The increase in breast cancer rates among younger patients is occurring as the incidence of other early-onset cancers — including colorectal, gastric, kidney and liver cancer — is also on the rise, though cancer among patients under 50 is still relatively rare overall, Dr. Partridge said.

The cancer death rate declined by more than one-third over a three-decade period in the United States, though incidence rates for many types of cancer are rising in women and young adults, according to a new report.