Photobleaching behavior of protoporphyrin IX during 5-aminolevulinic acid marked glioblastoma detection
The highly malignant brain tumor, glioblastoma multiforme (GBM), is difficult to fully delineate during surgical
resection due to its infiltrative ingrowth and morphological similarities to surrounding functioning brain tissue.
Selectiveness of GBM to 5-aminolevulinic acid (5-ALA) induced protoporphyrin IX (PpIX) is reported by other
researchers to visualize tumor margins under blue light microscopy. To allow objective detection of GBM, a compact
and portable fiber optic based fluorescence spectroscopy system is developed. This system is able to deliver excitation
laser light (405 nm) in both the continuous and pulsed mode. PpIX fluorescence peaks are detected at 635 and 704 nm,
using a fiber-coupled spectrometer. It is necessary to optimize the detection efficiency of the system as the PpIX quickly
photobleaches during the laser illumination. A light dose of 2.5 mJ (fluence rate = 9 mJ/mm2) is experimentally approved
to excite an acceptable level of fluourescence signal arising from glioblastoma. In pulsed illumination mode, an
excitation dose of 2.5 mJ, with a dark interval of 0.5 s (duty cycle 50%) shows a significantly shorter photobleaching
time in comparison to the continuous illumination mode with the same laser power (p < 0.05). To avoid photobleaching
(the remaining signal is more than 90% of its initial value) when measuring with 2.5 mJ delivered energy, the time for
continuous and pulsed illumination should be restricted to 2.5 and 1.1 s, respectively.
This paper was published in SPIE Proceedings Vol. 7161