The biological basis for changes in autofluorescence during neoplastic progression in oral mucosa
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Autofluorescence spectroscopy can improve the early detections of oral cancer. Biochemical and structural changes associated with dysplastic progression alter the optical properties of oral mucosa and cause diagnostically significant differences in spectra from normal and neoplastic sites. This dissertation describes experimental and modeling studies aimed at revealing biological reasons for the diagnostically significant differences observed in depth-resolved fluorescence spectra from normal and neoplastic oral mucosa. An experimental approach, based on high-resolution fluorescence imaging, is used to study the autofluorescence patterns of oral tissue. At UV excitation, most of the epithelial autofluorescence originates from cells occupying the basal and intermediate layers, while stromal signal originates from collagen and elastin crosslinks. With dysplasia, epithelial autofluorescence increases, while autofluorescence from subepithelial stroma drops significantly. Benign lesions also display a drop in autofluorescence from subepithelial stroma, but have different epithelium fluorescence patterns compared to dysplasia. Optical probes that measure mostly stromal fluorescence, may reveal a similar loss of fluorescence intensity and thus fail to distinguish benign inflammation from dysplasia. These results emphasize the importance of using probes with enhanced detection of epithelial fluorescence for improved diagnosis of different types of oral lesions. The second part of this work presents a Monte Carlo model that predicts fluorescence spectra of oral mucosa obtained using a depth-selective probe as a function of tissue optical properties. A model sensitivity analysis determines how variations in optical parameters associated with neoplastic development influence the intensity and shape of spectra, and elucidates the biological basis for differences in spectra from normal and premalignant oral mucosa. Spectra of oral mucosa collected with the depthselective probe, are affected by variations in epithelial optical properties and to a lesser extent by changes in superficial stromal parameters, but not by changes in the optical properties of deeper stroma. Changes in parameters associated with dysplastic progression lead to a decreased fluorescence intensity and a shift of the spectra to longer emission wavelengths. Decreased fluorescence is due to a drop in detected stromal photons, whereas the shift of spectral shape is attributed to an increased fraction of detected photons arising in the epithelium.