Laser-induced thermotherapy

Laser-induced thermotherapy, also referred to as laser ablation, consists of tissue destruction, induced by a local increase of temperature by means of laser light energy transmission.

Laser-tissue interactions

The irreversible necrosis of tissue irradiated by laser energy occurs for a combination of the temperature rise produced locally and the exposure time: cell death occurs within few seconds for temperatures exceeding 60 °C, while for lower temperatures are necessary longer exposure time.

The advantage of using laser light for thermotherapy, compared to other methods, is its ability to deposit a precise amount of energy in a well defined region. Thanks to its characteristics of monochromatic light (waves of the same wavelength), coherent (waves in phase) and collimated (parallel waves) it is possible to induce lesions of reproducible size, transmitting large amounts of energy at long distances. All applications of laser thermotherapy occur within a range of wavelengths, called 'therapeutic window', in which the light has a good tissue penetration. The light produced by a Nd:YAG laser with a wavelength of 1064 nm represents an ideal compromise to obtain a safe penetration depth, inversely proportional to the frequency of the wave, and a sufficient tissue absorption, depending on the tissue optical properties.

Most of the absorbed light is converted into heat, which causes changes in optical properties of tissue. Coagulation is defined as the thermal damage of the tissue proteins at temperatures in the interval about 55 and 95 °C. Its extension region depends mainly on the time during which the temperatures remain within the range. The most obvious example of coagulation is boiling the white egg, where the medium changes from transparent to white. If the temperature exceeds 100 °C causes the vaporization of the water contained in the tissue, which leads to the formation of steam.

Percutaneous technique

In the percutaneous technique (laser-induced interstitial thermotherapy (LITT) also referred to as percutaneous laser ablation (PLA) or interstitial laser-induced thermotherapy (ILT)) the heating takes place thanks to the insertion of optical fibers carrying the laser energy that is absorbed by the tissue and converted into heat, causing irreversible tissue destruction and proteins denaturation when using temperatures above 50 °C. The optical fibers are positioned within the region to be ablated by passing through the lumen of very fine needles (21/22 Gauge), whose diameter is less than 1 mm, inserted percutaneously.

Currently, the PLA is widely used in the treatment of benign thyroid nodules[1] and liver cancer,[2][3] when surgical resection is not feasible.

Applications

The therapeutic response of the laser ablation depends in a complex manner by the choice of the wavelength, the irradiation duration and the laser power. The wavelength of the laser light can be chosen such as the light is absorbed selectively by the target. The selective coagulation of varicose veins in cosmetic surgery can be accomplished by using laser light selectively absorbed by hemoglobin. The pulse is then chosen sufficiently short so as not to cause damage to surrounding normal tissue, but also long enough to allow the coagulation over the entire diameter of the vessel. For the non-selective heating is commonly used the near-infrared light, which penetrates more deeply into the tissue than others. The heating laser has been used to treat peptic ulcers bleeding, since the thermal contraction of the tissue seals the blood vessels. The laser light provides an excellent means to induce a local increase of temperature in the tissue, which can be used for cancer therapy. The tumor vaporization was effective for the palliative treatment of cancer of the esophagus, liver, pancreas and breast, with the primary purpose of concentrating the treatment locally in the tumor region of the body, trying to preserve the original structure of the parenchymal tissue. For Brain tumors, first cases have been performed in France in 2008 with excellent results (A.Carpentier et al. Neurosurgery 2008), so that more than 5.000 procedures have been performed in USA since then. Medtronic acquired the technology in 2014.

Curative potential

LITT is considered to be one of several alternatives for treating liver metastases when surgery, for various reasons, is not an option. An estimated 60 to 80 percent of all patients with liver metastases are not candidates for surgery. Annually in Sweden, approximately 600 to 700 patients with liver metastases from colorectal or breast cancers would be candidates for LITT. However, this estimate is highly uncertain.

Although LITT was originally developed to cause cell death locally, it has been suggested that laser therapy might also have favorable effects on patients’ immune defense. The hypothesis is that treatment could promote greater immune defense against tumors, and that the patient’s immune system could detect and destroy cancer cells hiding in the body. Hence, the treatment would have curative potential.[4]

References

  1. ↑ Valcavi, Roberto; Riganti, Fabrizio; Bertani, Angelo; Formisano, Debora; Pacella, Claudio M. (2010-11-01). "Percutaneous laser ablation of cold benign thyroid nodules: a 3-year follow-up study in 122 patients". Thyroid: Official Journal of the American Thyroid Association 20 (11): 1253–1261. doi:10.1089/thy.2010.0189. ISSN 1557-9077. PMID 20929405.
  2. ↑ Pacella, Claudio Maurizio; Francica, Giampiero; Di Lascio, Francesca Marta Lilja; Arienti, Vincenzo; Antico, Ettore; Caspani, Bruno; Magnolfi, Fabrizio; Megna, Angelo Salomone; Pretolani, Stefano (2009-06-01). "Long-term outcome of cirrhotic patients with early hepatocellular carcinoma treated with ultrasound-guided percutaneous laser ablation: a retrospective analysis". Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 27 (16): 2615–2621. doi:10.1200/JCO.2008.19.0082. ISSN 1527-7755. PMID 19332729.
  3. ↑ Pompili, Maurizio; Pacella, Claudio Maurizio; Francica, Giampiero; Angelico, Mario; Tisone, Giuseppe; Craboledda, Paolo; Nicolardi, Erica; Rapaccini, Gian Ludovico; Gasbarrini, Giovanni (2010-06-01). "Percutaneous laser ablation of hepatocellular carcinoma in patients with liver cirrhosis awaiting liver transplantation". European Journal of Radiology 74 (3): e6–e11. doi:10.1016/j.ejrad.2009.03.012. ISSN 1872-7727. PMID 19345541.
  4. ↑ "Laser-induced Interstitial Thermal Therapy for Liver Metastases: Summary and conclusions" (PDF). Swedish Agency For Health Technology Assessment and Assessment of Social Services. 2011-03-02.

Bibliography

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