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The effects of fluence rate are investigated in human glioma spheroids incubated in 5-aminolevulinic acid (ALA). It is shown that the response of glioma spheroids to ALA-mediated PDT depends strongly on the rate at which the light dose is delivered. At low doses, lower fluence rates are more effective. For example, at a dose of 50 JIcm2, near total spheroid kill is observed at fluence rates of as low as 1 0 mW/cm2. Below 1 0 mW/cm2, however, treatments are not as effective. The fluence rate effect is not as pronounced at higher doses where a favorable response is observed throughout the range of fluence rates investigated. The clinical implications ofthese findings are discussed.
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