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Health authorities often communicate cancer risks to the general public in terms of new cases that occur over time, sometimes referred to as “cancer clocks.” For example, according to the Surveillance, Epidemiology and End Results Program website of the National Cancer Institute, in 2018, “a new cancer is diagnosed every 30 seconds in the United States of America”1; in 2017, the Cancer in Australia in Brief, published by the Australian Institute of Health and Welfare, indicated that “Australians face one new case of cancer diagnosed every 4 minutes”2; and in 2016, the Taiwan Health Promotion Administration in Taipei suggested that “one new cancer case was diagnosed every 5 minutes and 18 seconds in the year 2013 in Taiwan, which is 8 seconds faster than in the previous year.”3 There are other examples of similar clocks used for specific cancers and other health conditions.
Although analogies between the risk of a health condition and a clock timer may have a strong emotional appeal, these analogies are epidemiologically inaccurate when used to compare the development of disease in different populations. A cancer clock expresses the mean time interval between the diagnosis of 2 consecutive cancer cases. When the time interval adopts the unit of year (alternatively, hour, minute, or second using the appropriate conversion factor), it is simply the reciprocal of the total number of new cancer cases diagnosed per year. Therefore, the mean time interval is not more meaningful than the number of cases diagnosed. Without a denominator to adjust for the differences in the population sizes and ages of the groups being compared, neither index applies to the comparisons between different populations or across different time periods.
Crude cancer clocks (mean time interval between diagnoses of new cases of cancer) in 7 selected countries that maintain a population-based cancer registry with complete national coverage are shown in the Table. These data suggest that this time interval varies substantially between countries, from more than 8 minutes in Sweden to less than 20 seconds in the United States. However, these clocks appear to run considerably “faster” (ie, shorter time intervals between new cancer cases) in more populous countries than in less populous ones (Figure). Therefore, these clocks are not an accurate representation of the situation; living in a more populous country compared with a less populous country does not necessarily substantially increase the likelihood of developing cancer.
Cancer clocks (as well as of clocks used for other health conditions, such as heart disease) should be standardized. One approach would be to use a “standard million” population, such as that suggested by the World Health Organization.4 Using this system, the standardized mean time interval in a country is the reciprocal of the total number of new cancer cases in 1 year among 1 standard million people in that country. Alternatively, the standardized mean time interval can be calculated as the reciprocal of the standardized incidence rate in that country multiplied by 1 million. Just as the mean time interval, the standardized time interval also adopts the time unit of year and is easily converted to other time units. For example, the age-standardized cancer incidence rate in Taiwan in 2016 was 300.0 cases per 100 000 population. The standardized cancer clock is then estimated as (300.0 [number of new cases] × 10−5 [per 100 000 population] × 106 [“standard million” population])−1 = 0.000333 years (2 hours, 55 minutes, and 21 seconds). The standardized rate would indicate that a new cancer was diagnosed every 2 hours, 55 minutes, and 21 seconds for 1 standard million people in Taiwan in 2016.
The standardized cancer clocks for the 7 selected countries are also presented in the Table and Figure. Using this method of measurement, the estimates from the standardized clocks for these countries can be compared. This comparison suggests that in these 7 countries, people in Australia had the highest rate of developing cancers (standardized mean time interval = 2 hours, 26 minutes, and 25 seconds) and people in South Korea had the lowest rate (standardized time interval = 3 hours, 2 minutes, and 38 seconds).
The readings taken at different time periods from a standardized clock in a country or a region are also directly comparable. For example, the standardized cancer clock in Taiwan was 13% “faster” (that is, the cancer risk was 13% higher) in 2016 (standardized time interval = 2 hours, 55 minutes, and 21 seconds) than in 2000 (standardized time interval = 3 hours, 21 minutes, and 4 seconds). The crude (that is, nonstandardized) clock used previously by the Taiwan Health Promotion Administration had overestimated the rate of the increase in cancer risk (a 44% increase in the speed of the crude clock; the time interval between new cancer diagnoses decreased from 8 minutes and 56 seconds to 4 minutes and 58 seconds during the same 17-year period) because these estimates were not adjusted for the changes in the population size and age structure in Taiwan during this period.
Cancer clocks and clocks used for other health conditions may be helpful in communicating the risks of developing a specific health condition. The concept behind these clocks is beneficial; however, by standardizing the measurement of incidence rates used in these clocks, the actual risk of developing certain health conditions could be more accurately represented.
Corresponding Author: Wen-Chung Lee, MD, PhD, Institute of Epidemiology and Preventive Medicine, College of Public Health, National Taiwan University, Xuzhou Road, Room 536, No. 17, Taipei 100, Taiwan (firstname.lastname@example.org).
Published Online: December 7, 2018. doi:10.1001/jama.2018.18756
Conflict of Interest Disclosures: None reported.
Funding/Support: This article is supported by grants from the Health Promotion Administration, Ministry of Health and Welfare, Taipei, Taiwan (grant A1051011; tobacco control and health care funds); the Ministry of Science and Technology, Taipei, Taiwan (grants MOST 105–2314-B-002–049-MY3 and MOST 104–2314-B-002–118-MY3); and the Public Health Research Center from the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education, Taipei, Taiwan.
Role of the Funder/Sponsor: The study supporters had no role in the preparation, review, or approval of the manuscript or decision to submit the manuscript for publication.
Lee W, Chiang C, Yang Y. Using Standardized “Cancer Clocks”. JAMA. Published online December 07, 2018. doi:10.1001/jama.2018.18756
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