Light technology for efficient and effective photodynamic therapy: A critical review

Simple Summary Photodynamic therapy (PDT) is a promising treatment for cancerous tumours in which light technologies play a crucial role. An attempt to answer a long-standing question about which light source and light parameters are superior is carried out by reviewing the works reporting their effects on PDT outcome. We have identified the light characteristics that primarily affect the PDT process, based on the current evidence found in the literature. This review also examines cutting-edge technologies aiming to surpass the main challenge of PDT: low light penetration through the tissue. Whereas these technologies overcome several initial technical issues, they generate new challenges and pose limitations. We hope this review may be of interest to a broad audience, from bioengineers to clinical oncologists. The overall contribution we hope to make is to identify key roadblocks and provide a broad overview of light-based technologies, to foster improved developments and new perspectives towards enhanced PDT. Abstract Photodynamic therapy (PDT) is a cancer treatment with strong potential over well-established standard therapies in certain cases. Non-ionising radiation, localisation, possible repeated treatments, and stimulation of immunological response are some of the main beneficial features of PDT. Despite the great potential, its application remains challenging. Limited light penetration depth, non-ideal photosensitisers, complex dosimetry, and complicated implementations in the clinic are some limiting factors hindering the extended use of PDT. To surpass actual technological paradigms, radically new sources, light-based devices, advanced photosensitisers, measurement devices, and innovative application strategies are under extensive investigation. The main aim of this review is to highlight the advantages/pitfalls, technical challenges and opportunities of PDT, with a focus on technologies for light activation of photosensitisers, such as light sources, delivery devices, and systems. In this vein, a broad overview of the current status of superficial, interstitial, and deep PDT modalities—and a critical review of light sources and their effects on the PDT process—are presented. Insight into the technical advancements and remaining challenges of optical sources and light devices is provided from a physical and bioengineering perspective.