Laser interstitial thermal therapy

The term laser interstitial thermal therapy (LITT, also referred to as laser-induced interstitial thermotherapy or laser-induced thermotherapy or interstitial laser therapy) 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-infraredwavelength 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. At the same time the good absorption in tissue limits the size of the lesion created by the laser irradiation. So a compromise between good penetration and good absorption has to be found. After initial absorption the temperature generated spreads through the tissue and enlarges the lesion somewhat, dependent on the perfusion of the tissue. Large vessels will transport the heat away from the site and the effective temperature achieved is reduced.

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 inthe 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 absorption 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 paperin 1958. A ruby laser as first constructed in 1960, and by the mid-1960smedical 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 1970s, butin the 1980s exploded as new laser technologies and new and cheaper fiberoptics became available. After an initial boom, lasers generally settled intoa 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 1990s a number of surgical applications and investigationsof 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

Throughout the history several Lasers have been used for LITT including Ruby lasers (690 nm) as well as visible gas lasers like Argon (514 nm). After the initial failures, the Lasers in the near infrared region and especially Neodymium:Yttrium-Aluminum-Garnite (Nd:YAG, 1064 nm, cw) were settled upon for best performance. These together with cooled applicators have created the largest lesions.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 areasinside the body. LITT may be performed using a simple bare-tip optical fiber or with a shaped (for example, ball-tip) fiber. However, a highpower 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 developedwhich 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 includeprovision 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.

Recent studies explored the feasibility of multiple applicators to generate even larger lesions in one session. The principle is to split the laser beam in 2 or 4 beams and supply the radiation to 2 or 4 applicators. Of course one needs a more powerful laser, but the serial application has the benfit of supplying more heat between the applicators due to less draining of heat by perfusion. (References in German) [ [http://www.lmtb.de/themen/litt.html LMTB +++ Laser- und Medizin-Technologie GmbH, Berlin ] ] [ [http://www.lmtb.de/themen/litcit.html LMTB +++ Laser- und Medizin-Technologie GmbH, Berlin ] ]

Application in breast cancer treatment

Interstitial laser thermotherapy is an innovative method of treating breast cancers in a minimally invasive manner and without the need for surgical removal, and with the absence of any adverse effect on the health and survival of the patient during intermediate followup [cite journal | authorlink = Kambiz Dowlatshahi | authorlink = Janice J. Dieschbourg | authorlink = Kenneth J. Bloom | title = Laser Therapy of Breast Cancer with 3-Year follow up | journal = The Breast Journal | volume = 10 | number = 3 | pages = 240-243 | date = May 2004 | publisher = Blackwell Publishing] . A forthcoming study of 24 patients has shown that small breast cancers can be effectively treated with ILT [cite journal | authorlink = Haraldsdóttir, K.H., Ivarsson, K., Götberg, S., Ingvar, C., Stenram, U., Tranberg, K.-G. | title = Interstitial laser thermotherapy (ILT) of breast cancer (in press) | journal = European Journal of Surgical Oncology | date = 2008 | publisher = Elsevier] .

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. MRIin 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.

References

ee also

*Laser surgery
*Laser ablation
*Photodynamic therapy
*EVLT