NIR laser irradiation - how does carbonisation influence the treated volume

The most frequent use of lasers in medicine is to heat tissue, as performed in, for instance, laser surgery. For this purpose, several different types of lasers are used, depending on the application. Diode and Nd:YAG lasers are frequently used due to the deep light penetration in tissue for those sources, while CO₂-lasers are widely used when very shallow light penetration is preferable. In this project you should concentrate on NIR sources offering deep light penetration, for most tissues. Once the tissue is heated beyond the threshold for carbonisation, also light in this wavelength region is, however,  absorbed in a shallow layer. In interstitial laser-induced thermotherapy (ILT), in which tissue coagulation is used as a means to destroy malignant tissue, it is controversial whether one should avoid tissue carbonisation or not. It has been shown that, despite the more shallow light penetration for light in carbonised tissue, larger coagulated lesions can be produced with as compared to without carbonisation for the same total light energy.

One of the purposes with the project is to quantitatively explain if, and in that case how, it may be possible to produce a larger lesion by carbonizing the tissue as compared to using the same fluence at a lower fluence rate, for which no carbonization results. You should also investigate how the illumination area effects the resulting lesion size.

The intention is that the project should include the following:

• You are supposed to briefly study the field of ILT to destroy malignant tissue. Perform a literature search. This background should be included in the introduction of the report.
• You should identify what happens with tissue when it is carbonized, with the focus on how the different parameters in the models are affected.
• You should measure the optical properties of muscle tissue at the wavelengths of interest (during the laboratory exercises). Try to obtain values also for coagulated and carbonized tissue (to obtain values for carbonized tissue is experimentally very difficult as the absorption is very high. This is thus probably not possible). Try then to find values in the litterature or estimate reasonable values with good motivation.
• You should calculate the light distribution for the wavelength of interest (during the computer exercises and by yourself, if necessary).
• You should obtain values for the parameters included in the bioheat equation before and after carbonization (can be difficult to find, but if not found estimate these numbers and motivate the estimation).
• You should calculate the temperature distribution following laser treatment with and without initial carbonisation (during the computer exercises and by yourself, if necessary).
• Model what would happen if you change the illuminated area, while keeping the laser power and/or energy.
• Discuss the pros and cons with initial carbonisation in ILT.

In the project, at least these parameters should be considered: The illuminated area, the temperature threshold for tissue coagulation and carbonisation, source wavelength and power, treatment time, optical and thermal properties of the chosen tissue type.

Suggested key-words: Intersitial laser-induced thermotherapy, ILT, hyperthermia, lasers, tissue optics, laser surgery