The Burden of Rare Cancers in North America.

Background: Rare cancers are difficult to study owing to their infrequent diagnosis. Using aggregate incidence data from population-based cancer registries in Europe, the Surveillance of Rare Cancers in Europe project compiled a list of clinically relevant, topography and morphology defined rare cancers operationally defined as having a crude annual incidence rate of <6 per 100,000 persons. In 2020, this list of rare cancers was updated. The objective of this study was to assess the utility of a rare cancer recode variable for use in the Cancer in North America (CiNA) dataset and to provide a first look at the burden of rare cancers in Canada and the United States.

Methods: Data were obtained from 62 registries in Canada and the United States that met North American Association of Central Cancer Registries (NAACCR) high-quality data standards. The list of rare cancers was programmed as a Rare Cancer Classification variable within SEER*Stat. SEER*Stat was used to estimate case counts and crude and age-specific incidence rates per 100,000 for cancers diagnosed 2015-2019 by age at diagnosis, country, and country-specific geographic regions in Canada and the United States, and by race/ethnicity in the United States.

Results: In Canada and the United States, 21% and 22% of all invasive cancers were classified as rare, respectively. The percentage of rare cancers ranged between 18% to 21% across geographic regions in Canada and the United States. Children (aged 0-14 years) had the highest percentage and lowest incidence rates of rare cancers. The percentage of rare cancers decreased, and incidence increased with increasing age. In the United States, Hispanics had the highest percentage (27%) and non-Hispanic Whites and non-Hispanic Blacks the lowest percentage (21%) of rare cancers.

Conclusions: While individual rare cancers are infrequently diagnosed, in aggregate, they account for a substantial percentage of all cancers diagnosed in the population and pose a substantial public health burden. We report variations in percentage of rare cancers by age, and race/ethnicity (United States only). Such variations in the burden of these cancers may suggest possible areas for public health research.