The Occurrence of Rare Cancers in US Adults, 1995-2004

Objective. Rare cancers have been traditionally understudied, reducing the progress of research and hindering decisions for patients, physicians, and policy makers. We evaluated the descriptive epidemiology of rare cancers using a large, representative, population-based dataset from cancer registries in the United States. Methods. We analyzed more than 9 million adult cancers diagnosed from 1995 to 2004 in 39 states and two metropolitan areas using the Cancer in North America (CINA) dataset, which covers approximately 80% of the U.S. population. We applied' an accepted cancer classification scheme and a published definition of rare (i.e., fewer than 15 cases per 100,000 per year). We calculated age-adjusted incidence rates and rare/non-rare incidence rate ratios using SEER*Stat software, with analyses stratified by gender, age, race/ethnicity, and histology. Results. Sixty of 71 cancer types were rare, accounting for 25% of all adult tumors. Rare cancers occurred with greater relative frequency among those who were younger, nonwhite, and of Hispanic ethnicity than among their older, white, or non-Hispanic counterparts. Conclusions. Collectively, rare tumors account for a sizable portion of adult cancers, and disproportionately affect some demographic groups. Maturing population-based cancer surveillance data can be an important source for research on rare cancers, potentially leading to a greater understanding of these cancers and eventually to improved treatment, control, and prevention.