Effect of laser intensity and exposure time on photothermal therapy with nanoparticles heated by a 793-nm diode laser and tissue optical clearing

Laser-induced thermotherapy is a promising method for cancer treatment, the outcome of which is affected by the exposure time. An inappropriate exposure time and laser intensity cause incomplete tumour destruction, tumour regrowth, and metastasis. Also possible is irreversible damage, i.e. death of healthy cells, and so numerical models are necessary to provide an optimised laser intensity and exposure time for different cancerous tumours. In this study, a model based on finite element method (FEM) is used for solving the bio-heat transfer equation and the Arrhenius equation describing tissue damage. The cancerous tumour is considered as a perfect cylinder with a diameter of 20 mm and a thickness of 2, 3, 4, and 5 mm, filled up by highly absorbing nanoparticles and surrounded by healthy cylindrical tissue with a diameter of 40 mm and a length of 10 mm, which ahs a low scattering coefficient due to optical clearing. The results show that 243 s is a safe and appropriate exposure time when a diode laser with a wavelength of 793 nm and intensity of 0.75 W cm^-2 together with gold nanorods of concentration 0.0001% is used for the treatment of a 3-mm-thick tumour. Then, the results are developed and extensive numerical simulations are used to reveal mathematical relationships between two critical parameters, input power and optimised exposure time, for a series of different tumour thicknesses. Treatment protocols are presented.