Increased number of fractionated irradiation sessions does not improve the cellular response to methyl aminolevulinate-mediated photodynamic therapy

Summary Background In the search to improve the efficacy of photodynamic therapy different approaches have been investigated until now. One of these approaches is the use of double fractionated irradiation schemes. This study aims to investigate the effect of increasing the numbers of fractionated irradiation sessions in methyl aminolevulinate-mediated photodynamic therapy on dysplastic oral keratinocytes by monitoring the changes in optical properties of cell culture. Methods Forty cell cultures were divided into four experimental groups (single, double and triple fractionated irradiation scheme and control). Cells were incubated with 1 mM methyl aminolevulinate and after 2 h they were exposed to laser radiation ( λ = 635 nm, P = 15 mW). The exposure time was 900 s for the single irradiation scheme, 300 s + 600 s for the double fractionated irradiation scheme with dark interval between sessions of 30 min and 300 s + 300 s + 300 s for the triple fractionated irradiation scheme with two dark intervals between sessions of 30 min. Two optical parameters (refractive index and extinction coefficient) were calculated from diffuse reflectance spectra of cell cultures using the Kramers–Kronig relations. Results The results demonstrate that the use of double fractionated irradiation scheme led to changes in optical parameters of cell cultures more important than the use of the single and triple fractionated irradiation schemes in photodynamic therapy. Conclusions In conclusion, increasing too much the number of irradiation sessions cannot lead to an improved photodynamic effect.