ScienceDaily — It is known that fluctuations in the treatment outcome of photodynamic therapy (PDT) between patients are related to the concentration of photosensitizer in target tissue. The purpose of a study, by Wang et al, published in the February 2010 issue of Experimental Biology and Medicine was to establish a fluorescence method not only providing a semi-quantitative measurement of photosensitizer concentration but also predicting the efficacy of PDT for Port Wine Stains (PWS).