All breast cancers initially form inside the milk duct near the area where the duct meets the milk gland, or lobule — a structure called the terminal duct lobular unit. As long as the abnormal cells remain inside the milk duct they are known as carcinoma in situ. When they break out of the milk duct and get into the fatty tissue of the breast, they become invasive breast cancers.
Invasive lobular breast cancers (ILCs) and invasive ductal cancers (IDCs) have very different growth patterns. Invasive lobular cancers tend to grow in single-file lines through the fatty tissue of the breast. Invasive ductal cancers, by contrast, tend to re-form the glandular structures of the breast and are more likely to form a mass.
ILC usually doesn't form a lump. If it's found by palpating (lightly pressing) the breast, it is more likely to resemble a fullness or thickening in one area that feels different from surrounding parts. On a mammogram, ILC often appears as an area of distortion. The diagnosis is confirmed by extracting a small piece of the abnormal tissue with a needle and examining it under a microscope. The majority of ILCs are estrogen receptor-positive (ER-positive), meaning they can use the hormone estrogen to grow.
Surgical treatment for invasive breast cancer follows the same approach whether the patient has an invasive lobular or invasive ductal cancer. Most women with invasive breast cancer first undergo surgery to remove the tumor. Depending on the size of the tumor, surgical options may include a lumpectomy (removing just the tumor and a margin of surrounding tissue) or a mastectomy (removing the whole breast). It is also important to determine whether cancer cells have spread from the breast to the lymph nodes under the arm.
Learn more about breast cancer surgery and lumpectomy vs. mastectomy.
Following surgery, treatment may involve radiation therapy to eliminate any remaining microscopic cancer cells at the site of the tumor, and/or chemotherapy to kill cancer cells that may have escaped into the bloodstream of lymph system. For patients with ER-positive tumors, endocrine therapy to reduce the amount of estrogen in the body is also a very effective form of treatment.
What Is LCIS?
Lobular carcinoma in situ (LCIS) is a term used to describe a change in which cells resembling those of invasive breast cancer are contained within the lobule. Although LCIS itself isn't a form of cancer and is not treated as cancer, it indicates a higher risk of developing breast cancer in the future, in either breast. Women with LCIS have a 20-25 percent chance of developing DCIS or invasive breast cancer in the first 15 years after the diagnosis of LCIS — a risk eight to 10 times higher than in women in the general population.
LCIS isn't detected by self-examination or routine mammograms. Most diagnoses come about incidentally: the abnormal cells are discovered as a result of a biopsy for another conditions in the breast, such as calcifications or benign lumps. For this reason, it's known as an "incidental finding" and is present in up to 4 percent of otherwise benign breast biopsies.
Women with LCIS may develop invasive ductal or invasive lobular cancers; but, importantly, when breast cancer occurs in women with LCIS, it is almost always ER-positive. This means that medications like tamoxifen, Evista®, and the aromatase inhibitors — which block the action of estrogen or prevent it from being produced — can be taken to reduce the risk of breast cancer in women with LCIS. Recent data show that these medications may reduce the risk by as much as 70 percent.