© 2005 Oxford University Press
CORRESPONDENCE |
RESPONSE: Re: Sun Exposure and Mortality From Melanoma
Affiliations of authors: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden (KES, H-OA); Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark (HH, MM); Department of Pathology and Oncology, Karolinska University Hospital, Stockholm, and Department of Oncology, Radiology and Clinical Immunology, Akademiska Hospital, Uppsala, Sweden (BG); Department of Epidemiology, Harvard University, Boston, MA (H-OA)
Correspondence to: Karin Ekström Smedby, MD, PhD, Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, SE-171 77 Stockholm, Sweden (e-mail: karin.ekstrom{at}meb.ki.se).
We agree with Dr. Kalish that the biologic mechanisms of an inverse association between sun exposure, an established skin cancer carcinogen, and malignant lymphomas (1) are unclear and that the discussion of alternative scenarios is warranted in light of the novelty of the findings (2). However, we do not believe that our simultaneous and confirmatory finding of a positive association between skin cancer history and lymphoma risk, according to Dr. Kalish, "conflicts with a direct link between sun exposure and lymphoma risk or a role for vitamin D" (3).
In our report (1), we noted that frequent sunbathing and sunburns were associated with increased risk of skin cancer when this malignancy was treated as outcome and lymphoma status was not considered. Such exploratory analyses were performed to check that frequent sun exposure was positively associated with skin cancer as would be expected (but the feasibility of such analyses may be questioned, as the study was not designed to analyze risk factors for skin cancer). However, we did not mean to convey that sun exposure was positively associated with lymphoma risk among the skin cancer patients, as opposed to the negative association observed overall.
In our study, few participants reported a history of skin cancer (approximately 2% among the control subjects and 4% among the non-Hodgkin lymphoma case patients). Thus, analyses of interaction, comparing the association between sun exposure measures and lymphoma risk in participants with and without skin cancer history, were hampered by the small number of individuals in the skin cancer group and were therefore not reported in the original article. In supplementary analyses restricted to participants without skin cancer, results were naturally much the same as those previously presented (Table 1) (1). Among skin cancer patients, frequent suntan habits at different ages in Denmark, Sweden, and abroad were also inversely associated with non-Hodgkin lymphoma risk, although the results did not reach statistical significance (Table 1). Relative risks for non-Hodgkin lymphoma after frequent sunburns were more difficult to interpret owing to even smaller numbers and very unstable estimates in categories of high exposure (data not shown). Thus, there was little to suggest that frequent sun exposure would be associated with an increased rather than a decreased lymphoma risk among skin cancer patients.
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We conclude that our overall findings appear to be independent of skin cancer history and that our concomitant observation of a positive association between skin cancer and lymphoma therefore does not contradict vitamin D as a plausible candidate factor, among other factors, for the observed overall inverse association. With respect to the intriguing link between skin cancer and lymphoma, we agree with Dr. Kalish that our results support the notion that skin cancer could be a marker for other deficiencies, such as deficiencies of immune function or DNA repair.
REFERENCES
(1) Smedby KE, Hjalgrim H, Melbye M, Torrang A, Rostgaard K, Munksgaard L, et al. Ultraviolet radiation exposure and risk of malignant lymphomas. J Natl Cancer Inst 2005;97:199–209.
(2) Egan KM, Sosman JA, Blot WJ. Sunlight and reduced risk of cancer: Is the real story vitamin D? J Natl Cancer Inst 2005;97:161–3.
(3) Kalish RS. Re: Sun exposure and mortality from melanoma. J Natl Cancer Inst 2005;97:1158.
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J Natl Cancer Inst 2005 97: 1158.
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