Breast cancer

Breast cancer is a malignant tumor arising from breast cells. The two primary types of breast cancer include ductal carcinoma and lobular carcinoma, originating in the milk-transporting ducts and the milk-producing lobules, respectively. It is classified as either noninvasive (carcinoma in situ) or invasive (infiltrating). Invasive breast cancer is further categorized into early breast cancer, locally advanced breast cancer, and advanced/metastatic breast cancer. Based on the expression of hormone receptors and HER2, tumors are classified into three main groups: HR-positive (luminal tumors), HER2-positive, and the remaining subtypes being collectively referred to as triple-negative breast cancer (including basal-like and claudin-low tumors).

High-penetrance genes BRCA1 and BRCA2 are associated with an increased predisposition to breast cancer. Mutations in these genes increase the risk of both breast and ovarian cancers. TP53 is another critical gene associated with breast cancer risk, often correlated with more aggressive disease and poor prognosis. Additionally, several single nucleotide polymorphisms, including FGFR2, TNRC9, MAP3K1, LSP1, and CASP8, have been identified as associated with an increased risk of breast cancer.

Breast cancer is the most common cancer worldwide, both in women and overall, with an estimated incidence rate of 119.2 per 100,000 woman-years in the US.

Risk factors for breast cancer include age, family history of breast or ovarian cancer, genetic mutations such as BRCA1 or BRCA2, history of breast atypical hyperplasia and carcinoma in situ, increased mammographic density, prior radiation exposure, and certain lifestyle factors such as alcohol consumption, obesity, and lack of physical activity.

Clinically, breast cancer may present with a palpable breast mass, nipple discharge, skin changes such as erythema, peau d'orange, and ulceration, and asymmetric thickening or nodularity. Focal breast pain and axillary mass may also be present.

The prognosis of breast cancer is influenced by the stage at diagnosis, with early-stage cancers generally having a better prognosis than late-stage cancers. The specific molecular characteristics of the tumor, such as hormone receptor status and HER2 status, also play a significant role in determining prognosis. Basal-like breast cancer, which is often triple-negative, is associated with a poor prognosis. The 5-year age-adjusted breast cancer mortality rate in the US is estimated at 19.6 per 100,000 female-years.