Figure 1.

Schematic of PDT for the treatment of glioblastoma with simplified energy diagram of the oxygen dependent photodynamic response. The photosensitizer in the ground state (⁰PS) is excited to one of two states by the appropriate wavelength and power photostimulation, the first excited singlet state (¹PS) or the second excited singlet state (²PS). The ¹PS may then convert to the excited triplet state (³PS) via intersystem crossing. In the presence of molecular oxygen, the ³PS may undergo Type I or Type II redox reactions producing reactive oxygen species which cause tumor cell death.