In 1998 one thousand men aged 65-80 years were examined for the presence of pros

Question

In 1998 one thousand men aged 65-80 years were examined for the presence of prostate cancer, of those 1,000 men, 197 had detectable prostate cancer in 1998. Of those free from prostate cancer in 1998 all were followed from Jan 1st 1999 through December 31, 2004. One hundred men were lost to follow-up and 388 died of other causes (assume they were lost or died midway through the follow-up period). Assume the lost-to-follow up subjects and the deaths occurred in the middle of the study period. During the 6-year follow-up 98 men developed prostate cancer. One hundred percent of the men who developed prostate cancer during the study period were still alive and included by the end of the study period. None of them were cured.

What is the incidence density (ID) for the six-year follow-up period (from Jan 1,1999 to Dec. 31, 2004) in person years?

Explanation / Answer

Number of people under risk of prostate cancer at the start of follow period = 1000 - 197 = 803

Total number of people lost lost or died midway through the follow-up period (3 years) = 100 + 388 = 488

Remaining people under risk for full follow up period of 6 years (Jan 1,1999 to Dec. 31, 2004) = 803 - 488 = 315

Total person-years under risk = 488 * 3 + 315 * 6 = 3354 person-years

Incidence density = Number of new cases of prostate cancer / Total person-years under risk

= 98 / 3354 = 0.029 or 29 per 1000 person/years

Hence a. is correct answer.

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