SEER*Stat's first step in prevalence calculations is determining the tumors included in the analysis. These tumors are then used for all calculations, including survival cohorts. Choosing All Tumors Matching Selection Criteria/One Tumor Per Statistic on the Selection tab may result in more than one tumor for an individual being included in the analysis. All other Multiple Primary Selection options reduce the tumors included in the analysis to, at most, one tumor per individual. Even with the All Tumors Matching Selection Criteria/One Tumor Per Statistic option, each prevalence statistic will still be person prevalence. That is, a person will not contribute more than one tumor diagnosis to a single prevalence estimate. However, different tumors for an individual may contribute to separate statistics shown in the results.
The first tumor for an individual matching the combination of table variables will contribute toward that statistic's known alive, lost, or dead count. For example, consider a person with a breast cancer diagnosis in 1980 and a lung cancer diagnosis in 1990. If prevalence is displayed by site (All sites, Breast, Lung), then the breast cancer would contribute to the "All sites" and "Breast" estimates, and the lung cancer would contribute to the "Lung" estimate.
If Display By Time Prior to Prevalence Date is checked on the Statistic tab, and you request discrete intervals, you can choose one of two possible methods for considering multiple tumors in the analysis. This is only applicable if Multiple Primary Selection is set to All Tumors Matching Selection Criteria/One Tumor Per Statistic. The two choices are:
First cancer per interval: Consider each interval separately and count the person's first tumor diagnosed within the interval's range of years prior to the prevalence date. With this method, tumors diagnosed in two different time periods may contribute to two separate intervals. Keep in mind that the prevalence statistics for the total duration period only considers one tumor for the person. Therefore, the sum of the prevalence counts for the intervals will not necessarily equal the count for the total duration period.
First cancer over all intervals: The sum of the prevalence counts for the intervals will equal the count for the total duration period. There is no overlap among the intervals. A person is counted once toward the overall prevalence and is counted in only one interval. The person's tumor with the earliest date of diagnosis will be counted in the appropriate interval.
The first tumor for an individual matching each survival cohort will contribute to that survival estimate. Since survival cohorts are defined separately from the prevalence table, tumors may be included for the survival calculations that are not used in the calculations of the known alive or lost cases. For example, suppose you wanted to calculate January 1st, 2000, 20-year limited-duration prevalence for malignant lung cancer in males. You could select male, malignant, and lung cancer on the Selection tab. In this example, we will define the survival cohorts by year of diagnosis (1980-1989 and 1990-1999).
Consider the impact of using All Tumors Matching Selection Criteria/One Tumor Per Statistic versus First Tumor Matching Selection Criteria on the survival cohorts. To simplify this example, no table variables are used. Therefore, the same tumors will contribute to the values for known alive and lost; that is, these statistics will be the same in both analyses. However, the All Tumors Matching Selection Criteria/One Tumor Per Statistic analysis will include additional tumors in the survival cohorts. Therefore, the prevalence estimates will differ. For example, a male that was diagnosed with a malignant lung cancer in the 1980s and again in the 1990s would contribute to two survival cohorts. In the First Tumor Matching Selection Criteria analysis, only one tumor (the 1980s diagnosis) would contribute to survival.