Detection and localization of pre-cancerous lesions and early lung cancer using tissue autofluorescence
Creators: name: Hung, Yip-Chan Jacyln
Publisher: name: University of British Columbia
Year: 2008

description: In this work, two different yet related hypotheses were tested by experimental means as follows: i) pre-cancerous and non-invasive (early) lung cancer can be detected and localized using the fluorescence properties of tumour localizing drugs at non-photosensitizing doses to skin tissue; ii) significant differences exist in laser-induced autofluorescence between normal, pre-cancerous and cancerous tissues such that these differences alone can be exploited to detect and delineate early lung cancer without using exogenous drug(s). For most cancers, including lung cancer, a five-year survival of over 90% can be realized if the malignancy is diagnosed at the carcinoma in situ stage. However, current techniques: chest x-ray, sputum cytology, and conventional bronchoscopy alone or in combination cannot detect these very small lesions which are usually only a few cell layers thick and a few millimeters in surface diameter. Exogenous fluorescent tumour markers such as hematoporphyrin derivatives (e.g. Photofrin) have been used to enhance the detection of these occult lung lesions. Photofrin is preferentially retained in tumour tissues compared to the surrounding normal tissues; it fluoresces at 630 rim and 690 run when excited at —405 rim. based on this principle several imaging and non-imaging devices have been developed. However, wider clinical applications were limited due to the skin photosensitivity property of Photofrin. We have postulated that this could be solved by employing a much lower dose of photofrin (0.25 mg/kg) which was believed to be less photosensitizing to human patients. This postulate was experimentally tested by ratio fluorometry and early lung cancers were detected with no false negative results and no apparent skin photosensitivity. An important finding in this study was that the mechanism for detection of early cancer was mainly due to the differences in the green autofluorescence between normal and malignant tissues, rather than fluorescence of tumour localizing drug. This discovery led to the second postulate of this thesis that tissue autofluorescence alone can be exploited for the detection of early lung cancer. In vivo spectroscopy using an optical multi-channel analyzer showed an overall decrease in autofluorescence in pre-cancerous and cancerous lesions when excited by 405 nm or 442 nm laser light. A stepwise discriminant function analysis was performed on a database of nearly 300 patients spectra to determine the optical emission wavelength(s) and algorithm(s) at which the normal, pre-cancerous and cancerous tissues can be differentiated. The results indicated that algorithm(s) could be developed to clearly delineate early lesions from the normal tissues. Several algorithms were then tested using a non-imaging ratio fluoronieter device and a prototype imaging fluorescence system to detect early lung cancer and dysplasia during standard bronchoscopy, therefore confirming the initial hypotheses even in a clinical setting. The major source of the autofluorescence in the normal bronchial tissue was determined to come from the sub-epithelial layers. The mechanism for the decrease in the autofluorescence in pre-malignant and malignant tissues was explored but not yet completely elucidated. Several factors such as decrease in fluorophores, increase in absorption of the excitation/fluorescent light or different redox state of the fluorophores may be responsible for the observed phenomena.