Image-guided Interstitial Photodynamic Therapy for Squamous Cell Carcinomas: Preclinical investigation.

OBJECTIVE:Photodynamic therapy (PDT) is a clinically approved minimally invasive treatment for cancer. In this preclinical study, using an imaging-guided approach, we examined the potential utility of PDT in the management of bulky squamous cell carcinomas (SCCs). METHODS:To mimic bulky oropharyngeal cancers seen in the clinical setting, intramuscular SCCs were established in six-to-eight week old female C3H mice. Animals were injected with the photosensitizer, 2-[hexyloxyethyl]-2-devinyl pyropheophorbide-a (HPPH; 0.4 μmol/kg, i.v.) and tumors were illuminated 24 hours post injection with 665 nm light. PDT as a single treatment modality was administered by surface illumination or by interstitial placement of fibers (iPDT). Magnetic resonance imaging was used to guide treatment and assess tumor response to PDT along with correlative histopathologic assessment. RESULTS:Interstitial HPPH-PDT resulted in a marked change on T2 maps 24 hours post treatment compared to untreated controls or transcutaneous illumination. Corresponding apparent diffusion coefficient maps also showed hyperintense areas in tumors following iPDT suggestive of effective photodynamic cell kill. Histologic sections (H&E) confirmed presence of extensive tumor necrosis following iPDT. CONCLUSIONS:These results highlight the potential utility of PDT in the treatment of bulky oropharyngeal cancers. The findings of our study also demonstrate the utility of MRI as a non-invasive tool for mapping of early tissue response to PDT.