Anisotropic scattering in photothermal therapy and beyond: a detailed evaluation of transport approximation for gold nanoshells and nanospheres

Photothermal therapy (PTT) utilizes thermal radiation to heat a targeted tissue for cancer treatment. Nanoparticles can be embedded in tissue regions to enhance radiation absorption, a practice known as plasmonic photothermal therapy (PPTT). Developing models for heat transfer simulation in PTT would be quite useful for supporting experiments and therapies. An important and complex part of such models is radiation scattering. The focus in this research is scattering phase functions of nanoparticles with the evaluation of the widely employed transport approximation (TA). Two TA approaches are considered: TA1 disregards the phase function asymmetry of nanoparticles, common in PTT applications, while TA2 takes it into account. Results obtained for gold nanospheres and nanoshells using the TA were compared with results accurately computed with the Monte Carlo method taking into account detailed Mie phase functions. Evaluation involved direct comparison of heat fluxes and transient temperature distribution. The impact of nanoparticle size variability was also investigated. Results clearly show that within scenarios encountered in real PTT, with nanoparticles ranging from 50 to 100 nm, the transport approximation is quite reliable. For a larger range of scenarios, which can occur in experimental conditions, imaging applications, and due to nanoparticle size variability, TA can still be accurate with the aid of the TA2 approach.