ARTICLE
Auteur(s) : H Hönigsmann
Photodermatology covers three distinct fields: 1. Skin damage
caused by exposure to sunlight (ultraviolet radiation, UV), 2. Skin
diseases induced by sunlight (UV and/or visible light), i.e.
photodermatoses, and 3. The use of coherent (laser) and incoherent
light (UV, visible light) for therapeutic purposes.
Light-induced skin damage
Definition
UV radiation is divided into three different wave length ranges
which have different biological effects (figure 1). UVC does not
reach the earth’s surface, being completely filtered out by the
ozone layer. The UV types relevant for skin biology are UVA and
UVB. UVB is responsible for the best known UV damage, sunburn
(“sunburn spectrum”), whereas UVA has long been considered
harmless.
Health Impact
UV radiation from sunlight or artifical sources may lead to at
least three harmful consequences: increased incidence of skin
cancer, premature skin ageing (photoaging) and reduced cutaneous
immune response. The major questions challenging photobiological
research regard the specific wavelengths and types of exposure that
incur risk, and the reasons why photo-induced skin cancers are at
the rise, including the impact of stratospheric ozone depletion,
tanning beds and sunscreens.
UVB is the single most important risk factor for skin
carcinogenesis, which is true for all of the three most common
forms of skin cancer: basal and squamous cell carcinoma, and
melanoma, the most aggressive form of skin cancer. New data suggest
that melanoma may also be associated with UVA. Both sunburn and
long-term tanning may predispose to skin cancers but the
association is better studied for sunburn. The increased incidence
of skin cancer is due to increasing exposure to UV radiation from
the sun, tanning beds, and sun lamps as well as changes in outdoor
behavior and recreational activities, travel to tropical areas and
changes in clothing habits. The rise of skin cancer is best
summarized in the “2003 Skin Cancer Fact Sheet”, published by the
American Academy of Dermatology (Appendix 1).
Although anyone can get skin cancer, individuals with certain
risk factors are particularly susceptible:
- – Light skin color, light hair or eye color (figure 2. Skin types);
- – Freckles, which indicate sun sensitivity and sun
damage;
- – Family history of skin cancer;
- – Personal history of skin cancer;
- – Chronic exposure to the sun (work outdoors
unprotected);
- – History of sunburns early in life;
- – Certain types and a large number of moles.
Premature skin ageing, i.e. a wrinkled, leathery appearance of
the skin, occurs in everyone who is repeatedly exposed to the sun
over a long time, although the damage is milder and takes longer to
appear in people with darker skin. Photoaging and skin cancer are
late UV effects which become apparent after many years only. Thus,
young people are commonly unaware of the dangers of tanning. People
who choose to tan over a period of years are greatly increasing
their risk for skin cancer. In addition, UV radiation can modify
the immune system, for example by inducing apoptosis in skin immune
cells.
Tanning parlors
People often associate suntan with good health and vitality;
sunlight is also considered beneficial because it stimulates
vitamin D production. However, only small doses of sunlight (far
less than it takes to develop a suntan) are needed to allow for
sufficient vitamin D synthesis. Nonetheless, tanning parlors have
become very popular as new means of supplemental UV exposure. It is
a common misconception that sun lamps and tanning beds are safer
than natural sunlight because they emit UVA. Although these
so-called “tanning rays” are less likely than UVB to cause sunburn,
they have many well-documented damaging effects. The tan produced
by UVA provides little protection against subsequent sunburn; on
the other hand, UVA may be linked to melanoma, accelerates skin
ageing, leads to phototoxic and photoallergic reactions, and is
linked to damage to the immune system. Dermatologists strongly
encourage people to avoid the use of tanning beds and sun lamps,
but political actions are required to supplement the public health
efforts to reduce the population’s exposure to these unnecessary
and harmful rays.
Stratospheric ozone depletion
The quantity of UV reaching the earth’s surface depends on several
factors, including the ozone layer and the presence of clouds and
atmospheric pollutants. Evidence indicates that man-made chemicals
released into the atmosphere have depleted the ozone layer in the
past quarter century. Ozone destruction has outpaced its natural
formation, resulting in increased UV radiation on the earth’s
surface, particularly in Antarctica, but also over populated areas.
It is currently under debate if increased UV radiation due to
ozone depletion represents a relevant health risk. The increase is
only a fraction of the exposure which occurs when visiting the
tropics or tanning parlors. The answer is clearly affirmative since
both increased cumulative lifetime exposure and a higher likelihood
of sunburns elevate the risk of skin cancer. Skin carcinogenesis is
slow to develop with latency periods of years to decades, which
means that these effects have practically no relevance in cancers
which are now appearing. There is little doubt, though, that skin
cancer rates may increase in future as ozone depletion continues to
occur.
