bacteria [ 39 – 42 ]. The applications of PDT include treatment of acne, age-related
macular degeneration, arteriosclerosis, and cancer.
As shown in Fig.6.21, the PDT process uses a specific light wavelength in
conjunction with a light-sensitive drug that is administered before exposure to the
light. The drug can be mixed into a drink, injected into the body, or administered
topically on a diseased skin area or a tumor. The selected wavelength (typically
from a laser emitting in the red spectral region) corresponds to the absorption peaks
of the photosensitive drug. The photosensitive agent accumulates strongly in
malignant cells but not in healthy tissue. The role of the drug is to absorb the light
energy and transfer it to the diseased cells. The process of the photosensitive agent
absorbing energy from the light results in the formation of singlet oxygen (^1 O 2 ).
The singlet oxygen is a highly toxic reactant that causes an irreversible oxidation of
cell structures. The reactant has a short lifetime (<0.04μs) in biological tissue and
thus has a short radius of interaction (<0.02μm). As a result, the oxidative damage
caused by the singlet oxygen takes place in the immediate vicinity of where the
photosensitive molecule is located. Thereby the photochemical process causes
chemical destruction of the malignant cells.
As Fig.6.8shows, the wavelength range from 620 to 760 nm is a region of high
penetration depth into tissue. Thus for treatment efficiency of the PDT process, it is
advantageous to select drugs that are sensitive to light in this spectral range. An
advantage of using laser diode sources is that the light can be transmitted directly
from the laser to an internal treatment site (such as the esophagus, the stomach or
intestines, the urinary bladder, and the lungs) by means of opticalfibers [ 39 ]. The
exit port of thefiber can be aflat or an angle-polished bare end or some type of
diffuser can be attached on thefiber tips for homogeneous spot or cylindrical light
distribution. The diffuser could be a microlens, a side-firingfiber, or a short length
of side-emittingfiber, as shown in Fig.6.22. A cylindrical side-emitting light
diffuser typically is a plastic or a specially constructed glassfiber that produces a
radial light pattern, which is homogeneous along its entire length. Standard
cylindrical diffuserfiber sections range from 10 to 70 mm in length, have a nominal
core diameter of 500μm, a NA of 0.48, and a minimum bend radius of 10 mm.
Red laser
light beam(^1) O 2 1 O 2
Drug expelled
from normal cells
Light sensitive
drug injection
(^1) O 2 created
by red light
(^1) O 2 destroys
malignant cells
Normal
cells
Malignant
cells
Drug injection
Fig. 6.21 Concept of photodynamic therapy procedure
178 6 Light-Tissue Interactions