An epidemiological study of prostate cancer among elderly men followed 1,400 men

Question

An epidemiological study of prostate cancer among elderly men followed 1,400 men, 65-79 years of age, without evidence of prostate cancer over several years. Every two years the investigators examined the men for prostate cancer. The results were as follows: 12 new cases at the first evaluation, 11 new cases at the second evaluation, 23 new cases at the third evaluation, and 28 new cases at the fourth (final) evaluation. The investigators also noted that 35 of the men had withdrawn from the study at the third evaluation. Based on this information, calculate and interpret the incidence density rate of prostate cancer in this group.

Explanation / Answer

Incidence density rate = No. of incidences / length of person-time

We eliminate the people as they develop the cancer.

Thus, contribution to study in person-years

= (12 * 1) + (11 * 3) + (23 * 5) + (28 * 7) + all those who never had prostate cancer

Thus, remaining people = 1400 - 74 = 1326, out of which 35 withdrew at 3rd stage

Thus, 1326 - 35 = 1291 completed the study for the entire 8 years

Thus, total man-years contribution

= (12 * 1) + (11 * 3) + (23 * 5) + (28 * 7) + (35 * 6) + (1291 * 8)

= 10894 man years

So,

incidence density rate = 74 / 10894 = 6.79 * 10-3

or 6.79 cases in 1000

i.e 7 cases of prostate cancer in 1000 people.

Hope this helps.

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