Introduction

In 1963, Goldman Blaney et al. wrote one of the first landmark papers about the use of lasers in dermatology entitled, “Pathology of the Effect of the Laser Beam on the Skin” [1]. Twenty years later, Anderson and Parrish published their seminal paper on the theory of selective photothermolysis [2]. That work paved the way for the use of lasers in dermatology, otolaryngology and oncology.

Selective photothermolysis is the premise for laser treatment of lesions. At selected laser wavelengths, the laser energy absorbed by the oxy- and deoxyhaemoglobin in the blood causes coagulation of blood cells and, by heat conduction, transmural coagulation of vessel walls (i.e. photocoagulation). To date, an increasing number of optically activated, metal-based nanoparticles are being investigated for therapeutic uses. These nanoparticles greatly increase the absorption of near infrared (NIR) lasers. Lasers can also be used for minimally invasive thermal ablation procedures, where the light energy is converted to heat by non-selective absorption of blood and connective tissue.

This special issue of laser thermal therapy includes papers that cover the above-mentioned use of lasers in the treatment of benign and malignant tumours. Following are short reviews of these papers per their main contributors:

Pearce reviews the use of mathematical models of laser-induced tissue thermal damage. The effect of laser wavelengths, water vaporisation and the classical thermal damage models are reviewed and discussed in details. He presents methods that quantitate thermal damage and teach how to validate these calculations in experimental settings.

Feng and Fuentes present a computational framework for treatment planning, real-time surgical monitoring, and controlling of laser interstitial thermal therapy (LITT) of prostate cancer. This framework is also applicable to other thermal ablation therapies such as radio-frequency, microwave, and high intensity focused ultrasound.

Shafirstein et al. present the use of a diffusion-based mathematical modelling for laser light tissue interaction that is used to examine the key variables that affect the temperature field within large blood vessels (0.1–2 mm). In this study, clinically approved NIR laser settings and concentrations of indocyanine green (ICG) dye were used to simulate selective photothermolysis in geometries that represent skin structures. From these simulations, specific ICG doses and NIR laser settings are recommended for future clinical trials.

Klein et al. provide an overview on laser treatment of benign skin tumours and recent developments in this field. They discuss the indications and contraindications of laser therapy for benign skin tumours. The use of fractional laser and combinational approaches such as laser treatment and photodynamic therapy (PDT) are also being presented as new therapeutic options. The review concludes that there is a lack of randomised, controlled trials comparing laser therapy with conventional therapy options in the treatment of benign skin tumours.

Stafford et al. use magnetic resonance temperature-imaging (MRTI) to investigate the use of gold-silica nanoshells (AuNS) with NIR laser for thermal ablation of prostate tumours in xenografts. This research demonstrates that passive uptake of intravenously injected AuNS in PC-3 xenografts converts the tumour's vasculature into a potent heating source for nanoparticle-mediated ablation at power levels which do not generate significant damage in normal tissue (i.e. selective photothermolysis).When used in conjunction with MRTI, this has implications for development and validation of more conformal delivery of therapy for LITT.

Rylander et al. study heat shock protein (HSP) expression and temperature distribution in prostate tumours treated with laser irradiation and nanoshells. They quantified HSP expression associated with laser irradiation of prostate tumours in combination with nanoshells. They then correlate this information with temperature distribution measured with MRTI. This work demonstrated that the inclusion of nanoshells in laser therapy can better enhance heat deposition capable of eliminating HSP expression within a larger tumour region than laser therapy alone can.

Helbig et al. summarise the current knowledge on PDT-mediated cell signalling, with a focus on the role of Hsp70 in PDT. In the case of PDT-induced apoptosis, cytoplasmic Hsp70 translocated to the cell surface has shown to have a crucial role in treatment response due to its inhibitory effect of both apoptotic and necrotic pathways. It is suggested that monitoring of Hsp70 expressions (and other influencing factors such as skin cooling) may be used to optimise treatment regimens for PDT of actinic keratoses, Bowen's disease, superficial basal cell carcinomas and skin rejuvenation.

Grunewald et al. investigate the in vivo wound-healing process and remodelling of an area prone to scarring using a fractional ablative CO₂ laser in a pilot clinical study. It was found that ablative fractional photothermolysis is safe and effective in non-facial skin within the range of 50–300 mJ. However, they found that dermal remodelling continues for up to 4 weeks, which should be the minimum time-frame between treatment sessions.

This issue highlights the importance of mathematical modelling in designing, monitoring and analysing laser thermal therapy. The use of nanomaterials and drugs in conjunction with laser therapy opens new possibilities for the treatment of diseases that presently have no good treatment options.

Laser thermal therapy is an active research field that has grown every year for the last 30 years. Translational studies and ongoing clinical trials supported by good reimbursement rates will continue to drive this field in medicine for years to come.