Significance: The diagnostic depth of photodynamic diagnosis (PDD) for gastric cancer with protoporphyrin IX (PpIX) is limited, which leads to missing intramucosal cancers in screening and surgery.
Aim: The reason is that the excitation light, whose wavelength is determined by the highest absorption peak of PpIX (∼405 nm), is strongly attenuated by mucosal tissues. We investigated an excitation wavelength that can extend the diagnostic depth of PpIX fluorescence at the mucosal subsurface.
Approach: By calculating the depth-dependent intensity of the excitation light in porcine gastric mucosa for each wavelength, relationships among the wavelength, fluorophore depth, and fluorescence intensity were assessed and fluorescence images of PpIX pellets located at different fluorophore depths were compared experimentally by changing the excitation wavelength.
Results: The numerical calculation showed that a 505-nm excitation light provided the highest fluorescence intensities at a fluorophore depth deeper than 1.1 mm. In the fluorescence observation, the fluorescence intensities at fluorophore depths of 0 and 1.0 mm at 405 nm were 5.4 × 103 and 1.0 × 103 arb. units, whereas those at 505 nm were 5.3 × 101 and 1.9 × 102 arb. units, respectively.
Conclusion: The experimental results suggest that the diagnosis depth of PDD with PpIX for intramucosal cancer can be extended by 505-nm excitation light.
Photodynamic diagnosis (PDD) of gastric cancer with protoporphyrin IX (PpIX) induced by 5-aminolevulinic acid (ALA) will lead to a method to estimate the invasive depth. The excitation light for ALA-based PDD is attenuated by absorption and scattering of the mucosal tissue according to its wavelength and the difference contains the fluorophore depth information. To realize the method to estimate the invasive depth for the intramucosal cancer, this study investigated the differences of the attenuation rate between two excitation wavelengths. Considering the optical properties of mucosa and PpIX, relationships between the invasive depths of cancer containing PpIX and the fluorescence intensity ratios detected from the mucosa were estimated using a numerical model. An experiment was also performed using a pellet made of epoxy resin containing PpIX. The excitation lights with wavelengths of 405 and 505 nm having the same power density were each used to irradiate the pellet placed at the fluorophore depth of 0 mm, 1.0 mm, and 1.7 mm from the mucosal surface to obtain fluorescence images, and the average fluorescence intensities of the images were evaluated. The numerical and experimental results suggested the possibility to estimate the fluorophore depth of the PpIX fluorescence source by comparing the fluorescence images with excitation lights at wavelengths of 405 and 505 nm.