CURRENT STATUS OF PHOTODYNAMIC THERAPY IN ONCOLOGY

Photodynamic therapy (PDT) is a cancer treatment based on the accumula tion in malignant tissue of a photosensitiser with low systemic toxici ty. Subsequent illumination induces a type II photochemical reaction w ith singlet oxygen production that results in destruction of biomolecu les and subcellular organelles. The first full clinical report of PDT dates from 1976. Haematoporphyrin derivative, a complex mixture of por phyrins, was initially used as a photosensitiser. An enriched fraction (porfimer sodium) is now the most commonly used clinical agent. After systemic administration porphyrins bind to albumin and lipoproteins. Accumulation occurs mainly in tumours and organs of the reticuloendoth elial system. The light of an argon-dye laser can be tuned to the appr opriate wavelength and delivered either superficially, interstitially or intraluminally. Light distribution can be assessed by using a radia tion transport model and tissue optical properties, or direct measurem ent with light detectors. The effects of PDT depend in a complex way o n: characteristics, tissue concentration and localisation of the photo sensitiser; the target tissue optical properties and oxygenation; acti vation wavelength, power density and treatment regimen. Future researc h is directed towards: better photosensitisers (i.e. phthalocyanines, chlorins or protoporphyrin IX endogenously produced from 5-aminolevuli nic acid); improved light generation and delivery; and combination wit h hyperthermia, chemotherapy, radiotherapy or surgery. Adjuvant intrao perative PDT is a promising approach to destroying residual tumour aft er surgery.