Trends in the proportion of second or later primaries among all newly diagnosed malignant cancers

Improvements in cancer survival mean that an increasing number of survivors may live long enough beyond their initial cancer to be diagnosed with additional independent primary cancers. The proportion of newly diagnosed cancers that are second- or higher-order primaries and how this proportion has changed over the past several decades were examined.

Data from the Surveillance, Epidemiology, and End Results (SEER) program were used to identify incident malignant primaries diagnosed between 1975 and 2017. Using the SEER sequence number, the authors tabulated the proportion of all cancers in each calendar year that were second- or higher-order primaries. The average annual percent change (AAPC) was then calculated to assess how this proportion has changed over time.

Analyses included nearly 4.9 million incident cancers diagnosed during 1975-2017. The proportion of all cancers that were second- or higher-order increased steadily from 9.77% during 1975-1984 to 21.03% during 2015-2017, reflecting an AAPC of 2.41% (95% CI, 2.16%-2.65%). In 2015-2017, second- or higher-order cancers were most prevalent among cancers of the bladder (28.79%), followed by lung and bronchus (28.07%), melanoma (27.88%), and leukemia (26.10%). The highest AAPCs over the study period were observed for melanoma (4.05%), leukemia (3.51%), and lung and bronchus (3.36%).

The proportion of newly diagnosed cancers that are second- or higher-order has grown rapidly over the past several decades and currently exceeds 20%. Continued monitoring of second and later primaries will be critical for anticipating the future impact on cancer treatment and survivorship care.