Invasive Mucinous Carcinoma of the Breast: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 7, 2026

Invasive mucinous carcinoma is a type of breast cancer in which the tumor cells are surrounded by large pools of a thick fluid called mucin. To be called mucinous carcinoma, at least 90% of the tumor must be made up of mucin. Compared with the more common invasive ductal carcinoma, invasive mucinous carcinoma tends to develop in older women (often over the age of 70), usually grows slowly, and is less likely to spread to lymph nodes or other parts of the body.

Most invasive mucinous carcinomas are hormone receptor-positive and HER2-negative (explained in the biomarker section below), which means they usually respond well to hormone-blocking therapy and have a favorable outlook. This article will help you understand the findings in your pathology report, what each term means, and why it matters for your care.

What causes invasive mucinous carcinoma?

The exact cause of invasive mucinous carcinoma is not fully understood. Like most breast cancers, it develops after genetic changes occur in breast cells that allow them to grow in an uncontrolled way. Because these tumors are usually hormone receptor-positive, lifetime exposure to the hormone estrogen is thought to play a role, which is one reason the cancer is more common in older women. General breast cancer risk factors such as increasing age, family history, and inherited gene changes (including BRCA1 and BRCA2) may contribute, but many cases arise without a clear cause.

What are the symptoms of invasive mucinous carcinoma?

Invasive mucinous carcinoma usually appears as a lump in the breast that may be felt or seen on imaging. Because these tumors are often soft and well-defined, they can appear as benign (non-cancerous) growths on a mammogram or ultrasound, and the diagnosis is sometimes made only after the tissue is examined under the microscope. Some tumors are found on a screening mammogram before any symptoms develop.

How is the diagnosis made?

The diagnosis of invasive mucinous carcinoma is usually made after a small sample of the tumor is removed by biopsy, or after the tumor is removed by surgery and examined under the microscope by a pathologist. Under the microscope, invasive mucinous carcinoma consists of small clusters and nests of cancer cells that appear to float within large pools of pale, bubbly mucin. The cells are usually small and uniform (low grade). For a tumor to be classified as mucinous carcinoma, at least 90% of it must be composed of mucin; when mucinous areas make up less than 90%, the tumor is classified as a mixed mucinous carcinoma.

Pathologists sometimes divide invasive mucinous carcinoma into type A (fewer cells and more mucin) and type B (more cells, sometimes showing neuroendocrine differentiation, meaning the cells share features with hormone-producing cells). After the diagnosis is made, the tissue is also tested for hormone receptors, HER2 (described in the biomarker section below), and breast imaging is used to measure the size and extent of the tumor and to plan treatment.

Histologic grade

Invasive mucinous carcinoma is given a histologic grade using the Nottingham grading system, which describes how closely the cancer cells resemble normal breast tissue and how quickly they are growing. The pathologist scores three features, each from 1 to 3:

Tubule formation — How much of the tumor forms round, gland-like structures called tubules. More tubules is scored 1; very few is scored 3.
Nuclear pleomorphism — How variable and abnormal the cell nuclei look compared with normal cells. Small, uniform nuclei are scored 1; large, pleomorphic (varied) nuclei are scored 3.
Mitotic rate — How many cells are actively dividing, seen as mitotic figures. Few dividing cells are scored 1; many are scored 3.

The three scores are added together (total 3 to 9) to give grade 1 (score 3 to 5, low grade), grade 2 (score 6 or 7, intermediate grade), or grade 3 (score 8 or 9, high grade). Most invasive mucinous carcinomas are grade 1 or 2, which is one reason they tend to grow slowly and have a favorable outlook.

Tumor size

The size of the tumor is used to determine the pathologic tumor stage (pT) and is an important predictor of outcome; larger tumors are more likely to metastasize (spread) to lymph nodes and other parts of the body. Because mucinous carcinoma contains large pools of mucin, the tumor can feel or appear larger than the actual extent of cancer present. The final size can only be measured after the entire tumor has been removed at surgery, so it does not appear in a biopsy report.

Tumor extension

Invasive mucinous carcinoma starts inside the breast, but the tumor can grow into the overlying skin or the muscles of the chest wall. This is called tumor extension. Its presence is associated with a higher risk of local recurrence and of spread to distant sites, and it raises the pathologic tumor stage to pT4. Tumor extension is uncommon in this slow-growing cancer.

