Phototesting And Dosimetry For Photochemotherapy

Photochemotherapy is a recently developed approach to the treatment of psoriasis (Parrish et al., 1974) which may have initiated a new era in photomedicine. It is based on the interaction of longwave ultraviolet light (UVA) and a systemically administered photoactive compound, 8-methoxypsoraIen, within the skin; hence the designation PUVA (PUVA = 8-methoxy-Psoralen plus UVA) (WolfFer a/., 1976). As the penetration of UVA is restricted to the superficial layers of the skin and as the photoactivadon of 8-methoxypsoralen occurs only under the influence of these wave-lengths ('365 nm), PUVA represents a successful attempt to localize systemic chemotherapy to a diseased organ, the skin, sparing other tissues from cytotoxic effects (Parrish et al., 1974; Wolff er al., 1975b, 1976). The dramatic effectiveness of this treatment, both in clearing psoriatic lesions and maintaining patients in a state of remission, has been demonstrated in several studies (Parrish ei ai, 1974; Wolfl' al., 1975a, b, 1976). The rationale of PUVA therapy is to bring psoriasis into remission by repeated, controlled photosensitization reactions which are monitored to remain within a therapeutically desired range. Since phototoxic erythema is a limiting factor, careful attention to dosimetry is essential and it is therefore the dosimetry system which is a key to both the effectiveness and safety of PUVA. The following account outlines and discusses criteria for dosimetry that have proved useful in 230 patients with severe, generalized psoriasis.