Interstitial Laser Therapy

ILT is a minimally invasive, high-temperature ablative technique designed for localized tumor tissue coagulation. The laser energy is delivered through optical fibers implanted interstitially. The treated volume "per fiber" is limited owing to the small penetration depth of the laser. Its extent also depends on the fiber tip, laser wavelength, and optical properties of the target tissue. The latter are highly variable even among tumors with the same histological grading (5) and also change during therapy. The difficulty presented by a limited treatment volume per fiber can be addressed by using multiple optical fibers or specially designed diffusing tips.

The laser-induced tissue damage follows a typical pattern, with a central necrosis zone and a shapely demarcated, reversible edema zone at the tumor periphery, containing viable tumor cells that represent the origin of recurrent tumor growth after treatment (27,28). An effective method for separating the reversible and irreversible changes areas has still to be developed.

Since the original description of MRI monitoring and control of laser-tissue interactions (8), MRI-guided tissue ablation has become a clinically tested and accepted minimally invasive treatment option. It is a relatively simple straightforward method, which can be well adapted to the interventional MRI environment.

After the original publication by Jolesz et al. (8), Ascher et al. (29) performed the first ILT treatment under MRI. Later, this approach was further developed by Bettag et al. (28), Kahn et al. (30,31), and Jolesz et al. (32-34).

Despite these early trials, the role of ILT as a therapeutic alternative for brain tumors has yet to be established. There are preliminary reports in the literature on this subject (5,30,35-37). All showed a low incidence of postoperative morbidity and no mortality. However, transient neurologic deficits are not uncommon in the early postoperative period due to perifocal edema, although full recovery is expected within a few weeks. One study also reported permanent neurologic deficits in glioblastoma patients, in whom administration of high-energy doses was attempted (36). Overall, these early results suggest that ILT is a safe therapy method. Although no definitive conclusion can be drawn based on the currently available data, it appears that ILT can be of benefit in patients with low-grade gliomas. In malignant gliomas, thermal therapy has been essentially unsuccessful, a predictable outcome, since such tumors extend far beyond the area of MRI contrast enhancement.

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