Lymphovascular invasion

Lymphovascular invasion means cancer cells have entered small blood vessels or lymphatic channels near the tumor. These vessels can serve as pathways for cancer cells to travel to lymph nodes or other parts of the body. The pathologist reports lymphovascular invasion as “present” or “absent.” It is uncommon in invasive mucinous carcinoma, but when present, it is associated with a higher risk of spread and may influence decisions about additional treatment.

Margins

A margin is the edge of the tissue removed during surgery. The pathologist examines the margins to determine whether the entire tumor was removed. Margins are assessed only after surgery that removes the whole tumor, not after a biopsy.

Negative margin — No cancer cells at the cut edge. The report may also state how close the nearest cancer cells came to the edge; a wider clear margin lowers the risk of local recurrence.
Positive margin — Cancer cells present at the cut edge, meaning some cancer may remain. Another surgery or radiation therapy may be considered.

Lymph nodes

Lymph nodes are small immune organs that filter fluid and can trap cancer cells. When breast cancer spreads, it usually travels first to the axillary lymph nodes (under the arm). During surgery, a sentinel lymph node (the first node in the drainage path from the breast) may be removed and examined; if it contains cancer, more nodes may be removed. Your report will include the number of nodes examined, the number that contain cancer, and the size of the largest deposit. It may also mention extranodal extension, meaning cancer has broken through the outer capsule of a node into the surrounding tissue. Invasive mucinous carcinoma reaches the lymph nodes less often than most other breast cancers.

Isolated tumor cells — Clusters no larger than 0.2 mm. Not counted as positive for staging and recorded as pN0(i+).
Micrometastasis — A deposit larger than 0.2 mm but not larger than 2 mm, reported as pN1mi.
Macrometastasis — A deposit larger than 2 mm, associated with a higher risk of spread and often leading to more intensive treatment.

Biomarker and molecular testing

Biomarker testing is an essential part of the workup for every invasive mucinous carcinoma. The results determine which treatments are likely to work and help estimate the risk of recurrence. Every invasive mucinous carcinoma is tested for the estrogen receptor, the progesterone receptor, and HER2.

Estrogen receptor (ER) and progesterone receptor (PR)

Estrogen receptor (ER) and progesterone receptor (PR) are proteins that allow breast cancer cells to grow in response to the hormones estrogen and progesterone. They are tested by immunohistochemistry and reported as the percentage of positive cells and the staining intensity (weak, moderate, or strong). The vast majority of invasive mucinous carcinomas are hormone receptor-positive, meaning at least 1% of cells express ER or PR. This is favorable, because hormone receptor-positive cancers usually respond well to hormone-blocking therapies such as tamoxifen or aromatase inhibitors (anastrozole, letrozole, exemestane), which lower the risk of recurrence.

HER2

HER2 is a protein that helps control cell growth. In some breast cancers, the HER2 gene is amplified and the cells make too much HER2 protein. Invasive mucinous carcinoma is usually HER2-negative. HER2 is tested first by immunohistochemistry (reported as 0, 1+, 2+, or 3+), with fluorescence in situ hybridization (FISH) used to settle a borderline 2+ result. A result of 3+ or FISH-amplified is HER2-positive; 1+ or 2+/FISH-negative is HER2-low; and 0 is HER2-negative. In the uncommon event that a mucinous carcinoma is HER2-positive, HER2-targeted therapy may be considered.

Ki-67

The Ki-67 labeling index is the percentage of cancer cells that are actively dividing. Invasive mucinous carcinoma usually has a low Ki-67, reflecting its slow growth. Ki-67 is one of several factors that may be considered when deciding whether chemotherapy would add benefit.

Genomic testing

Because most invasive mucinous carcinomas are hormone receptor-positive and HER2-negative, a genomic test that measures the activity of a panel of genes may be used to estimate the risk of recurrence and whether chemotherapy is likely to help beyond hormone therapy. The most commonly used is the 21-gene recurrence score (Oncotype DX). These tests are usually ordered by the oncologist, and the results often come separately from the pathology report. For more information, visit our Biomarkers and Genetic Testing section.

