Glossary — I

Immunotherapy is a category of cancer treatment that uses or strengthens the body's natural immune defenses to recognize and attack cancer cells. Unlike chemotherapy, which directly kills rapidly dividing cells, immunotherapy works by training or activating immune cells. It is now used for several cancer types, including some breast cancers, and is often combined with other treatments.

In situ describes a condition in which abnormal or cancerous cells are present but have not grown beyond the tissue layer where they originated. In breast pathology, the two main in-situ diagnoses are ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS). Because the cells have not yet invaded surrounding tissue, in-situ conditions are generally considered earlier and more treatable than invasive cancers.

Inflammatory breast cancer (IBC) is a rare but fast-moving form of breast cancer in which cancer cells block the lymph vessels in the skin of the breast, causing the breast to look red, swollen, and feel warm — symptoms that can easily be mistaken for an infection. IBC accounts for a small percentage of breast cancers but tends to be diagnosed at a more advanced stage and requires prompt, aggressive treatment.

Invasive breast cancer refers to cancer that has broken through the walls of the milk ducts or lobules and grown into the surrounding breast tissue. Unlike in-situ cancers, invasive breast cancers have the potential to reach lymph nodes and other organs. Invasive ductal carcinoma and invasive lobular carcinoma are the two most common types.

Invasive ductal carcinoma (IDC) is the most frequently diagnosed type of breast cancer, accounting for the majority of cases. It begins in the cells lining the milk ducts and grows through the duct walls into the surrounding breast tissue. IDC can vary widely in its characteristics — including grade, hormone receptor status, and HER2 status — which shape its behavior and treatment.