The role of medical imaging in the context of photodynamic therapy

Photodynamic therapy (PDT) is a modality with promising results for the treatment of various cancers. PDT is increasingly included in the standard of care for different pathologies. The role of medical imaging in this context is crucial to better understand how and where to deliver the therapy but also to observe the different mechanisms involved in the effects on tumors. At different stages of delivery, PDT requires imaging to plan, evaluate and monitor treatment. In this paper, we review the contribution of Magnetic Resonance Imaging or Positron Emission Tomography for planning and therapeutic monitoring purposes. Several solutions have been proposed to plan PDT from imaging. MRI and dedicated segmentation algorithm have been recently proposed to plan interstitial PDT with stereotactic localization and light diffusion simulation capabilities. Additionally, photosensitizer biodistribution has been evaluated with radiolabeled photosensitizers. The effects of PDT delivery have also been explored with specific Magnetic Resonance Imaging or Positron Emission Tomography radiopharmaceuticals to evaluate the effects on cells (apoptosis, necrosis, proliferation, metabolism) or vascular damage. The contribution of medical imaging in the context of photodynamic therapies is important and continues to increase. Using morphological or molecular imaging has to be considered for future developments of PDT.