Invasive lobular carcinoma (ILC) is the second most common type of breast cancer, accounting for approximately 15% of all breast cancer cases. Unlike other breast cancers, ILC is characterized by cancer cells that grow in single-file patterns and often do not form a distinct lump, making them harder to detect during physical exams or mammography imaging. Common symptoms can include subtle changes in the breast such as thickening, swelling, pain, dimpling, changes in the nipple, or a sense of firmness, though many people may not notice any symptoms at all.

In the United States, ILC affects an estimated 47,500 people each year, making it more common in women than either kidney, brain, pancreatic, liver, or ovarian cancers. Most ILC cases are diagnosed in women over the age of 60.

When it’s diagnosed and treated at an early stage, ILC can be managed effectively, and most people with the condition lead healthy lives. However, ILC, as any other breast cancer, can recur in some patients years later.

Breast lobules are glands that produce milk after pregnancy. The cells of the lobules are held together by proteins that help maintain the structure and function of the tissue. ILC is characterized by the loss of one of these proteins, called E-cadherin.

ILC is a type of breast cancer that begins in the breast lobules. From the lobules, the cancer can invade nearby breast tissue and spread to other parts of the body via the blood and lymph systems. Unlike other forms of breast cancer, ILC is characterized by cancer cells that lose their ability to stick together, causing them to grow in single-file lines.

This unique growth pattern makes ILC harder to detect during physical exams or with standard imaging, as the tumor often does not form a distinct mass and can spread through the breast in a subtle, diffuse way. This can hinder early detection of the cancer. Because of this, ILC may not be diagnosed until the tumors are larger and the cancer is at a more advanced stage.

ILC can also be more likely than other types of breast cancer to occur in both breasts and to spread to unusual sites in the body, such as the gastrointestinal tract, ovaries, uterus, peritoneum (the lining of the abdomen), and, more rarely, the lining of the brain and spinal cord and/or the orbital tissue (the tissues within the orbit, or eye socket).

Most ILCs are hormone receptor (HR)-positive and HER2-negative. HR-positive means the cancer cells have receptors for the estrogen and/or progesterone hormones. When these hormones bind to the receptors, they help the cancer cells grow. Medications that lower estrogen hormone levels or block the estrogen receptor activity are used to treat HR-positive breast cancers. HER2-negative means that the cancer cells have little or no HER2 (human epidermal growth factor receptor-2), a protein involved in cell growth.

The exact causes of ILC are not fully understood, but researchers have identified several factors and genetic changes that play a role in its development. A key feature of ILC is the loss of a protein called E-cadherin, which normally helps breast cells stick together and maintain normal tissue structure. This loss is most often due to changes, or mutations, in the CDH1 gene that contains the instructions to make the E-cadherin protein. When this gene is altered or inactivated, cells lose their cohesion, allowing them to spread in the characteristic single-file pattern seen in ILC.

In rare instances, mutations in the CDH1 gene could be inherited, which increases the risk of developing ILC and a rare form of stomach cancer called hereditary diffuse gastric cancer. However, most cases of ILC are not inherited. Older age, use of estrogen plus progesterone hormone replacement therapy, family history of breast cancer, and regular alcohol consumption may increase the risk of developing ILC, but they do not directly cause the disease.

Other genetic changes are also frequently found in ILC, including mutations in genes such as PIK3CA, PTEN, AKT1, FOXA1, ERBB2, FGFR1, and FGFR2. These mutations also contribute to the cancer’s ability to grow, resist certain treatments, or spread to other parts of the body. Some of these mutations can be targeted by drugs that could be used to treat ILC.

Risk factors for ILC include:

• Older age
• Use of hormone replacement therapy, especially combined estrogen and progesterone therapy
• Family history of breast cancer
• Inherited mutations in the CDH1 gene
• Inherited mutations in other genes such as BRCA2, CHEK2, PALB2, and ATM
• Personal history of atypical lobular hyperplasia or lobular carcinoma in situ
• Regular alcohol consumption
• Low parity (having fewer children) or nulliparity (never having children)
• Late age at first birth
• Early onset of menstruation (before age 12)
• Late menopause (after age 55)

Symptoms of ILC may include:

• A thickening or hardening in the breast
• Swelling of all or part of the breast
• Changes in the appearance of the nipple or breast, such as dimpling or pulling in of the skin
• Pain in the breast or nipple
• A sense of fullness or firmness rather than a distinct lump
• Changes in breast size or shape
• Nipple discharge
• Redness or changes in the skin texture of the breast
• A mass or lump in the underarm area

In many cases, people with ILC do not have any symptoms. In such cases, ILC may be found incidentally during imaging or screening tests.

If the cancer has metastasized, that is, has spread from the breast to other parts of the body, it can cause additional symptoms such as bone pain, arm swelling, abdominal pain, gastrointestinal symptoms, or neurological symptoms, depending on the parts of the body to which the cancer has spread.

To diagnose ILC, your doctor will review your medical history, conduct a physical exam, and order imaging tests and a diagnostic needle biopsy. The diagnosis of ILC is made by a pathologist who examines the tissue that was removed from the breast during the biopsy.

Your doctor may ask you about any changes you have noticed in your breasts, such as thickening, swelling, pain, or changes in appearance, as well as any family history of breast cancer or use of hormone replacement therapy. During the physical exam, your doctor will check for areas of thickening, firmness, or changes in the skin or nipple, and may also feel for lumps or swelling in the underarm area.

