PDT represents another modality for the treatment of human
malignancy. Photoactivated
hematoporphyrins have definite antitumor activity in both in vitro and in vivo experimental systems. Much of the early clinical work involved treatment of patients with advanced, recurrent disease who had not responded to conventional
therapy. Because good responses with acceptable toxicity have been obtained in these patients, active investigation continued and is aimed at defining the most appropriate sites and applications for the technique. Because of the limited depth of light penetration in tissue, the most promising sites may be those where there is limited thickness of
tumor, such as in superficial skin lesions or
carcinomas in situ involving the aerodigestive tract, bronchial tree, or genitourinary tract. Other potential uses include those where
PDT could be combined with surgical or chemotherapeutic debulking, such as pleural
mesothelioma or advanced stage
ovarian cancer. Whether
PDT can be of benefit in surgical cases where the margins of resection are close is an interesting but speculative notion at the present time. Clinical trials with
hematoporphyrin derivative PDT in the sites mentioned are in progress. Laboratory work to better understand HpD also continues, as well as investigations into alternative
photosensitizers with improved
tumor localization, less cutaneous photosensitivity, and absorption peaks at deeper penetrating wavelengths of light. Attempts at measuring
singlet oxygen, if successful, will permit the development of more meaningful dosimetry in order to correlate response with actual tissue levels of the purported
cytotoxic agent. Hopefully, these and other developments in the field of
PDT will improve the treatment for patients with
cancer.