Sunscreens
The ability of sunscreens to prevent or reduce sunburn is
well-established, but the prevention of sunburn is not equivalent
to the prevention of all UV radiation-induced effects. We now know
that the “sun protection factor” (SPF), which quantitates the
ability of a given sunscreen to block sunburn, tells little about
its capacity to protect against immune suppression, photoageing or
induction of skin cancer.
UVA and UVB radiation both irreversibly damage DNA and protein
structures in skin. Many joint research projects involving
dermatologists, cosmetic chemists, and photobiologists are
currently trying to better define the contributions of various
solar wavelengths to the UV damage. Formulation of new sunscreens
which specifically maximize sun protection and minimize adverse
effects is an area of active research in photobiology and the
sunscreen industry.
Education and skin cancer prevention
The danger of uncritical UV exposure is still highly underestimated
by the public, and there is a clear need for further educational
campaigns with recommendations how to minimize sunlight-induced
skin damage including:
- 1) Avoiding sun exposure at peak times of UV radiation
(around midday).
- 2) Wearing protective clothing and hats particularly for
children younger than 6 months.
- 3) Applying broad-spectrum (blocking UVA and UVB)
sunscreens with SPF 30.
- 4) Not employing sunscreens to prolong sun
exposure.
- 5) Not employing artificial tanning devices.
A European agency popularizing sun protection would be helpful
in skin cancer prevention. This agency could perform
epidemiological research and monitor national trends in sun
protection behavior and attitudes towards sun exposure. The data
collected could serve to better target and evaluate prevention
efforts and to coordinate national activities by developing a
European skin cancer prevention and education plan which should
include:
- 1) Establishing policies to reduce exposure to UV
radiation.
- 2) Maintaining an environment that supports sun-safety
practices.
- 3) Providing health education to students.
- 4) Training of health care professionals.
- 5) Evaluating school programs for skin cancer
prevention.
A grave deficit for all educational and preventive efforts is
the lack of adequate skin cancer registries in most European
countries. A European skin cancer registry is urgently
needed.
Skin diseases caused by sunlight (photodermatoses)
Photodermatoses are skin diseases elicited or aggravated by
exposure to sunlight or artificial radiation. They are uncommon,
except for polymorphous light eruption and phototoxic drug
reactions. Severe photosensitivity can be a devastating condition
that may result in disruption of professional, social, and private
life. Photosensitizing substances may be of external origin or
arise in the body proper (porphyrins). In most cases, the causes
are not fully elucidated but immunologic mechanisms are likely.
Clinical examples
Polymorphous light eruption (PLE). PLE is the most common
idiopathic photodermatosis, with prevalences of 10-20% in Western
Europe and in the USA. It is often incorrectly referred to as “sun
allergy”. The typical patient is a fair-skinned young woman who
develops a transitory eruption following her first sun exposure of
the season. The main symptom is itch and the rash’s appearance is
quite variable. The causative light is usually in the UVA range.
Chemically-induced photosensitivity. Photosensitivity may
develop when skin is exposed to light while a topical or systemic
photosensitizing chemical is present. Photosensitizers include
drugs (oral antidiabetic agents, diuretics and many others),
industrial products, cosmetics, plants and even sunscreen
ingredients. Porphyrins are endogenous photosensitizers.
Photosensitivity may manifest as phototoxic or photoallergic
reactions. Phototoxic reactions resemble severe sunburns. They
result from direct tissue injury following UV-induced activation of
photosensitizers and can occur in all exposed individuals at first
contact, if adequate doses of light and photosensitizer are
present. Photoallergy, in contrast, is a delayed-type
hypersensitivity reaction which appears as an itchy dermatitic rash
resembling allergic contact dermatitis. It first appears after a
sensitization period of 7 to 10 days, and recurs following
subsequent challenge in sensitized individuals only. Photoallergic
reactions are much less common and more often found following
topical than systemic exposure to photosensitizers.
Porphyrias. Porphyrias are a group of hereditary metabolic
diseases caused by enzyme deficiencies in hemoglobin biosynthesis.
