To the Editors: The recent article on incidence of cancer after diagnosis of Parkinson's disease is strongly confirmatory that risk of melanoma is significantly increased whereas risk of smoking-related cancers is reduced, albeit nonsignificantly (1). This finding was thought to suggest a gene-environment interaction, and one hypothesis was given related to the detoxifying enzyme P450 D6. However, no clear biological explanation was given for the melanoma finding.

It should be noted that smoking has been found inversely related to risk of melanoma in four observational studies (2-5). This is in contradistinction to the case for basal cell carcinoma and squamous cell carcinoma of the skin, for which smoking is a risk factor (4). Again, no clear biological explanation has been provided. However, combining the results of ref. 1 with those of refs. 2-5, it seems that the same mechanism related to Parkinson's disease that reduces the risk of smoking-related cancers also increases the risk of melanoma. Perhaps this suggestion will be useful for both sets of studies.

I receive funding from the UV Foundation (McLean, VA) and the Vitamin D Society (Canada) and expect funding from the European Sunlight Association.

1
Driver JA, Logroscino G, Buring JE, Gaziano JM, Kurth T. A prospective cohort study of cancer incidence following the diagnosis of Parkinson's disease.
Cancer Epidemiol Biomarkers Prev
2007
;
16
:
1260
–5.
2
Freedman DM, Sigurdson A, Doody MM, Rao RS, Linet MS. Risk of melanoma in relation to smoking, alcohol intake, and other factors in a large occupational cohort.
Cancer Causes Control
2003
;
14
:
847
–57.
3
Odenbro A, Gillgren P, Bellocco R, Boffetta P, Hakansson N, Adami J. The risk for cutaneous malignant melanoma, melanoma in situ and intraocular malignant melanoma in relation to tobacco use and body mass index.
Br J Dermatol
2007
;
156
:
99
–105.
4
Grant WB. A meta-analysis of second cancers after a diagnosis of nonmelanoma skin cancer: additional evidence that solar ultraviolet-B irradiance reduces the risk of internal cancers.
J Steroid Biochem Mol Biol
2007
;
103
:
668
–74.
5
Grant WB. An ecologic study of cancer mortality rates in Spain with respect to indices of solar UV irradiance and smoking.
Int J Cancer
2007
;
120
:
1123
–7.