EXPERIMENTAL STUDIES TO ASSESS THE POTENTIAL OF PHOTODYNAMIC THERAPY FOR THE TREATMENT OF BRONCHIAL CARCINOMAS

Background-Photodynamic therapy (PDT) is a technique for producing loc alised tissue necrosis with light after prior administration of a phot osensitising drug. There is some selectivity of uptake of photosensiti sers in malignant tissue, although this is difficult to exploit. Full thickness necrosis in normal and neoplastic colon heals without perfor ation because of a lack of effect on collagen, making local cure a pos sibility. The experiments described here aim to establish whether thes e conclusions are also valid for bronchial tumours. Methods-In pharmac okinetic studies normal rats were given 5 mg/kg of the photosensitiser aluminium sulphonated phthalocyanine (AISPc) intravenously and killed up to one month later. The distribution of AISPc in the trachea was m easured by chemical extraction and fluorescence microscopy. In subsequ ent experiments sensitised animals were treated with light delivered t o the tracheal mucosa through a thin flexible fibre and the resultant lesions were studied for their size, mechanical strength, and healing. A series of resected human bronchial carcinomas were examined histolo gically for their collagen content. Results-The tracheal concentration of AISPc in normal rats was maximum 1-20 hours after administration. Fluorescence microscopy revealed that most was in the perichondrium an d submucosal stroma, with little in the cartilage. Light exposure show ed necrosis of the soft tissues which healed by regeneration, but no e ffect on cartilage and no reduction in the mechanical strength of the trachea at any stage. Histological examination of resected human bronc hial carcinomas showed more collagen in the tumour areas than would be found in normal regions. Conclusions-PDT leads to necrosis of the sof t tissues of the normal trachea but there is complete healing by regen eration, no risk of perforation (due to collagen preservation), and no effect on cartilage. Human bronchial carcinomas apparently contain mo re collagen than normal bronchi which may give protection against perf oration following necrosis induced by PDT. PDT may have a role in erad icating small volumes of tumour tissue in situ and could be valuable f or treating (1) small carcinomas in patients.unfit for resection, (2) tumour remaining after surgical resection, (3) stump recurrences, or ( 4) to prolong palliation of tumours after debulking with the NdYAG las er.