Imaging protoporphyrin IX photoproduct accumulation as a dosimetry reporter for monitoring photodynamic therapy of oral cancer

Significance: Photodynamic therapy (PDT) for the treatment of oral cancers and oral potentially malignant lesions can be enhanced by the capability of the photosensitizer to serve as a fluorescence contrast agent for treatment guidance. The development of image-based dosimetry reporters can inform treatment progress in real time to avoid under-treatment, leading to incomplete response and recurrence. Aim: We investigate the hypothesis that imaging of protoporphyrin IX (PpIX) photoproduct (PP) accumulation may be leveraged as an implicit PDT dosimetry reporter for PDT using 5-aminolevulinic acid (ALA)-induced PpIX photosensitization. Approach: In initial spectroscopy studies, we investigate dose-dependent changes in absorption and fluorescence spectra of PpIX corresponding to PP accumulation during red light (635 nm) delivery. We use spectral analysis to select fluorescence excitation and spectral filtering components for PP imaging during treatment. We evaluate the capability for imaging PP accumulation concomitant with PpIX photobleaching in tissue phantoms, 3D oral squamous cell carcinoma (OSCC) models, and in murine xenografts. Results: Spectroscopy shows fluence-dependent changes in PpIX optical properties, and that excitation of photobleached PpIX with 450 nm light produces fluorescence emission associated with PpIX PPs. An existing handheld intraoral probe is shown to be capable of imaging dose-dependent PP accumulation with the addition of a spectral filter to isolate fluorescence emission longer than 650 nm. PP signal increases concomitant with PpIX photobleaching in a fluence-dependent manner and correlates with the extent of cytotoxic response in 3D cultures. PP accumulation is also observed to occur concomitantly with photobleaching in OSCC subcutaneous xenografts. Conclusions: Overall, the results show that imaging of PP accumulation is feasible by adapting traditional photodiagnosis optical components and may serve as a dosimetry reporter for ALA-PDT, which is complementary to the measurement of PpIX photobleaching. Keywords: 5-aminolevulinic acid (ALA); oral cancer; photobleaching; photodynamic therapy; photodynamic therapy dosimetry; protoporphyrin IX photoproducts.