A cohort study of breast cancer among elderly women followed 1,400 women (65–79

Question

A cohort study of breast cancer among elderly women followed 1,400 women (65–79 years of age) with no evidence of breast cancer at baseline over several years. Follow-up visits occurred every two years at which time, investigators screened all women for breast cancer. The results were as follows: 12 new cases at the first follow-up visit, 11 new cases at the second follow-up visit, 23 new cases at the third follow-up visit, and 28 new cases at the fourth (final) follow-up visit. The investigators also noted that 35 of the women had withdrawn from the study after the third evaluation (these women were present for visits 1 - 3 but missed the 4th visit). Based on this information, calculate and interpret the incidence density (per 1,000 person-years) of breast cancer in this group.

Explanation / Answer

Answer: For calculation of incidence density (per 1,000 person-years) of breast cancer cases:

Incidence density = No. of new cases during stated period/No. of person-years of observation

1) 12 new case in first follow up, ID = 12/1000*2 = 0.006 person-year

2) 11 new cases in second follow up, ID = 11/1000*2 = 0.0055 person-year

3) 23 new cases in third follow up , ID= 23/1000*2 = 0.0115 person-year

4) 28 cases in fourth follow up, ID = 28/1000*2 = 0.014 person-year

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