Additional tests are necessary to make a diagnosis. Tests may include:

• Mammography: An X-ray of the breast used to look for abnormal areas, though ILC can be difficult to detect with this test due to its diffuse growth pattern.
• Ultrasound: Uses sound waves to create images of the breast and can help identify areas that may not be visible on mammography.
• Magnetic resonance imaging (MRI): Provides detailed images of the breast and is especially useful for detecting the extent of ILC, multifocal or bilateral disease, and for evaluating dense breast tissue.
• Core needle biopsy or surgical biopsy: Involves removing a sample of breast tissue for examination under a microscope to confirm the diagnosis and determine the specific type of breast cancer.
• Checking for proteins to guide treatment decisions: Laboratory tests on biopsy tissue to look for the presence or absence of certain proteins, such as E-cadherin, estrogen and progesterone receptors, and HER2, which help confirm the diagnosis and guide treatment decisions.
  
  In some cases, additional imaging tests such as computed tomography (CT), positron emission tomography (PET), or bone scans may be used to check for cancer metastases to other organs. These tests are usually only requested if there are symptoms or findings that suggest metastasis.

Treatments for ILC include:

• Surgery: The primary treatment for most people with ILC is to remove the cancer from the breast. Surgical options include breast-conserving surgery (removal of the tumor and a margin of healthy tissue) or mastectomy (removal of the entire breast). Lymph nodes are also frequently removed to check for cancer spread. A sentinel lymph node biopsy procedure involves removing only one, or a few, lymph nodes that are directly connected to the area of the breast where the cancer was located. An axillary lymph node dissection is a procedure in which several lymph nodes (usually six to 10) are removed.
• Radiation therapy: High-energy X-rays are used to destroy any remaining cancer cells in the breast or surrounding area after surgery, reducing the risk of cancer recurrence in the breast.
• Endocrine (hormone) therapy: Used for tumors that are HR-positive. Medications such as aromatase inhibitors or tamoxifen block or lower the amount of estrogen in the body, slowing or stopping the growth of HR-positive cancer cells.
• Chemotherapy: Uses drugs to kill cancer cells or stop them from growing. ILC is generally less responsive to chemotherapy than some other types of breast cancer, such as invasive ductal carcinomas, but some ILC can benefit from chemotherapy in addition to endocrine therapy. There are molecular tests, such as the Oncotype DX Recurrences Score or MammaPrint assay, that can help to determine if your cancer is chemotherapy sensitive, or not. In chemotherapy-sensitive cancer, chemotherapy can improve cure rates.
• Targeted therapy: For cancers that have specific genetic changes, such as HER2-positive or PIK3CA-mutated tumors, targeted drugs can block the growth signals of cancer cells. Examples include HER2-targeted therapies and PI3K inhibitors. These treatments are usually used together with endocrine therapy and chemotherapy.
• Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors: These drugs may be used in combination with endocrine therapy for advanced or metastatic hormone receptor-positive, HER2-negative ILC to help slow cancer growth and prolong patients’ lives.
• Immunotherapy: Drugs that activate the immune system to fight cancer, so far have not been found beneficial in ILC.

The choice of treatment is based on a number of factors, including the stage of the cancer (which is determined by the size of the tumor and whether the lymph nodes in the armpit contain cancer), its hormone receptor and HER2 status, and the patient’s overall health and preferences. Treatments may be used in combination.

People with ILC may be at increased risk for certain complications, including:

• Local recurrence: The cancer may return in the same breast or nearby tissues after treatment, or it may spread to other sites in the body.
• Distant metastasis: ILC can spread to other parts of the body, including the bones, liver, lungs, gastrointestinal tract, ovaries, uterus, peritoneum, and, less commonly, the lining of the brain and spinal cord or the orbital tissues.
• Bilateral breast cancer: ILC is more likely than other breast cancers to occur in both breasts.
• Positive surgical margins: Because ILC often grows in a diffuse pattern, it can be difficult to remove all cancer cells during surgery, increasing the risk of needing additional surgery.
• Late recurrence: ILC can recur many years after the initial diagnosis, sometimes more than 10 years later.
• Leptomeningeal carcinomatosis: Rarely, ILC can spread to the lining of the brain and spinal cord, causing neurological symptoms.

Complications can also arise from ILC treatments including surgery, radiation, chemotherapy, hormone therapy, and targeted therapy. These may include fatigue, infection, and changes in body image. After lymph node removal or radiation therapy, lymphedema (swelling of the arm or hand) can occur. Endocrine therapies can cause calcium loss from bones called osteopenia or osteoporosis, and menopause-like symptoms such as hot flushes and joint stiffness. Chemotherapy drugs can cause hair loss, low blood counts, skin rash, mouth sores, diarrhea, and numbness or tingling in the fingers and toes. The severity of symptoms varies from person to person, and also by the type of specific treatment.

Most patients who are diagnosed with stage I-III ILC are cured of their disease. The probability of cure varies depending on several factors, including the stage at which the cancer is diagnosed, the tumor’s hormone receptor and HER2 status, the person’s age and overall health, and how well the cancer responds to treatment. Patients who are diagnosed with cancer that has spread to organs other than the breast and lymph nodes in the armpit, called stage IV disease, cannot be cured with current therapies, but life can be prolonged with treatment.

Regular follow-up care and monitoring are important, as ILC can sometimes recur many years after initial treatment.

“The Yale Breast Center has a multidisciplinary team of surgeons, medical oncologists, and radiation oncologists who specialize in the treatment and research of ILC,” says Yale Medicine medical oncologist Lajos Pusztai, MD, DPhil. “It provides opportunities not only to receive the best current standard of care but also to participate in clinical trials that could mean receiving tomorrow’s best therapies today.”