Biomedical Engineering Reference
In-Depth Information
Three types of phototherapy are in common use today, on the basis of
the light source in use for treatment. The type of lamp selected for use in
treatment depends upon the patient's diagnosis, the disease location and the
patient's skin type. All types of phototherapy are less effective in dark-skinned
individuals because melanin blocks some of the therapeutic light transmission.
The most widely available is UVB therapy, using lamps that emit light pri-
marily in the wavelengths from 280 to 320 nm. Because of its similarity to solar
UVB emission, treatment can be administered daily, taking advantage of the
body's adaptive responses to UVB light. In use since the early part of the last
century, it is a well-characterized treatment method. Because the wavelengths
of light emitted are the same wavelengths responsible for photocarcinogenesis,
it is used sparingly. Studies examining whether UVB phototherapy produces
increased cancer risk suggest that the increase is modest, due to physician
supervision of light treatment and the fact that its use is generally intermit-
tent for disease therapy. Tanning also occurs when patients are treated with
this light source, which decreases the e
cacy of light treatment. Disease lo-
cated deep within the skin is di
cult to treat because of absorption of UVB
wavelengths primarily in the epidermis [9].
In addition to UVB phototherapy, two other light sources have been in-
troduced in the more recent past for phototherapy treatment. The creation
of UVA light sources (320-400 nm) triggered an investigation of whether this
light source could be useful for disease located deep in the skin, which is re-
sistant to UVB treatment because of poor skin penetrating capacity. To make
UVA wavelengths e
cacious as a treatment method, light exposure occurs
after patients have taken the photosensitizing drug psoralen. The treatment
method is, therefore, termed “PUVA,” meaning psoralen + UVA. When pso-
ralen is present in the skin during UVA irradiation, psoralen becomes cross-
linked to DNA. This initiates the repair pathways discussed earlier, and has
a beneficial effect on a number of diseases. Because the treatment produces
more inflammatory change than does UVB therapy, it is only given 2-3 times
per week to allow ample time for repair. As this treatment also produces
crosslinks in DNA, the risk of photocarcinogenesis is quite high; studies have
demonstrated that the rate of squamous cell skin cancer occurring after PUVA
increased eightfold and that melanoma rates are increased as well. Nonethe-
less, PUVA treatment is significantly more e
cacious than UVB treatment,
and therefore in cases of refractory disease, it is used [9]. The lamps used
in tanning parlors are the same as those used for PUVA treatment, but the
public does not take a photosensitizing drug prior to their visit!
To try and reduce the undesirable side effects of UVB and PUVA treat-
ment, lamps that emit light most intensely at 312 nm have been brought to
market, termed as narrow-band UV lamps. This light source avoids some of
the cancer risk of broad-band UVB lamps, and can work well without pro-
ducing intense tanning of patients. This form of treatment has been found to
be nearly as effective as PUVA, and is becoming the standard of care in many
treatment centers. The diseases most responsive to phototherapy using any
