

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