Invasive Mucinous Carcinoma of the Breast: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 7, 2026

Invasive mucinous carcinoma is a distinct subtype of breast cancer in which the tumor cells are surrounded by large amounts of a gel-like substance called mucin. To be classified as mucinous carcinoma, at least 90% of the tumor must consist of mucin-containing tissue. Compared to the more common invasive ductal carcinoma, invasive mucinous carcinoma tends to be diagnosed in older women, grows more slowly, and is less likely to spread to lymph nodes or other parts of the body. It is associated with a more favorable prognosis than most other breast cancer subtypes.

This article will help you understand the findings in your pathology report — what the terms mean, what the numbers indicate, and why each piece of information matters for your care. If you had a breast biopsy or surgery, you may also find our guide to understanding your breast biopsy report helpful.

What causes invasive mucinous carcinoma?

The exact cause of invasive mucinous carcinoma is not fully understood, but it shares many risk factors with other types of breast cancer. These include increasing age (it is more common in women over 70), a personal or family history of breast cancer, long-term estrogen exposure (from factors such as late menopause or hormone replacement therapy), and inherited gene mutations such as BRCA1 or BRCA2. Most cases arise without a clearly identifiable inherited cause. Invasive mucinous carcinoma may sometimes develop from a precancerous change called ductal carcinoma in situ (DCIS), which is non-invasive breast cancer confined within the milk ducts.

What are the symptoms?

The most common symptom is a palpable lump in the breast, which is often soft or gelatinous in texture — reflecting the mucin content of the tumor — rather than the hard, irregular lump more typical of invasive ductal carcinoma. Some people have no symptoms, and the tumor is found incidentally on a screening mammogram or breast ultrasound. On imaging, mucinous carcinoma often appears as a well-defined, rounded mass, which can sometimes resemble a benign cyst.

How is the diagnosis made?

The diagnosis is made after a tissue sample is examined under the microscope by a pathologist. The sample is usually obtained during a biopsy — a procedure in which a small amount of tissue is removed from the suspicious area using a needle or during surgery. After the biopsy confirms cancer, surgery to remove the entire tumor is typically recommended.

Under the microscope, invasive mucinous carcinoma has a distinctive appearance: clusters of tumor cells appear to float in large pools of pale blue or pink mucin. The cells themselves are typically small and uniform with relatively few dividing cells — features that reflect the generally slow-growing nature of this tumor. The pathologist confirms the diagnosis by verifying that mucin makes up at least 90% of the tumor volume. If mucin comprises less than 90%, the tumor is classified as invasive ductal carcinoma with mucinous features, which may behave more like conventional invasive ductal carcinoma.

Pure versus mixed mucinous carcinoma

Your pathology report may describe the tumor as pure mucinous carcinoma or mixed mucinous carcinoma. This distinction is clinically important.

• Pure mucinous carcinoma — At least 90% of the tumor consists of cancer cells floating in mucin pools, with no other invasive tumor component present. Pure mucinous carcinoma carries the most favorable prognosis of the two types, with a very low rate of lymph node spread and excellent long-term outcomes.
• Mixed mucinous carcinoma — The tumor contains areas of mucinous carcinoma alongside areas of another invasive carcinoma type, most commonly invasive ductal carcinoma. The mucinous component still accounts for a substantial portion of the tumor, but non-mucinous areas are also present. Mixed mucinous carcinoma tends to behave more like the non-mucinous component and may carry a higher risk of lymph node spread and recurrence than pure mucinous carcinoma.

Nottingham histologic grade

The Nottingham histologic grade describes how abnormal the cancer cells look and how quickly they are growing — information used to predict how aggressive the tumor is likely to be. The grade is calculated by scoring three microscopic features, each on a scale of 1 to 3:

1. Tubule formation — The proportion of the tumor forming round, gland-like structures called tubules. A score of 1 means most cells form tubules; a score of 3 means very few tubules are present.
2. Nuclear pleomorphism — How variable and abnormal the nuclei look. A score of 1 means nuclei are small and uniform; a score of 3 means they are large and markedly irregular.
3. Mitotic count — How many cells are actively dividing (mitotic figures) in a defined area. Fewer dividing cells = score of 1; more dividing cells = score of 3.

