Laser and photodynamic therapy in the management of cutaneous malignancies

Abstract

Laser therapy for cutaneous malignancies dates back to 1964, when Goldman and Wilson first described the use of the ruby laser in the treatment of a basal-cell carcinoma (BCC).1 During that same year argon and carbon dioxide (CO2) lasers were developed, and by the 1980s the neodymium yttriumaluminum garnet (Nd:Yag) and the superpulsed CO2 lasers were all reported to have been used in skincancer management.2–4 In the last decade, the emergence of new lasers such as the ultrapulsed CO2 laser and the Erbium yttrium-aluminum garnet (Erbium: Yag) laser have provided alternatives for ablating superficial skin cancers and precancerous lesions with possibly fewer complications.5 The development of lasers (an acronym for Light Amplification by the Stimulated Emission of Radiation) is based on the concept of stimulated emission predicted by Einstein’s quantum theory.2–4 Each laser emits a specific wavelength(s) of light in the visible or infrared spectrum (Table 1). Depending on its wavelength, the laser energy is absorbed by different tissue chromophores and converted into heat. For visible and infrared light the principal chromophores are melanin, hemoglobin, and water.6 Lasers can deliver energy in either continuous, pseudocontinuous, pulsed, ultrapulsed, or Q-switched beams.2 In continuous or pseudocontinuous modes the laser-induced heat deposited in the absorbing chromophores diffuses into the surrounding tissues, leading to nonspecific damage. More precise, spatially localized thermal damage can be achieved with pulsed irradiation. The absorbing structures (e.g., a hemoglobin-filled blood vessel) have characteristic thermal relaxation times (TRT), which depend on their size. The TRT is the time required for the heated structure to lose half of its heat to the surrounding tissue.2,3,6 If the duration of the laser pulse is less than the TRT, the thermal energy will remain localized to the target. Thus, to achieve optimal damage of a selected lesion and avoid nonspecific thermal damage, the laser pulse duration or exposure time should be equal to or slightly less than the tissue’s TRT. Note that for visible wavelengths absorbed by chromophores such as melanin and hemoglobin or tattoo inks, the selective thermal damage can occur within the skin. For lasers producing infrared wavelengths absorbed by water (e.g., CO2, Erbium:YAG, Nd:YAG), the thermal energy is deposited at the surface of the skin. The argon laser emits a continuous wavelength between 488 to 514 nm of visible blue-green light6 with tissue penetration of 1 to 2 mm.2 The chromophores for the argon laser are melanin and oxyhemoglobin.6 Because argon lasers may cause nonspecific thermal damage leading to scarring, their use has been largely limited.2,6 The argon pumped dye laser, which produces a red light at 630 nm, is predominantly used in photodynamic therapy.4 The CO2 laser produces a continuous wave of 10,600 nm in the infrared spectrum with skin penetration of 0.6 mm.2 The main chromophore for all the CO2 lasers is water. The continuous wave CO2 laser can be used in the focused mode for excising lesions, or in the defocused mode for vaporizing or ablating superficial tissue. In the focused mode, the laser can seal small blood vessels ,0.5 mm in diameter, as well as seal lymphatics and small nerve endings.2 Nonselective thermal damage can be produced by continuous wave CO2 lasers, which can result in scarring. In an attempt to decrease scarring, the superpulsed and ultrapulsed lasers were developed to “pulse” the laser beam close to the skin’s ('70 msec), thereby minimizing heat conduction and reducing overall thermal damage. Superpulsed lasers can deliver pulses of 200 to 400 msec3 while ultrapulsed lasers can achieve pulses between 250 msec and 1 msec2 with tissue penetration between 20 and 30 mm. The continuous-wave Nd:Yag laser emits an invisible, near-infrared wavelength of 1,064 nm with no specific chromophore. The laser beam can be focused for use as an incisional surgical tool by adding contact fibers with sapphire or quartz tips.3,6 Tissue penetration From the Roswell Park Cancer Institute, Buffalo, New York, USA. Address correspondence to Nathalie C. Zeitouni, MD, State University of New York at Buffalo, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NY 14263, USA. E-mail address: rpci@med.buffalo.edu