Aug. 29 (HealthDay News) -- Being screened for prostate cancer more often does improve disease detection overall, but it doesn't help doctors spot aggressive cancers better, a new study finds.

The researchers looked at more than 17,000 men who had prostate specific antigen (PSA) testing every two years or every four years. Among 4,202 Swedish men screened every two years, the overall incidence of prostate cancer diagnosis over 10 years was 13.14 percent, compared to 8.41 percent among the 13,301 Dutch men who were screened every four years, said the researchers from Erasmus Medical Centre in Rotterdam, The Netherlands.

The total number of interval cancers -- those diagnosed based on symptoms during the years between screening tests -- was 31 (0.74 percent) among the Swedish men and 57 (0.43 percent) among the Dutch men.

The differences in the interval cancer rates and aggressive interval cancer rates between the two groups were not statistically significant, the study authors said. This indicates that two-year screenings don't reduce the number of interval cancers, as might be expected.

"It is therefore more realistic to believe that each screening will again lead to prostate cancer diagnoses among some men from a large pool harboring small and often clinically insignificant disease, and thus that more cancers were detected in Sweden because screening was more frequent," the authors wrote.

They also noted that two- and four-year screenings seem equally effective at detecting larger cancers.

The study was published in the Aug. 28 online issue of the Journal of the National Cancer Institute.

SOURCE: Journal of the National Cancer Institute, news release, Aug. 28, 2007