The three scores are added together (total range 3–9) to determine the overall grade:

• Grade 1 (low grade) — Total score 3–5. The tumor closely resembles normal cells and tends to grow slowly.
• Grade 2 (intermediate grade) — Total score 6–7: moderate abnormality and intermediate growth rate.
• Grade 3 (high grade) — Total score 8–9. Cells look markedly abnormal and tend to grow more rapidly.

Most invasive mucinous carcinomas are grade 1 or 2, reflecting their typically slow-growing nature. Grade 3 mucinous carcinoma is uncommon and may carry a higher risk of recurrence than grade 1 or 2.

Tumor size

Tumor size is used to determine the pathologic tumor stage (pT) and is an important predictor of outcome — larger tumors are more likely to metastasize to lymph nodes and other parts of the body. The final tumor size can only be accurately measured after the entire tumor has been surgically removed, so it will not appear in a biopsy report — only in the surgical specimen report.

Tumor extension

Invasive mucinous carcinoma begins inside the breast, but in some cases the tumor may spread 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 distant spread, and it raises the pathologic tumor stage (pT4). Your report will note whether tumor extension is present.

Lymphovascular invasion

Lymphovascular invasion means cancer cells have entered small blood vessels or lymphatic channels near the tumor. These channels can carry cancer cells to the lymph nodes or, through the bloodstream, to distant organs. Lymphovascular invasion is less common in pure mucinous carcinoma than in most other breast cancer subtypes. Your report will state whether it is “present” or “absent.” When present, it may increase the risk of lymph node involvement and influence decisions about additional treatment.

Surgical margins

A margin is the edge of tissue removed during surgery. The pathologist examines the cut surfaces to determine whether the entire tumor was removed.

• Negative margin — No tumor cells at the cut edge. This suggests the tumor was completely removed and is associated with a lower risk of local recurrence.
• Positive margin — Tumor cells present at the cut edge, raising concern that some cancer remains. Additional surgery or radiation therapy is usually recommended.

Even when all margins are negative, the report may include a measurement of the closest distance between tumor cells and the cut edge — a wider negative margin generally lowers the risk of recurrence. Margins are only assessed in surgical specimens, not in biopsy samples.

Lymph nodes

Lymph nodes are small immune organs that filter lymphatic fluid. When breast cancer spreads, it typically first reaches the axillary (underarm) lymph nodes. During surgery, these nodes are removed and examined under the microscope. Your pathology report will state the total number of lymph nodes examined, the number containing cancer, and the size of any cancer deposits.

Pure invasive mucinous carcinoma has one of the lowest rates of lymph node involvement of any breast cancer subtype — studies report lymph node positivity in fewer than 15% of cases. This is one of the reasons it carries such a favorable prognosis. Mixed mucinous carcinoma has a somewhat higher rate of lymph node spread.

There are three levels of lymph node involvement:

• Isolated tumor cells (ITCs) — Clusters no larger than 0.2 mm. Not counted as positive for staging purposes.
• Micrometastasis — Clusters between 0.2 mm and 2 mm. Reported as pN1mi. May slightly increase recurrence risk.
• Macrometastasis — Clusters larger than 2 mm. Associated with a higher risk of distant spread and typically prompts more intensive treatment recommendations.

Your report may also mention extranodal extension — when cancer cells break through the outer wall of a lymph node into surrounding tissue — and whether a sentinel lymph node (the first node in the drainage chain from the breast) was examined.

Biomarker and molecular testing

Biomarker testing is an essential part of every breast cancer workup. The results directly determine which treatments are most likely to be effective and help estimate the risk of recurrence.

