Photodynamic therapy for treatment of solid tumors--potential and technical challenges

Abstract

Photodynamic therapy (PDT) involves the administration of photosensitizer followed by local illumination with visible light of specific wavelength(s). In the presence of oxygen molecules, the light illumination of photosensitizer can lead to a series of photochemical reactions and consequently the generation of cytotoxic species. The quantity and location of PDT-induced cytotoxic species determine the nature and consequence of PDT. Much progress has been seen in both basic research and clinical application in recent years. Although the majority of approved PDT clinical protocols have primarily been used for the treatment of superficial lesions of both malignant and non-malignant diseases, interstitial PDT for the ablation of deep-seated solid tumors are now being investigated worldwide. The complexity of the geometry and non-homogeneity of solid tumor pose a great challenge on the implementation of minimally invasive interstitial PDT and the estimation of PDT dosimetry. This review will discuss the recent progress and technical challenges of various forms of interstitial PDT for the treatment of parenchymal and/or stromal tissues of solid tumors.

Figure 1

Interstitial PDT using a cylindrical diffuser. (A) Implantation of a cylindrical diffuser with 2-cm active length. (B) Cylindrical pattern of light emission. (C) Zone of tumor necrosis.

Figure 2

Cellular PDT versus vascular PDT. (A) Long drug to light interval (DLI, > 24 h) allows the photosensitizers to be accumulated by target cells and therefore causes intracellular damages. (B) At short DLI (< 30 min), the light irradiation takes place while the photosensitizers are still circulating in the vascular compartment and therefore mainly causes direct vascular damages.