What does photodynamic therapy have to offer radiation oncologists (or their cancer patients)?

Major advances have recently been made in photodynamic therapy (PDT) for cl inical application, including the development of more powerful photosensiti zers and light sources and suitable light applicators. PDT is emerging as a n attractive new form of cancer therapy, suitable for treating superficial lesions (less than I cm in depth) and carcinoma in situ, or as an adjuvant to surgery for more bulky disease. PDT is therefore complementary to radiot herapy which is better suited to treating larger rumours. There are some qu alitative similarities between light distribution in tissue during superfic ial illumination and ionizing radiation dose distributions during external beam irradiation, or between interstitial PDT and brachytherapy, although t he geometric scale is very different (visible light penetrates a maximum of 5-10 mm in tissue). The contribution of scattered light to tissue irradian ce is much greater than for ionizing radiation and in situ light dosimetry is very important (although rather complicated) to ensure adequate illumina tion without over-treating. Dosimetry and treatment planning are highly adv anced for ionizing radiation and are routine in all radiotherapy department s. Proper in situ light dosimetry and dose distribution calculation for PDT is in its infancy. Physicists have an important role to play in the furthe r optimization of clinical PDT and much of the infrastructure and expertise present in the radiotherapy department is ideally suited to accommodate PD T. In this review, parallels and contrasts are made between PDT and ionizin g radiation for both mechanistic and dosimetric aspects of the therapies. A summary of the most interesting clinical applications is also given. (C) 1 998 Elsevier Science Ireland Ltd. All rights reserved.