Tumors Included in Prevalence Estimates

When estimating prevalence there are several methods for determining which tumors to include, depending on the question of interest.

First Malignant Primary Only (non-reported assumed to be malignant) - this method would include only the first malignant tumor. The SEER registries collect the number (but not the site or behavior) of cancers that occur prior to the start of the registry, or prior to the person moving to a SEER catchment area. We make the assumption that these cancers are malignant, as is true of the majority of SEER cancers. Thus, if SEER indicates that the first SEER registered tumor is the person's second tumor (the other was a non-SEER cancer) this person's cancers are excluded from this method. This is the standard method used to calculate prevalence statistics at the NCI. It only counts a person once, and is useful if the user would like to sum various prevalence estimates across cancer sites without double counting individuals (i.e., the prevalence of specific cancer sites adds up to the prevalence of all cancers combined). Prevalence statistics for different tumor inclusion criteria are available at Table I-17 of the Cancer Statistics Review, 1975-2015 (PDF) .

This method can be implemented in SEER*Stat by setting the Multiple Primary Selection option on the Selection tab to First Malignant Primary Only (Non-reported Assumed Malignant).

First Malignant Tumor per Site in the Last x Years (observation period) - this method includes the first malignant tumor per cancer site diagnosed in SEER during the entire observation period. If 20-year prevalence on January 1, 1999 is being calculated, only the first malignant tumor per cancer site diagnosed from 1979 through 1998 would contribute to prevalence. For this method, the sum of the prevalence counts across specific sites may be greater than the prevalence for all sites combined. This is true because a person with multiple cancers may contribute to more than one estimate.

This method can be implemented in SEER*Stat by following these key steps (only the steps to implement the method are included, proper settings for the survival corhorts and other options are required to generate meaningful estimates):

  • Select malignant cases in the "Other (Case files) selection box" on the Selection tab.
  • Set the Multiple Primary Selection option to All Tumors Matching Selection Criteria on the Selection tab.
  • Use site recode as a table variable.

First Malignant Tumor per Site and Years since Diagnosis - this method includes the first malignant tumor diagnosed in SEER for each considered site and each period since diagnosis. The sum of the prevalence counts across specific sites and across non-overlapping periods since diagnosis may be greater than the prevalence for all sites combined and for the total period, respectively. This is because a person with multiple cancers may contribute to more than one site-specific prevalence and to more than one period since diagnosis for the same site.

This method can be implemented in SEER*Stat by following these key steps (only the steps to implement the method are included, proper settings for the survival cohorts and other options are required to generate meaningful estimates):

  • Select malignant cases in the "Other (Case files) selection box" on the Selection tab.
  • Set the Multiple Primary Selection option to All Tumors Matching Selection Criteria on the Selection tab.
  • Use site recode as a table variable.
  • Select Display By Time Prior to Prevalence Date on the Statistic tab and
    • Enter 5,10,15 in the Cut-Points (Number of Years Prior) edit box.
    • Select Discrete (Non-Overlapping) Intervals.
    • Select First Cancer Per Interval (Intervals May Not Sum to Total).

Examples

The following examples illustrate the situations, using the 3 methods, in which a person would be included in the prevalence estimates. Suppose we are estimating 20-year prevalence at January 1, 1999.

  1. A person had a malignant melanoma in 1980 and a malignant breast cancer in 1990.
    • First Malignant Tumor Only: That person's first cancer would contribute to the prevalence of melanoma and all sites prevalence, but the second cancer would not contribute to the prevalence of breast cancer.
    • First Malignant Tumor per Site in the Last x Years (observation period): The melanoma would contribute to the melanoma prevalence and, since it was the earlier diagnosis, it would contribute to the all sites cancer prevalence. The breast cancer would contribute to the breast cancer prevalence.
    • First Malignant Tumor per Site and Years since Diagnosis: The melanoma would contribute to the melanoma prevalence and, since it was the earlier diagnosis, it would contribute to the all sites cancer prevalence. This person's breast cancer would only contribute to the breast cancer prevalence.
  2. A person had a malignant breast cancer in 1980 and another malignant breast cancer in 1990.
    • First Malignant Tumor Only: If prevalence is computed on January 1, 1997, the person's second cancer would not contribute to the 10-year limited-duration prevalence of malignant breast cancer. The person's first cancer would contribute to the 15 to < 20 year and the 0 to < 20 year limited-duration malignant breast cancer prevalence.
    • First Malignant Tumor per Site in the Last x Years (observation period): If prevalence is computed for January 1, 1997, then the person's second cancer would contribute to the 10-year limited-duration prevalence of malignant breast cancer. The person's first cancer would contribute to the 15 to < 20 year limited-duration prevalence of malignant breast cancer. Only the first cancer would contribute to the 0 to < 20 year limited-duration prevalence of malignant breast cancer.
    • First Malignant Tumor per Site and Years since Diagnosis: If prevalence is computed in January 1, 1999, then the person's second cancer would contribute to the 5 to < 10 year limited-duration prevalence of malignant breast cancer. The person's first cancer would contribute to the 15 to < 20 year limited-duration prevalence of malignant breast cancer. Only the first cancer would contribute to the 0 to < 20 year limited-duration prevalence of malignant breast cancer.
  3. A person had a malignant breast cancer in 1990. Although this is the first cancer they had in a SEER registry, it is indicated that this is their second cancer (the other was a non-SEER cancer). This person would not be included for the prevalence of malignant breast cancer because we assume that the non-SEER cancer was malignant. Since this cancer occurs prior to SEER, we do not know the type of cancer or when it occurred.
    • First Malignant Tumor Only: Although this is the first cancer they had in a SEER registry, it is indicated that this is their second cancer (the other was a non-SEER cancer). This person would not be included for the prevalence of malignant breast cancer because we assume that the non-SEER cancer was malignant. Since this cancer occurs prior to SEER, we do not know the type of cancer or when it occurred.
    • First Malignant Tumor per Site in the Last x Years (observation period): Although this is the first cancer they had in a SEER registry, it is indicated that this is their second cancer (the other was a non-SEER cancer). This person would be included for the prevalence of malignant breast cancer because we do not consider the pre-SEER cancer when using this method.

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