Various types of porphyrins accumulate, depending on which enzyme
is defective. Liver damage is a typical complication of some
porphyrias; this may include acute liver failure, cirrhosis or even
hepatocellular carcinoma. Sensitivity to visible light is a major
clinical manifestation of several porphyrias. It may be so severe
as to necessitate complete avoidance of daylight
Photoaggravated dermatoses. Photoaggravation, the worsening of
skin disorders by exposure to sun or to artificial UV light, is not
uncommon. Examples include psoriasis, lupus erythematosus, herpes
simplex virus infection (“herpes solaris”) and even some forms of
acne. These are not true photodermatoses since light is not a
necessary precondition for their development. The underlying
mechanisms are mostly undetermined. Photoaggravation occurs only in
some of the affected individuals.
Diagnostic procedures
Phototesting is a diagnostic procedure which can be used to
diagnose photodermatoses by provoking typical skin lesions, to
measure the degree of photosensitivity, to determine the action
spectrum, and to evaluate treatment effects. Phototesting is a
time-consuming procedure which requires considerable expertise.
Sophisticated and expensive light sources are sometimes needed to
provoke lesions, but adequate information can often be obtained
with simple light sources. Photopatch testing (“irradiated patch
test”) is used to identify photoallergic substances. It is the only
procedure which has been standarized, as a result of efforts by
Scandinavian and then German-language photopatch groups.
A European Task Force for Photopatch Testing has just formed.
There is no general agreement on other phototesting protocols.
Therapy and prevention
Treatment of photodermatoses is aimed at minimizing UV exposure by
sun avoidance or the use of sunscreens, or by removing
photosensitizers if possible. In moderate to severe PLE, sunlight
avoidance and broad-spectrum sunscreens are often disappointing.
A springtime course of prophylactic hardening with
phototherapy (usually PUVA) will often allow the patients to better
tolerate sunlight. Treatment of porphyrias depends on the
particular type. In some, effective measures exist which may result
in clinical and biochemical remission; in others, the available
modalities are disappointing. Oral beta-carotene may reduce light
sensitivity in some types of porphyrias, but does not improve the
metabolic defect.
Phototherapy
The use of light, in particular UV, in the treatment of skin
diseases has a long tradition. Modern phototherapy began around
1900 when Niels Ryberg Finsen (Nobel Laureate 1903) showed that UVB
irradiation could cure skin tuberculosis. In the 1920s, topical
treatment with tar plus subsequent UV irradiation became a standard
therapy of psoriasis. Broadband UVB phototherapy and
photochemotherapy have been used for common skin diseases such as
psoriasis, atopic dermatitis and others for more than 30 years.
More recently, selective spectra of the UVB and UVA range such as
narrowband UVB and UVA1 have been introduced, and new indications
for phototherapy identified. Phototherapy is usually administered
under medical supervision, although some forms may be used at home.
Visible light in combination with photosensitizers (porphyrins),
known as photodynamic therapy (PDT), is now employed for diagnosis
and treatment of selected skin tumors. Extracorporeal
photochemotherapy has proven effective beyond dermatological
indications, in particular, in transplantation medicine.
Since the advent of the first laser, scientists and physicians
have been working together to develop medical applications. The
specificity of laser wavelengths and the precision of fibre optic
light delivery have greatly enhanced the evolvement of non-invasive
therapeutic techniques.
Forms
UVB: The use of narrowband UVB (312 nm) phototherapy is
superior to conventional broadband UVB with respect to both
clearing and remission times for several diseases (such as
psoriasis) and nearly as effective as PUVA. In Europe, narrowband
UVB has replaced conventional broadband UVB phototherapy in most
institutions.
Photochemotherapy (PUVA): Psoralen photochemotherapy denotes the
combination of psoralen (P) and long-wave ultraviolet radiation
(UVA) which results in a phototoxic reaction. Remission of skin
disease is induced by repeated controlled exposures. Psoralens can
be administered orally or topically as solutions, creams or baths,
with subsequent UVA exposure. Two major forms of PUVA are in
use-oral PUVA and bath PUVA. Diseases which may respond to PUVA
include psoriasis, mycosis fungoides (a malignant skin lymphoma),
vitiligo, atopic dermatitis and PLE.
UVA1: UVA1 (340-400 nm) phototherapy represents an
investigational treatment that was originally developed for atopic
dermatitis. Its efficacy has not yet been systematically compared
to other phototherapeutic modalities. Controlled multicenter
studies are currently ongoing.
Photodynamic therapy (PDT): PDT denotes photochemotherapy with
porphyrins or porphyrin precursors plus irradiation with visible
light. PDT aims at selectively destroying target cells while
minimizing damage to neighboring tissues. In dermatology, PDT is
used for the treatment of superficial tumors such as actinic
keratoses, superficial basal and squamous cell carcinomas, as well
as some inflammatory and infectious conditions (psoriasis, viral
warts).