Estrogen receptor (ER) and progesterone receptor (PR)

The vast majority of invasive mucinous carcinomas — over 90% — are hormone receptor-positive, expressing estrogen receptor (ER) and/or progesterone receptor (PR). These receptors allow the cancer cells to use estrogen and progesterone to fuel their growth.

Testing is done by immunohistochemistry (IHC). Your report will include the percentage of positive cells (e.g., “90% ER-positive”), staining intensity (weak, moderate, or strong), and possibly an overall score (Allred or H-score). A cancer is considered hormone receptor-positive if ER or PR is present in at least 1% of cells. Tumors with ER positivity between 1% and 10% are classified as ER low positive and still generally benefit from hormone-blocking therapy.

Hormone receptor-positive mucinous carcinomas respond well to endocrine (hormone-blocking) therapy such as tamoxifen or aromatase inhibitors (anastrozole, letrozole, or exemestane), which are given after surgery to reduce the risk of recurrence.

HER2

HER2 (human epidermal growth factor receptor 2) gene amplification and protein overexpression are uncommon in invasive mucinous carcinoma — the large majority of mucinous carcinomas are HER2-negative. Testing follows the standard two-step approach:

• Immunohistochemistry (IHC) — measures HER2 protein on tumor cell surfaces, reported as 0, 0+, 1+, 2+, or 3+. Scores of 0 and 1+ are negative; 3+ is positive; 2+ is equivocal and requires confirmatory testing by in situ hybridization.
• Fluorescence in situ hybridization (FISH) — performed when IHC is 2+, counting HER2 gene copies to determine if the gene is amplified. A positive result confirms HER2-positive status.

Tumors with an IHC score of 1+ or 2+/FISH-negative are classified as HER2-low, which may be eligible for trastuzumab-deruxtecan in the metastatic setting. Tumors with IHC 3+ are HER2-positive and respond to HER2-targeted therapies such as trastuzumab. In the rare cases where mucinous carcinoma is HER2-positive, treatment is approached similarly to other HER2-positive breast cancers.

Genomic testing (gene expression profiling)

Some patients with hormone receptor-positive, HER2-negative invasive mucinous carcinoma may be considered for genomic tests that analyze gene activity in the tumor to estimate the risk of recurrence and guide decisions about whether chemotherapy is needed in addition to hormone therapy. The most commonly used tests are the 21-gene recurrence score (Oncotype DX) and the 70-gene signature (MammaPrint).

Because pure mucinous carcinoma is typically low grade and slow growing, many cases receive a low recurrence score, which supports using hormone therapy alone without chemotherapy. Your oncologist will discuss whether genomic testing is appropriate for your specific situation. For more information about breast cancer biomarkers, visit our Biomarkers and Molecular Testing section.

Treatment effect and residual cancer burden

If you received chemotherapy or hormone therapy before surgery (called neoadjuvant therapy), your pathology report will describe how much tumor remains in the breast and lymph nodes after treatment.

The Residual Cancer Burden (RCB) index combines the size of the tumor bed, the percentage of remaining cancer cells, and the extent of lymph node involvement into a single score:

• RCB-0 (pathologic complete response) — No residual invasive cancer in the breast or lymph nodes. The most favorable result.
• RCB-I (minimal residual disease) — Very little cancer remains.
• RCB-II (moderate residual disease) — A moderate amount of cancer remains.
• RCB-III (extensive residual disease) — A large amount of cancer remains, associated with a higher risk of recurrence.

Invasive mucinous carcinoma is generally hormone receptor-positive and grade 1–2, which means it is typically less sensitive to neoadjuvant chemotherapy than triple-negative or HER2-positive breast cancers. Neoadjuvant hormone therapy may be considered in some cases as an alternative approach.

Pathologic stage (pTNM)

The pathologic stage describes how far the cancer has spread, using the TNM staging system. The pathologist from the surgical specimen determines the pT and pN stages; the M stage is determined by imaging.

