1. ¹Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark
2. ²International Epidemiology Institute, Rockville, MD, USA
3. ³Thames Cancer Registry, King's College London, London, UK
4. ⁴London School of Hygiene & Tropical Medicine, London, UK
5. ⁵Vanderbilt University Medical Centre, Vanderbilt-Ingram Cancer Center, Nashville, TN, USA

Correspondence: Dr JH Olsen, Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagen Ø, Denmark. E-mail: jorgen@cancer.dk

Received 6 August 2004; Revised 18 October 2004; Accepted 20 October 2004; Published online 7 December 2004.

Abstract

Among 14 088 patients, with a primary diagnosis of Parkinson's disease during the period 1977–98 identified from the National Register of Patients, 1282 cancers were subsequently recorded in the Danish Cancer Registry, compared with 1464 expected, with a standardised incidence ratio (SIR) of 0.88 (95% confidence interval (CI), 0.8–0.9). Significantly reduced risks were found for smoking-related cancers, for example, cancers of the lung (SIR, 0.38), larynx (0.47) and urinary bladder (0.52), although moderate reductions in risk were also seen for several nonsmoking-related cancers. In contrast, increased risks were seen for malignant melanoma (SIR, 1.95; 95% CI, 1.4–2.6), nonmelanocytic skin cancer (1.25; 1.1–1.4) and breast cancer (1.24; 1.0–1.5). The observed cancer pattern supports the hypothesis that constituents of tobacco smoke inhibit or delay the development of Parkinson's disease, but a low smoking prevalence appears to be only part of the explanation for the decreased cancer incidence. The increased relative risks of melanoma and nonmelanoma skin cancer are not likely to be artefactual, but further investigations of potential mechanisms are warranted.