Two recent studies have reported a significantly elevated risk of prostate cancer among vasectomized men. To assess whether the new results conflict with earlier studies that found no significant overall association, and, if so, whether such a conflict could have a methodological basis, we reviewed the six major epidemiological studies of this topic. Statistical analysis revealed significant (p < 0.01) heterogeneity among the associations in the six studies, attributable to one of the recent studies. Scrutiny of the studies for fulfillment of eight methodological standards for scientific validity revealed that no study completely fulfilled more than four standards, and that all studies were deficient in avoiding detection bias and obtaining accurate vasectomy histories. Our review indicates that the evidence on this topic is indeed conflicting, that the quality of the evidence does not resolve the conflict, and that future studies of this topic, designed to ensure scientific credibility of results, are needed.
PIP: 2 recent studies have reported a significantly elevated risk of prostate cancer among vasectomized men. 6 major relevant epidemiological studies were reviewed to determine the new results conflicted with earlier studies finding no significant association and whether such conflict was attributable to methodological deficiencies. Statistical analysis indicated significant heterogeneity among the associations in the 6 studies (p 0.01) imputed to one of the studies. Examination of 8 methodological standards for scientific validity showed that none of the studies fulfilled more than 4 standards, and all were deficient in avoiding detection bias and obtaining accurate vasectomy histories. The evidence is conflicting concerning prostate cancer, and future studies are needed to ensure scientific credibility of results. The literature search included the MEDLINE bibliographic database from January 1970 to December 1991, which yield 6 studies on the link of vasectomy and risk of prostate cancer. The 8 criteria were prior hypothesis, single underlying suitable validated histories of vasectomy, protection against detection bias, valid diagnoses of prostate cancer, and incident prostate cancer outcomes. 1 was a cohort and 5 were case-control investigations during 1982-88. The follow-up after vasectomy varied: 15+, 20+, 30+, and 44 years with 40-86 years of documented ages. Analysis of homogeneity indicated similarity for 5 studies. Estimation of overall effect by odds ration estimates and confidence intervals suggested a slight risk of prostate cancer associated with vasectomy. Control for confounding by age was done in 5 studies, and no study was given credit for adequate protection against detection bias. 3 studies reported histological confirmation of prostate cancer, but none reported a review of diagnostic evidence validated with vasectomy histories.