Photopheresis: Extracorporeal photochemotherapy (ECP,
photopheresis) was originally introduced for the palliative
treatment of erythrodermic cutaneous T-cell lymphoma but is now
also used experimentally for T-cell mediated autoimmune diseases.
Excellent results have been reported for refractory acute and
chronic graft-versus-host disease. In ECP, psoralen is administered
orally. UVA irradiation is targeted on the white blood cell
fraction prepared by apheresis; it is performed outside the body in
an UVA exposure system, and the cells are re-infused into the body
after irradiation. The mode of action of ECP is not yet fully
understood.
Laser treatment: Laser therapy is a rapidly growing field in
which new types of lasers and applications are continually being
introduced. Each laser has distinct uses depending on its
wavelength and energy output. Better understanding of laser-tissue
interactions has led to the development of high-energy lasers that
can selectively target different structures of the skin, such as
blood vessels, pigment particles, and hair follicles without
damaging the surrounding tissue. For years, lasers have provided
safe and effective treatment for vascular lesions (port wine
malformations and hemangiomas). Other conditions amenable to lasers
are AIDS-related Kaposi’s sarcoma, superficial neoplastic lesions
(actinic keratoses, actinic cheilitis), viral warts and tattoos.
Cosmetic uses include epilation and skin resurfacing. Recently an
experimental UV laser (excimer laser) has been shown to clear
psoriatic lesions more rapidly than standard UVB phototherapy; such
treatment is applied exclusively to lesions, thus avoiding UV
damage to the adjacent tissue.
Future needs and prospects
Most forms of phototherapy including photodynamic therapy are easy
to perform, clean and relatively cheap. Except for photopheresis,
they are mostly used in outpatients, thus reducing treatment costs,
which are quite large for common diseases such as psoriasis and
atopic dermatitis. Safe and effective phototherapy and PUVA
requires an adequately trained staff and safe, correctly
maintained, and adequately monitored equipment.
There is a definite need for more specialized phototherapy
facilities in Europe. Currently, there are not enough centers both
for treating all deserving patients and training adequate numbers
of photodermatologists. Standard treatment guidelines have been
published for most types of phototherapy; they should be adopted
throughout Europe. There are urgent unresolved questions which
should be addressed in multicenter studies, such as the hazards of
skin cancer in patients receiving long-term PUVA, UVB or UVA1.
Laser treatments may be performed in physician offices,
ambulatory centers or hospital. Guidelines do exist in some
countries but European treatment standards are not available. Safe
and effective use of laser therapy requires:
- – Special training, including knowledge of basic laser
physics and laser-tissue interactions
- – Acquaintance with the special safety requirements of
the laser, prevention of complications and their management.
- – Maintenance of laser systems by trained personnel to
assure proper energy output and safety.
2003 Skin cancer fact sheet
- – Nearly half of all new cancers are skin cancers.
- – More than 1 million new cases of skin cancer will
be diagnosed in the United States this year*.
- – About 80 percent of the new skin cancer cases will be
basal cell carcinoma, 16 percent are squamous cell carcinoma, and 4
percent are melanoma.
- – Both basal cell carcinoma and squamous cell carcinoma
have a better than 95 percent cure rate if detected and treated
early.
- – An estimated 9,800 people will die of skin cancer this
year, 7,600 from melanoma and 2,200 from other skin cancers*.
- – There will be about 91,900 new cases of melanoma in
2003 – 37,700 in situ (noninvasive) and 54,200 invasive (29,900 men
and 24,300 women)*. This is a 4 percent increase in new cases of
melanoma from 2002. In 2003, at current rates one in 39 Americans
have a lifetime risk of developing melanoma and one in 67 Americans
have a lifetime risk of developing invasive melanoma.
- – One person dies of melanoma every hour. In 2003, 7,600
deaths will be attributed to melanoma – 4,700 men and 2,900 women*.
Older Caucasian males have the highest mortality rates from
melanoma.
- – The incidence of melanoma more than tripled among
Caucasians between 1980 and 2003.
- – More than 77 percent of skin cancer deaths are from
melanoma.
- – Melanoma is more common than any non-skin cancer among
women between 25 and 29 years old.
- – Melanoma is the fifth most common cancer in men and
the seventh most common cancer in women**.
*Source: American Cancer Society’s 2003 Facts & Figures.
** Excluding basal cell carcinoma and squamous cell carcinoma,
which together are the most common cancers in both sexes.
© 2003 American Academy of Dermatology
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