Photodynamic therapy is a photochemical technique for the local destruction of tumours, entailing the interaction of light with an administered photosensitiser to produce a cytotoxic effect. We investigated the tissue distribution of the photosensitiser aluminium sulphonated phthalocyanine (AlSPc) in dimethylhydrazine induced colonic tumours and adjacent normal colon in rats. Forty eight hours after intravenous injection, most tumours contained twice as much AlSPc as normal colon. Tumour size and position in the colon did not affect AlSPc concentration. Microscopic fluorescence localisation of AlSPc showed significant photosensitiser accumulation in tumour stroma, whereas tumour and normal mucosa contained similar amounts. Thus, some normal tissue damage, where malignant cells invade normal areas, would inevitably accompany eradication of tumours. Tumour destruction and healing of colon after tumour eradication were examined histologically. There was sharp demarcation between necrotic areas (tumour or normal) and adjacent tissue and, whether the treated area was tumour or normal, healing occurred by regeneration of normal tissue. Some incompletely eradicated large tumours showed evidence of delayed bleeding. The possibility of selective uptake or preferential retention of the photosensitiser in tumours formed the initial basis for investigation of photodynamic therapy, but it is now clear that this is seldom the most important factor for tumour eradication. Of far greater importance is the nature of the biological effect of photodynamic therapy as necrosis of small tumours involving the full thickness of the bowel wall can be achieved with safe healing by regeneration of normal colon. The maximum depth of necrosis produced was only a few millimetres, so this technique is unlikely to be of value as the primary treatment for large colonic tumours but may prove of value for eradicating small lesions or as adjunctive therapy for eradication of small nests of tumour remaining or recurring in the tumour bed after conventional surgery.
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