Tumor stage (pT)

• pT0 — No residual invasive tumor in the surgical specimen.
• pT1mi — Tumor 1 mm or smaller.
• pT1a — Tumor larger than 1 mm but 5 mm or smaller.
• pT1b — Tumor larger than 5 mm but 10 mm or smaller.
• pT1c — Tumor larger than 10 mm but 20 mm or smaller.
• pT2 — Tumor larger than 20 mm but 50 mm or smaller.
• pT3 — Tumor larger than 50 mm.
• pT4a — Tumor has grown into the chest wall.
• pT4b — Tumor has spread to the skin, causing ulceration or satellite nodules.
• pT4c — Both pT4a and pT4b.
• pT4d — Inflammatory breast cancer.

Nodal stage (pN)

• pN0 — No cancer in any lymph nodes examined.
• pN0(i+) — Isolated tumor cells only (≤0.2 mm) — not counted as positive.
• pN1mi — Micrometastases only (0.2–2 mm) in axillary lymph nodes.
• pN1a — Cancer in 1–3 axillary lymph nodes, at least one deposit larger than 2 mm.
• pN1b — Cancer in ipsilateral internal mammary sentinel nodes (excluding isolated tumor cells).
• pN2a — Cancer in 4–9 axillary lymph nodes.
• pN2b — Cancer in internal mammary lymph nodes without axillary involvement.
• pN3a — Cancer in 10 or more axillary lymph nodes, or in infraclavicular lymph nodes.
• pN3b — Cancer in internal mammary and axillary lymph nodes.
• pN3c — Cancer in supraclavicular lymph nodes.

What is the prognosis for invasive mucinous carcinoma?

Invasive mucinous carcinoma — particularly the pure subtype — has one of the most favorable prognoses of all invasive breast cancer subtypes. Several features contribute to this:

• Low grade in most cases — The majority of mucinous carcinomas are grade 1 or 2, reflecting their typically slow rate of growth.
• Low rate of lymph node involvement — Pure mucinous carcinoma spreads to the axillary lymph nodes in fewer than 15% of cases, considerably lower than most other breast cancer subtypes.
• Strong hormone receptor positivity — Over 90% are ER-positive, which means most patients benefit from long-term hormone therapy, further reducing the risk of recurrence.
• Low rate of HER2 amplification — The large majority are HER2-negative, which is associated with a more indolent course.
• Excellent long-term survival — Studies consistently report 10-year and 15-year survival rates above 85–90% for pure mucinous carcinoma, even for tumors that present at a larger size.

Prognosis is less favorable when the tumor is mixed mucinous (containing a non-mucinous invasive component), higher grade (grade 3), or associated with lymph node involvement. As with all breast cancers, factors such as tumor size, stage, surgical margin status, and response to treatment also influence individual outcomes. Your treatment team will use all of the findings in your pathology report together to estimate your personal prognosis and recommend the most appropriate treatment plan.

Questions to ask your doctor

Your pathology report contains important information that will guide your care. The following questions may help you prepare for your next appointment.

• Is my tumor classified as pure mucinous carcinoma or mixed mucinous carcinoma, and does that affect my prognosis?
• What was the tumor size and Nottingham grade?
• What is the pathologic stage of my cancer (pT and pN)?
• Were any lymph nodes involved? If so, how many were there, and what sizes were the deposits?
• Were the surgical margins clear? Was the tumor completely removed?
• Was lymphovascular invasion present?
• What are my hormone receptor (ER and PR) results, and which hormone-blocking therapy do you recommend?
• What is my HER2 status — negative, low, or positive — and does it change my treatment?
• Will genomic testing (such as Oncotype DX) be performed to help decide whether chemotherapy is needed?
• Was DCIS found alongside the invasive cancer, and does that affect my surgical recommendations?
• If I received neoadjuvant therapy, what was my residual cancer burden score?
• What additional treatment is recommended — radiation, chemotherapy, hormone therapy, or a combination?
• How long will I need to take hormone therapy?
• What follow-up imaging and appointments will I need, and how often?