The Effects of Ultra Low Fluence Rate Single and Repetitive Photodynamic Therapy on Glioma Spheroids

Background and Objective: Achieving local control of gliomas with photodynamic therapy (PDT) requires the delivery of adequate light fluences to depths of 1-2 cm in the resection margin where the majority of local recurrences originate. This is clinically impractical with current single-shot, intraoperative PDT treatments due to the length of time required to deliver adequate fluences. Multiple or extended treatment protocols would therefore seem to be required. The response of human glioma spheroids to 5-aminolevulinic acid (ALA)-mediated PDT using single or, repetitive light delivery protocols was investigated at both low and ultra low fluence rates. Study Design/Materials and Methods: Human glioma spheroids (400 mu m diameter) were subjected to sub-threshold light fluence (1.5, 3, or 6 J cm(-2)) ALA-PDT consisting of four light delivery schemes: single treatment given over either 1 or 24 hours, repetitive treatment given either as four 1 hour light treatments separated by a 4 day interval, or 24 hours light delivery, consisting of four 24 hours treatments separated by a 3 day interval. Treatment efficacy was evaluated using a growth assay. In some cases, confocal microscopy was used to image cell viability. Results: The repetitive and single light treatment protocols were most effective when delivered at ultra low (mu W cm(-2)) fluence rates. In all cases, growth inhibition was light dose-dependent. The repetitive ultra low fluence rate treatment (1.5 J cm(-2); irradiance = 17 mu W cm-2) light delivery protocol was the most effective resulting in total growth inhibition during the 2-week observation period. Conclusion: Ultra low light fluence rate ALA-PDT results in significant spheroid growth inhibition. Repeated administration of ALA was required during repetitive and/or protracted single PDT treatment protocols. The existence of a lower fluence rate limit, below which the efficacy of threshold light fluences diminish was not found in these studies. Lasers Surg. Med. 41:578-584, 2009. (C) 2009 Wiley-Liss, Inc.