[Determining the tumor-cell density required for macroscopic observation of 5-ALA-induced fluorescence of protoporphyrin IX in cultured glioma cells and clinical cases].

We investigated the association between the cell density and intensity of 5-aminolevulinic acid-induced fluorescence of protoporphyrin IX in 3-dimensionally cultured C6 glioma cells. The ratio between 636-nm red fluorescence excited by a 405-nm laser and 513-nm green autofluorescence of the tissue was measured as the fluorescence intensity. A ratio exceeding 0.68 was macroscopically judged as fluorescence-positive by observers, and the cell density at this fluorescence intensity was 1×107< cells/mL. In clinical surgical fields, the fluorescence ratio was about 0.65 in cases judged as fluorescence-positive, similarly to that in cultured cells. Neurosurgeons and pathologists should recognize that tumor cells are present in fluorescence-negative regions. It is necessary to develop a device which measures fluorescence more simply than macroscopic observation for cutting-edge brain surgery.