Protoporphyrin IX (
PpIX), a biochemical converted from 5-aminolevulinc
acid (5-ALA) in living cells, is useful for intraoperative fluorescent detection of
cancer metastasis in lymph nodes (LNs). However, unknown is whether the fluorescence of
PpIX can be detected in the LNs when they coexist with
indigo carmine, a blue
dye commonly used for identification of sentinel LNs during surgery. To address this issue, we sought to evaluate the diagnostic usefulness of
PpIX fluorescence in the presence of
indigo carmine in a mouse LN
metastasis model of
rectal cancer after administration of 5-ALA. Spectral analysis of pure chemicals revealed that the absorption spectrum of
indigo carmine widely overlapped with the fluorescence spectrum of
PpIX specifically at the peak of 632nm, a common emission wavelength for detecting
PpIX, but not at the other peak of 700nm. Due to such spectral overlap, the
PpIX fluorescence intensity was significantly attenuated by mixture with
indigo carmine at 632nm, but not at 700nm. Accordingly, fluorescent measurements of the mouse metastatic LN revealed more intense presentation of
PpIX at 700nm than at 632nm, indicating that the diagnostic usefulness is greater at 700nm than at 632nm for the
indigo carmine-dyed LNs after administration of 5-ALA. From these observations, we propose that the fluorescence measurement is more efficient at 700nm than at 632nm for detection of
PpIX in metastatic LNs stained with
indigo carmine.