Laser interstitial thermal therapy (LITT)

The term laser interstitial thermal therapy (LITT, also referred to as laser-induced interstitial thermotherapy or laser-induced thermotherapy) is a surgical procedure in which destruction of soft tissues in the body is effected through high temperatures generated by the local absorption of laser energy. LITT is also sometimes referred to as laser ablation or laser thermal ablation, but this terminology is inaccurate since the goal of LITT is to destroy tissue through thermal coagulation and thermal necrosis rather than by removal (ablation).

Laser-tissue interactions
LITT is generally performed using optical radiation in the near-infrared wavelength range (from about 700 - 2000 nm), though when appropriate chromophores are available, visible wavelengths (e.g. green) can also be used. Photons launched into tissue meet one of three fates: scattering, absorption, or exit from the tissue. When photons are absorbed, the energy from the photon is converted into inter- and intra-molecular energy and results in generation of heat within the tissue. As heating continues and tissue temperature is elevated, several processes occur which lead to the destruction or death of the tissue.

At temperatures of 100 degrees Celsius or more, water in the tissue and in the intracellular compartments may vaporize and lead to rupture or explosion of cells or tissue components.

At temperatures above 60 degrees Celsius, proteins and cellular components of the tissue become severely denatured and coagulate leading to cell and tissue death.

At somewhat lower temperatures, generally above 45 degrees Celsius, prolonged exposure leads to the thermal denaturation of non-stabilized proteins such as enzymes. Though cell death is not immediate, destruction of critical enzymes leads eventually to cell death.

Optical absorpition in the near-infrared range is generally due to combination and overtone bands of fundamental molecular stretches. For wavelengths near 1000 nm, water is a primary absorber of optical energy.

History
The laser was invented at Bell labs and first described in a paper in 1958. A ruby laser as first constructed in 1960, and by the mid-1960's medical applications were beginning to be explored. One of the first publications of what was called laser endoscopy (which could essentially be called LITT) was by S.G. Brown in 1968.

Medical laser applications grew somewhat steadily throughout the 1970's, but in the 1980's exploded as new laser technologies and new and cheaper fiber optics became available. After an initial boom, lasers generally settled into a few niche areas in surgery and medicine.

In the 1990s, availability of new high-power and compact semiconductor (diode) lasers increased the convenience of laser surgery, in particular LITT. Since the early 1990's a number of surgical applications and investigations of LITT have been described primarily for the destruction of malignant or benign tumors. Organs or tissues in which LITT has been used for this purpose include the brain, head and neck, liver, kidneys, and prostate, among others.

Lasers used for LITT
Lasers in the near infrared region, including Ruby lasers (690 nm) and flash-pumped Neodymium:Yttrium-Aluminum-Garnite (Nd:YAG, 1064 nm) as well as visible gas lasers like Argon (514 nm) have been used for LITT. More recently, high-power semi-conductor lasers developed for the telecommunications industry have become popular owing to their reduced power and cooling requirements and greatly reduced size. Diode laser sources are commonly found at 810, 940, and 980 nm.

Applicators
A particular advantage of LITT is that large amounts of energy may be delivered through small, flexible optical fibers to reach remote areas inside the body. LITT may be performed using a simple bare-tip optical fiber or with a shaped (for example, ball-tip) fiber. However, a high power density immediately adjacent to the fiber tip often leads to char formation which limits penetration of optical radiation into the tissue.

As an alternative, diffusing optical fiber applicators have been developed which emit light circumferentially into tissue over some length. Such fibers have the advantages of reduced power density and an increased optical delivery area.

In addition to a diffusing tip, some laser applicators may also include provision for cooling of either the applicator and/or the tissue adjacent to it. Cooled applicators can support higher power deposition rates and may be less likely to fail or burn up than un-cooled applicators.

Image guidance
Laser applicators for LITT may be inserted into target tissue using a number of image-guided techniques including x-ray fluoroscopy, ultrasound imaging, magnetic resonance imaging, or stereotaxic approaches. MRI in particular, is attractive because dynamic MRI can be used to infer temperature changes and/or other tissue changes as a potential means of feedback during the LITT treatment.