Surface dosimetry in phototherapy: comparison of three ultraviolet B lamps used in the treatment of psoriasis

The quantitation (dosimetry) of ultraviolet B irradiation (UVB) administered during phototherapy for psoriasis is fundamental to this modality of treatment because only absorbed irradiation can cause a photochemical reaction and, hence, a phototherapeutic effect. Significant differences may exist in surface dosimetry achieved with different UVB irradiators. and this could have important consequences for the observed clinical efficacy of phototherapy as well as practical implications for avoiding burning when transferring patients between systems. Multiple anatomically referenced polysulphone photodosimeters were applied to the skin of five subjects receiving treatment for severe psoriasis. Three subjects were irradiated with both a free-standing hot quartz lamp and a fluorescent cabinet. Two others were irradiated with a fiat bank of fluorescent tubes. Topographical maps of surface irradiation were constructed, ‘Cold’ areas (<5% of stated dose) included the neck, upper chest, shoulders, and anterolateral thighs for the hot quartz lamp; the medial arms, medial legs, and feet for the cabinet: and the lateral arms, forearms, lower legs, and feet for the bank of tubes. ‘Hot’ areas (>100% of stated dose) included the forearms and posterior thighs for the hot quartz lamp; the lateral arms, dorsal forearm, and abdomen for the cabinet; and the central trunk (anterior and posterior) for the bank of tubes, Thus, striking differences in surface irradiation exist among the three UVB irradiators studied, and this has an impact on the results of phototherapy for psoriasis. Knowledge of these differences is of practical importance in achieving maximum therapeutic potential as well as minimizing the risk of burning reactions, particularly if patients are to be transferred between systems.