Tumor specific response to photodynamic therapy.

The exact mechanism by which photodynamic therapy (PDT) causes tumor destruction has not been elucidated. Early reports indicated that PDT causes direct cellular effects probably mediated by unstable oxygen species, resulting in cellular oxidation and death. More recently, PDT effects on tumor blood flow have been implicated, and there are questions as to whether the PDT response is specific to tumor tissue. Rats were implanted with a window chamber containing either a mammary adenocarcinoma or a piece of inert surgical sponge. After growth of the tumor was ascertained, all rats were given 5 mg/kg Photofrin intraperitoneally, and then were irradiated with 630 nm light 24 hours post-injection. Caliper thickness and reflectance measurements were performed before and after irradiation; all animals were sacrificed 72 hours post-PDT and the chambers submitted for histologic analysis. Animals implanted with tumors demonstrated marked edema of the chamber with an associated decrease in reflectance. No edema response was noted in the chambers containing inert sponge, or in any controls. Nonselective PDT effects (characterized by a marked foreign body response) in chambers containing sponge was not seen. Histologic analysis of treated specimens corroborate the above data.