MASS-SCREENING FOR PROSTATE-CANCER AND THE BIAS RELATING TO SURVIVAL RATE

Since 1981 we have been studying prostate cancer (Pca) by mass screeni ng (MS) in the Gunma Prefecture, Japan. From 1981 to 1990, 9,067 subje cts (total 15,451) were examined in connection with this project and 1 21 subjects were diagnosed as having Pca. The presence of Pca in 87 ca ses was confirmed at their initial MS (initial group); 34 cases were c onfirmed either the year after or several years after the first MS (re peat group). To evaluate the significance of MS for Pca, in this group of patients, the effects of 'lead time bias', 'self-selection bias' a nd 'length bias' on the survival rate were compared to Pca patients de tected in the hospitals of the Gunma Prefecture during the same period . The survival curves of MS cases for each stage of the disease were b etter than those of the controls. Only in stage D was there a signific ant difference between the two groups. However, this MS curve decrease d from the 4th year in the same manner as the control curve from the s tart. The clinical characteristics (age, pathological differentiation, prostatic acid phosphatase, gait disturbance, erythrocyte sedimentati on rate, chronic disease, and pain in the patients with stage D diseas e) were compared between both groups. All characteristics studied, exc ept age, in the MS group were clinically more favorable compared to th ose of the controls for each stage or in total. Moreover, the relative survival rate of subjects examined by MS was greater than 1.0. No sig nificant difference was found in stage distribution between the initia l group and the repeat group. During the follow-up study, 15 Pca patie nts were found who had a history of MS examination but were diagnosed in standard hospitals. Only 2 of these 15 patients were diagnosed with Pca within 1 year following their last MS examination, and they had s tage A disease. We conclude that MS should be performed annually and r ecommend that people who want an early diagnosis should receive MS onc e a year.