Multiple Primary Cancers in the United States

The distribution of multiple primary cancers has been described previously using data from the Surveillance, Epidemiology, and End Results (SEER) Program. However, a complete picture regarding the distribution of multiple primary cancers in the United States is still lacking. The objective of the current study is to present a comprehensive description of multiple primary cancers in the United States.

Invasive cancer cases (including in situ bladder cancers) diagnosed between 2001 and 2016 from 49 population-based state cancer registries in the United States were evaluated for this study. The sequence number central assigned to each tumor was used to determine whether a tumor was a first primary cancer or a subsequent multiple primary cancer. Tumors with a sequence number 00 or 01 were classified as the first primary cancer, while tumors with a sequence number 02 or above were classified as a multiple primary cancer. The percentage of multiple primary cancers was calculated by sex, age, race/ethnicity, cancer site, registry, and diagnosis year. In addition, the percentage of cancers diagnosed at a local stage among multiple primaries was compared with that among first primaries.

Overall, about 19.0% of cases were reported as multiple primary cancers; the percentage was higher among non-Hispanic Whites and among older patients. Bladder, melanoma of the skin, and lung cancers had the highest percentage of cases reported as multiple primaries. The percentage of multiple primary cancers also varied by registry and has been increasing over time. Cancers reported as multiple primaries were more likely to be diagnosed at a local stage than those reported as first primaries.

Cancers registered as multiple primaries are common in the United States, showing an increasing trend over time and wide variation by race/ethnicity, age, cancer type, and registry. The findings have some practical implications for cancer registries that collect data and for researchers conducting investigations using information on multiple primary cancers.