title: Effect of sunscreen application on UV-induced thymine dimers.
abstract: BACKGROUND
  Exposure to UV radiation is a major risk factor for skin cancer, including malignant melanoma. Photocarcinogenesis is caused largely by mutations at sites of incorrectly repaired DNA photoproducts, of which the most common is the thymine dimer. Over the past decade, controversy has arisen over the use of sunscreens to prevent UV-induced skin cancer.
  
  
  OBJECTIVE
  To determine if daily application of a broad-spectrum sunscreen with a sun protection factor (SPF) of 15 protects human skin against UV-induced DNA damage as determined by the formation of thymine dimers after repeated exposures to simulated solar light and, if so, to determine whether daily applications are required to achieve this protective effect.
  
  
  METHODS
  Over 4 consecutive days, an SPF 15 sunscreen was applied homogeneously to each of 4 buttock sites of 18 women 30 minutes before exposure to 2 minimum erythemal doses of UV radiation. Of these 4 sites, 1 was treated with SPF 15 daily, and the remaining 3 were treated on 3 of the 4 irradiation days, skipping application on day 2, 3, or 4. A fifth site served as the untreated control and was also not irradiated. The number of cells per square millimeter positive for thymine dimer formation was determined using immunohistochemical and image analyses.
  
  
  RESULTS
  There was no significant difference in thymine dimer formation between nonirradiated and irradiated skin when application of sunscreen preceded each irradiation. However, when sunscreen application was omitted even once prior to irradiation, a statistically significant increase in thymine dimer formation was apparent. At 48 hours after irradiation of unprotected skin, 50% of epidermal dimers present 24 hours after irradiation had been removed; at 72 hours, more than 75% were gone.
  
  
  CONCLUSIONS
  Our study indicates that regular use of a broad-spectrum sunscreen is effective in preventing a major form of UV-induced DNA damage. Irregular and inadequate use of sunscreen during exposure to UV radiation results in thymine dimer formation, which may lead to mutation and subsequent cancer development.
authors.affiliation: Department of Dermatology, Salmaniya Medical Complex, Manama, Bahrain.
authors.forename: Mona
authors.initials: M
authors.lastname: Al Mahroos
authors.forename: Mina
authors.initials: M
authors.lastname: Yaar
authors.forename: Tania J
authors.initials: TJ
authors.lastname: Phillips
authors.forename: Jag
authors.initials: J
authors.lastname: Bhawan
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Gilchrest
mesh_terms: D000287:Administration, Topical; D000328:Adult; D001707:Biopsy, Needle; D005260:Female; D006801:Humans; D007150:Immunohistochemistry; D008875:Middle Aged; D011446:Prospective Studies; D011740:Pyrimidine Dimers / Q000378:metabolism* / Q000528:radiation effects*; D012016:Reference Values; D012680:Sensitivity and Specificity; D013473:Sunscreening Agents / Q000494:pharmacology*; D014466:Ultraviolet Rays / Q000009:adverse effects*
chemical_list: D011740:Pyrimidine Dimers; D013473:Sunscreening Agents