5-aminolevulinic acid-induced porphyrin contents in various brain tumors: implications regarding imaging device design and their validation

BACKGROUND: Fluorescence-guided resections using 5-aminolevulinic acid (5-ALA)-induced tumor porphyrins have been established as an adjunct for malignant glioma surgery based on a phase III study using specifically adapted microscopes for visualizing fluorescing protoporphyrin IX (PPIX). New hardware technologies are being introduced, which claim the same performance as the original technology for visualizing fluorescence. This assumes that qualitative fluorescence detection is equivalent to the established standard, an assumption that needs to be critically assessed.

OBJECTIVE: To determine PPIX concentrations (cPPIX) in tissue that can be detected visually using the established BLUE400 filter system (Carl Zeiss Meditec, Oberkochen, Germany) as a basis for defining the performance of this system.

METHODS: Utilizing a hyperspectral imaging system, tumor samples from patients harboring different tumor tissues, with or without visible fluorescence, were analyzed. Absolute values of cPPIX were calculated after calibrating the system with fluorescence phantoms with known cPPIX.

RESULTS: A total of 524 tumor samples from 162 patients were analyzed. Visual fluorescence under the BLUE400 filter was documented by experienced neurosurgeons. A 0.9 μg/ml threshold of cPPIX was defined as the minimal concentration required to detect and discriminate visual fluorescence.

CONCLUSION: This is the first report providing data on the threshold of cPPIX, which is visually detected using the current generation of microscopes, thus defining the specificity and sensitivity of this technology as initially tested in a randomized trial. Novel technologies should show similar characteristics in order to be used safely and effectively. If more sensitive, such technologies require further assessments of tumor selectivity.