Evaluation of photodynamic therapy-induced edema in the rat brain using magnetic resonance imaging

Failure of treatment for high-grade gliomas is usually due to local recurrence at the site of surgical resection indicating that a more aggressive form of local therapy should be used. Photodynamic therapy (PDT) shows promising results in the treatment of gliomas as it has been shown to cause significant destruction of tumor cells through localized necrosis and/or apoptosis. An unfortunate complication of this therapy is the induction of cerebral edema which causes increased intracranial pressure and associated morbidity. A potential solution is the administration of steroids to reduce PDT-induced edema, associated morbidity, and mortality in treated animals. In the research described here, high field strength (3 T) magnetic resonance imaging (MRI) was used for screening and detecting the extent and time evolution of PDT-induced edema. Results using inbred Fischer 344 rats subjected to PDT showed that this noninvasive imaging technique was very useful for studying and evaluating treatment-induced edema, its dependence on the PDT light fluence, and the effects of steroids. The data suggest that higher light fluences cause greater edema formation and higher mortality, while the addition of postoperative steroid treatment reduces the incidence of treatment-induced morbidity and mortality. The images also provided information about blood-brain barrier patency, which is important in therapeutic regimens involving the delivery of drugs (e.g. chemotherapeutic agents) to the brain. PDT appeared to open the BBB for a period of 24-48 hours after which it was restored