Photodynamic therapy (PDT) uses photooxygenation to destroy cancerous tissue.  A photosensitizer is injected into the tumor and then specific wavelengths of light are exposed to the tissue to excite the Sens. The excited Sens generally follows a type I or II photooxygenation mechanism to result in oxidative damage to cells. Extensive oxidative damage to tumor cells will kill tumor cells. Also oxidative damage to nearby blood vessels will cause local agglomeration and cut off nutrient supply to the tumor, thus starving the tumor.
An important consideration when selecting the Sens to be used in PDT is the specific wavelength of light the Sens will absorb to reach an excited state. Since the maximum penetration of tissues is achieved around wavelengths of 800 nm, selecting Sens that absorb around this range is advantageous as it allows for PDT to be affective on tumors beneath the outer most layer of the dermis. The window of 800 nm light is most effective at penetrating tissues because at wavelengths shorter than 800 nm the light starts to be scattered by the macromolecules of cells and at wavelengths longer than 800 nm water molecules will begin to absorb the light and convert it into heat.