Pathologic stage (pTNM)

Invasive mucinous carcinoma is staged using the TNM system of the American Joint Committee on Cancer (AJCC), 8th edition, based on the tumor (T), lymph nodes (N), and distant metastasis (M). The pathologist determines the pT and pN stages from the removed tissue; the M stage is determined by imaging.

Tumor stage (pT)

pT1 — Tumor 2 cm (20 mm) or smaller. Subdivided into pT1mi (1 mm or smaller), pT1a (more than 1 mm up to 5 mm), pT1b (more than 5 mm up to 10 mm), and pT1c (more than 10 mm up to 20 mm).
pT2 — Tumor larger than 2 cm but not larger than 5 cm.
pT3 — Tumor larger than 5 cm.
pT4 — Tumor of any size that has grown into the chest wall or skin. pT4a is growth into the chest wall; pT4b is growth into the skin causing ulcers or swelling; pT4c is both; pT4d is inflammatory breast cancer.

Nodal stage (pN)

pN0 — No cancer in the lymph nodes. Isolated tumor cells only are recorded as pN0(i+).
pN1 — Cancer in 1 to 3 axillary lymph nodes, or micrometastases only (pN1mi), or spread to internal mammary sentinel nodes.
pN2 — Cancer in 4 to 9 axillary lymph nodes, or in internal mammary nodes without axillary involvement.
pN3 — Cancer in 10 or more axillary lymph nodes, in nodes below or above the collarbone, or in a combination of internal mammary and axillary nodes.

What is the prognosis for invasive mucinous carcinoma?

Invasive mucinous carcinoma has a favorable prognosis, generally better than that of invasive ductal carcinoma of the same stage. Most tumors are low grade and hormone receptor-positive, grow slowly, and reach the lymph nodes less often than other breast cancers. The five-year survival rate is over 90%, and many people live much longer without the cancer returning. Several features on the report influence the outlook:

Pure versus mixed — Pure mucinous carcinoma (at least 90% mucin) has a better outlook than mixed mucinous carcinoma, which contains areas of ordinary invasive ductal carcinoma and tends to behave more like that tumor.
Grade — Most mucinous carcinomas are grade 1 or 2; a higher grade is associated with a higher risk of recurrence.
Lymph node status — Spread to lymph nodes is uncommon but, when present, is associated with a higher risk of recurrence.
Hormone receptor status — Being hormone receptor-positive allows treatment with effective, well-tolerated hormone-blocking therapy.

What happens after this diagnosis?

After a diagnosis of invasive mucinous carcinoma, care is usually coordinated by a team that may include a breast surgeon, a medical oncologist, a radiation oncologist, and a pathologist. The pathology findings guide which options the team considers, rather than dictating a single path. Surgery removes the tumor, either by breast-conserving surgery (lumpectomy) or removal of the whole breast (mastectomy), and lymph nodes are usually sampled at the same time. Radiation therapy is often considered after breast-conserving surgery.

Because most invasive mucinous carcinomas are hormone receptor-positive, hormone-blocking therapy is a mainstay of treatment. Chemotherapy is often not needed for small, low-grade, node-negative tumors, and a genomic test such as the 21-gene recurrence score may help decide whether chemotherapy would add benefit. If treatment is given before surgery, the pathologist reports the extent of residual cancer using the residual cancer burden (RCB) index. After treatment, follow-up includes regular examinations and imaging to watch for recurrence.

Questions to ask your doctor

Is my tumor a pure mucinous carcinoma, or a mixed mucinous carcinoma?
What was the tumor size and Nottingham grade?
What is the pathologic stage of my cancer (pT and pN)?
Were any lymph nodes involved, and if so, how many and what size were the deposits?
Were the surgical margins negative, and was the tumor completely removed?
Was lymphovascular invasion present?
What were my ER and PR results, and is my tumor hormone receptor-positive?
What is my HER2 status?
Will I need a genomic test such as Oncotype DX, and how would the result affect my treatment?
Is hormone-blocking therapy recommended, and for how long?
Will I need chemotherapy or radiation therapy?
What follow-up and imaging schedule will I need?