First-generation male immigrants to Sweden have a lower risk for all stages of prostate cancer (PCa) compared with native-born men, a study found.

Researchers had speculated that lesser use of PSA testing among immigrants would result in lower rates of localized PCa and similar or higher risk of metastatic disease. Although the study revealed that immigrants were less likely than native-born men to have PCa detected by PSA testing, immigrants had a lower risk for metastatic disease, suggesting that PSA testing only partly explains their lower PCa risk, according to an online report in Cancer Causes and Control.

The study, by Stacy Loeb, MD, of New York University, and colleagues, included 117,328 men with PCa diagnosed from 1991 to 2008. Of these men, 8,332 were foreign born. For each case, investigators randomly selected five cancer-free matched controls from the Swedish population registry. The control population included 505,561 Swedes and 57,083 immigrants.

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Compared with native-born men, Middle Eastern, Southern European, and Asian immigrants had a 53%, 52%, and 47% decreased risk of PCa, respectively, in unadjusted analyses. These risk estimates remained virtually unchanged after adjusting for socioeconomic and marital status, educational attainment, and comorbidities, Dr. Loeb’s group reported.

Additionally, the study showed that older age at immigration and short time since arriving in Sweden were significantly associated with lower PCa risk. Compared with native-born men, immigrants who were younger than 20, 20-30, 30-40, and 40 years and older when they arrived in Sweden were at 16%, 24%, 28%, and 49% decreased risk of PCa, respectively. Immigrants who had been in Sweden less than 5, 5-25, 25-50, and 50 or more years had a 53%, 50%, 27%, and 15% decreased PCa risk, respectively.

Compared with native-born men, Dr. Loeb’s group found, foreign-born men, including those from other Nordic countries, were significantly less likely to be diagnosed through PSA testing as part of an asymptomatic checkup. Middle Eastern men, for example, had a 74% lower risk of diagnosis through PSA testing.

Except for immigrants from North America and Northern Europe, all immigrant groups had lower risks of low-, intermediate, and high-risk disease as well as regionally metastatic and distant metastatic disease compared with native-born men.

The researchers noted that their initial hypothesis was that lower use of PSA testing and subsequent lower frequency of work up among asymptomatic immigrants would result in a lower risk of localized PSA, but they pointed out that immigrants had a lower risk of advanced and metastatic disease compared with native-born men, “suggesting that disparities in screening and diagnostic testing could only partly explain the observed trends.”

With respect to the factors that might contribute to the lower PCa risk among immigrants to Sweden, Dr. Loeb and her colleagues pointed to a possible role for diet. Observational data have implicated intake of calcium, dairy products, and charred meat in PCa etiology, progression, or both, the researchers pointed out. Sunlight and vitamin D also could be involved. Previous studies have demonstrated an inverse relationship between geographic ultraviolet light exposure and PCa mortality. The investigators said this could help explain their observation of lower PCa risk among men born in southern regions and increased risk with longer duration of residence in Sweden.