Risk factors for squamous cell carcinoma of the penis--population-based case-control study in Denmark.

Few etiologic studies of squamous cell carcinoma (SCC) of the penis have been carried out in populations where childhood circumcision is rare. A total of 71 patients with invasive (n=53) or in situ (n=18) penile SCC, 86 prostate cancer controls, and 103 population controls were interviewed in a population-based case-control study in Denmark. For 37 penile SCC patients, tissue samples were PCR examined for human papillomavirus (HPV) DNA. Overall, 65% of PCR-examined penile SCCs were high-risk HPV-positive, most of which (22 of 24; 92%) were due to HPV16. Penile SCC risk was positively associated with measures of early and high sexual activity, including lifetime number of female sex partners, number of female sex partners before age 20, age at first intercourse, penile-oral sex, a history of anogenital warts, and never having used condoms. Histories of phimosis and priapism at least 5 years before diagnosis were also significant risk factors, whereas alcohol abstinence was associated with reduced risk. Our study confirms sexually transmitted HPV16 infection and phimosis as major risk factors for penile SCC and suggests that penile-oral sex may be an important means of viral transmission. The association with priapism was unexpected and needs replication.