Frequency of sensitization to methyl aminolaevulinate after photodynamic therapy

Allergic contact dermatitis to methyl aminolaevulinate (Metvix) after topical application in photodynamic therapy (PDT) has previously been described in case reports.

To compare the frequency of sensitization to Metvix cream in a group of patients previously treated at least five times with Metvix-PDT with the frequency observed in an unexposed control group.

Twenty patients treated five times or more with Metvix-PDT and 60 controls with no prior exposure to Metvix were patch tested with Metvix cream and Metvix placebo cream. Subsequently, the patients were interviewed to determine the relevance of a positive patch test reaction to Metvix.

Of 20 patients treated with Metvix-PDT, 7 were sensitized to Metvix cream, giving a sensation risk of 35%. In the control group, 1 of 60 became sensitized after a single exposure to Metvix cream (1.7%). There was no reaction to the placebo cream. The positive patch tests to Metvix were considered relevant in four of seven patients (57%).

This study demonstrates a considerable risk of sensitization after Metvix-PDT. We suggest that the patients are interviewed to detect late or persistent local reactions after PDT. These reactions are often considered to be local infections but may represent allergic contact dermatitis, and therefore, patients should be offered patch testing with Metvix cream.