The relative risk for a second primary cancer after the diagnosis of prostate cancer and the prognostic impact of the association of multiple primary malignant neoplasms (MPMNs) in patients with prostate cancer were analyzed in a retrospective study. The development of MPMNs was examined in 312 consecutive patients who had been diagnosed with prostate cancer between 1966 and 1992. The population-based cancer incidence rates in Japan were utilized to calculate the expected number of MPMNs. Of the 312 patients, 60 fulfilled the diagnosis of MPMNs. In 13 men, prostate cancer and other malignancies were diagnosed simultaneously. In 35 of the 312 patients, prostate cancer was the second or third cancer diagnosis. In the remaining 287 patients, prostate cancer developed initially. Of the 287 patients, 12 developed a second primary malignancy compared with 17 expected (relative risk 0.71, 95% confidence interval 0.45–1.4). No single anatomic site showed a significantly increased risk above that expected either. The overall survival of patients with prostate cancer was not reduced by the association with MPMNs. This may be explained by the fact that the stage of the prostate cancer was lower in patients with MPMNs than in patients without MPMN
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