SEER Logo

SEER Training Modules

Introduction to Breast Cancer

Breast cancer is a malignant cell growth in the breast. If left untreated, the cancer spreads to other areas of the body. Excluding skin cancer, breast cancer is the most common type of cancer in women in the United States, accounting for one of every three cancer diagnoses.

An estimated 211,240 new invasive cases of breast cancer were expected to occur among women in the United States during 2005. About 1,690 new male cases of breast cancer were expected in 2005.

The incidence of breast cancer rises after age 40. The highest incidence (approximately 80% of invasive cases) occurs in women over age 50.

In addition to invasive breast cancer, 58,590 new cases of in situ breast cancer are expected to occur among women during 2005. Of these, approximately 88% will be classified as ductal carcinoma in situ (DCIS). The detection of DCIS cases is a direct result of the increased use of mammography screening. This screening method is also responsible for detection of invasive cancers at a less advanced stage than might have occurred otherwise.

An estimated 40,870 deaths (40,410 women, 460 men) were anticipated from breast cancer in 2005. Breast cancer ranks second among cancer deaths in women. According to the most recent data, mortality rates declined significantly during 1992-1998, with the largest decreases in younger women, both white and black.