Prevalence Corrected Prostate Cancer Incidence Rates and Trends

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Mark K. Morris and Dr. Ray Merrill, Health Science

Conventional cancer incidence rates reported in the United States represent the number of newly diagnosed cases in a given year divided by the mid-year population for that year (1). Incidence rates have been reported on an annual basis in the United States since 1973, when the National Cancer Institute initiated the Surveillance, Epidemiology, and End Results (SEER) Program (2). Population-based site-specific cancer incidence rates are commonly thought of as measures reflecting the average risk of developing the disease (3). However, this assumes that the rate calculation includes new cases of the cancer in the numerator and the at-risk population for developing the cancer in the denominator. In other words, a measure of the risk of developing a given cancer should be new cases of the disease divided by those who have never had the disease but are at risk of developing it (4).

Unlike other cancers, such as of the breast, in which multiple primary tumors may be diagnosed at the same site, multiple diagnosed cancers of the prostate are extremely rare (4). On the other hand, the prevalence of prostate cancer exceeds that of any other cancer in men in the United States (5). The prevalence of this disease has been high because of a combination of high incidence and good survival of the disease (1). There has also been a steady increase observed in prostate cancer prevalence in recent years (6). The extensive pool of prostate cancer cases in the population and the increasing growth of prevalent cases suggest that failure to remove these men from the denominator in the rate calculation may give inaccurate prostate cancer incidence rates and trends. Correcting the denominator in the rate calculation for prevalent cases of the disease will increase the magnitude of the rates and may also change the trend in rates. The purpose of this study is to obtain corrected prostate cancer incidence rates and to assess the change in magnitude and tend of the rates according to age.

A correction is made of prostate cancer incidence rates based on data from the Surveillance, Epidemiology, and End Results (SEER) Program of the United States National Cancer Institute. Unlike conventional incidence rates reported by SEER, corrected rates remove from the population the estimated number already having been diagnosed with the disease. The corrected rates reflect the average prostate cancer risk for men in the at-risk population. Because of the high incidence and relatively good survival of prostate cancer, the prevalence of this disease is higher than that of any other cancer in men. Corrected prostate cancer incidence rates were higher in magnitude, particularly in older age groups and among black men. For example, in 1997 for whites the corrected rates were 3.8% higher in cases aged 60-69, 9.3% higher in cases aged 70-79, and 13.1% higher in cases aged 80+. Corresponding percentages for blacks were 5.9%, 18.9%, and 16.9%, respectively. Percent changes over calendar time were very similar between corrected and uncorrected prostate cancer incidence rates according to age and race (white and black). Failure to account for high levels of prostate cancer prevalence in conventional incidence rates of the disease results in underestimation of the rates but little practical difference in the trends.

This study suggests that corrected prostate cancer incidence rates differ noticeably in magnitude from conventionally derived rates, more so in older age groups, among black men, and in recent years because of PSA screening. Although corrected incidence rates may have a trivial influence on many cancer sites with low prevalence, cancers of the breast, colon and rectum, and prostate are possible exceptions. In addition, the high prevalence of hysterectomy in the population has removed a large segment of the female population from risk of developing certain cancers of the female genital system (4,19). The American Cancer Society calculates annual estimates of new cancer cases for the United States using conventional SEER-based cancer incidence rates. Hence, these national estimates of new cases will be underestimated. When estimating the number of cancer cases in the united States, at least for the cancer sites just specified, basing the estimates on corrected rates seems appropriate. Methods for correcting rates for cancer sites affected by hysterectomy have been discussed previously (4,20). In the near future, the National Cancer Institute is planning to include with the annual cancer data made available through the SEER *Stat Program (21), site- and age-specific cancer point prevalence proportions. Ready availability of these prevalence estimates can make it easier to obtain corrected cancer incidence rates.

The experience of working with Dr. Merrill has been extremely beneficial to me. He has helped guide me in the field of Epidemiology and heightened my understanding of cutting-edge research. Our results will undoubtedly help other colleagues in their epidemiological work. We have submitted our results into the American Journal of Epidemiology and has recently been accepted for publishing.

References

  1. Ries LAG, Eisner MP, Kosary CL, et al., SEER cancer statistics review, 1973-1997. Bethesda, MD: National cancer Institute, 2000.
  2. Miller BA, Ries LAG, Hankey BF, et al. Cancer statistics review 1973-1990. Rockville, MD: National Cancer Institute, 2000.
  3. Rothman KJ, Greenland S. Modern Epidemiology. 2nd ed. Philadelphia, PA: Lippincott- Raven Publishers, 1998.
  4. Merrill RM, Feuer EJ. Risk-adjusted cancer incidence rates. Cancer Causes Control, 1996, 7:544-52.
  5. Merrill RM, Capocaccia R, Feuer EJ, et al. Cancer prevalence estimates bases on tumor registry data in the Surveillance, Epidemiology, and End Results (SEER) Program. Int J Epidemiol 2000; 29:197-207.
  6. Merrill RM. Partitioned prostate cancer prevalence estimates: an informative measure of the disease burden. J Epidemiol Community Health 2001;; 55:191-7.
  7. Lyon JL, Gardner JW. The rising frequency of hysterectomy: its effect on uterine cancer rates. Am J Epidemiol 1977; 105:439-43.
  8. Merrill RM. Prevalence corrected hysterectomy rates and probabilities in Utah. Ann Epidemiol 2001; 11:127-35.
  9. SEER *Stat for Windows 95/NT Versin 3.0 SEER cancer incidence public-use database, 1973-1997. Silver Spring, MD: Information Management Services, Inc., 2000.