Photodynamic therapy consisted of two components: 1) a photo sensitiser that made
the tissue sensible for light and 2) intense light with a specific wavelength that caused
the destruction of infected cells. There were two types of treatment that both used
the property that cancer cells would more easily absorb certain photo sensitisers and
were more reluctant to release them than healthy cells would. With one method, the
photo sensitiser or its metabolic precursor (a substance that only within the tissue
was transferred into a photo sensitiser) was applied to the affected tissue in the form
of an ointment and covered with a light-tight bandage. When after a few hours the
photo sensitiser was formed and/or absorbed by the cancer cells, the treated area
was radiated with light.
With an other method, the photo sensitiser was injected into the bloodstream and
absorbed by cells all over the body. When after a period of 24 to 48 hours the photo
sensitiser had disappeared from the healthy cells, the infected cancer cells (that still
contained some of the photo sensitiser) were radiated with light. The first method,
using an ointment, could be used externally. The second method could also be used
internally, using fiber optics to bring the light to the area to be treated.
The exact wavelength of the light component of both methods depended on the type
of photo sensitiser but was mostly in the range of 620 to 780 nanometer, so red to
nearby infrared. Under the influence of the light, the photo sensitiser reacted with
oxygen present in the tissue, releasing active singlet oxygen, an aggressive chemical
that reacted with the infected cell in a destructive way. The exact result of the
reaction depended strongly on the composition of the applied photo sensitiser. Since
photo sensitisers tended to build up in tumours more than in healthy tissue, especially
cancer cells would die and damage to healthy tissue would be limited. A part of the
cells would die by apoptosis, a natural form of decease. An other part would die by
necrosis, leaving the remainder of the cell for the immune system to be expelled. This
could cause inflammation, swelling, pain and even scarring, so photodynamic therapy
was only allowed to be executed by a physician.