Invasive Apocrine Carcinoma of the Breast: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 7, 2026

Invasive apocrine carcinoma is a rare type of breast cancer in which the tumor cells closely resemble apocrine cells — the specialized cells that make up apocrine sweat glands in the skin. It accounts for approximately 1% of all breast cancers. Under the microscope, apocrine carcinoma cells are large and have abundant pink cytoplasm, with prominent central nucleoli (the dark spot inside the nucleus), giving them a distinctive appearance that the pathologist uses to identify this cancer type.

Invasive apocrine carcinoma has a distinctive biomarker profile that sets it apart from most other breast cancers: it is almost always negative for estrogen and progesterone receptors and positive for androgen receptor (AR). This means it is usually not treated with standard hormone-blocking therapies like tamoxifen, but may respond to treatments that target androgen signaling. When testing shows HER2 positivity, HER2-targeted therapy is also an option.

This article will help you understand the findings in your pathology report. If you had a breast biopsy, you may also find our guide to understanding your breast biopsy report helpful.

What causes invasive apocrine carcinoma?

The exact cause is not fully understood. Like most breast cancers, apocrine carcinoma develops after genetic mutations occur in breast cells — in this case, cells with apocrine features. Unlike most breast cancers, the tumor cells are driven by androgen receptor signaling rather than estrogen, suggesting that androgens (male hormones such as testosterone, which are also present in women) play a role in its growth. General breast cancer risk factors such as age, obesity, alcohol use, radiation exposure, and inherited gene mutations (including BRCA1 and BRCA2) may contribute, but many cases arise without a clear identifiable cause.

What are the symptoms?

Invasive apocrine carcinoma presents similarly to other types of invasive breast cancer. The most common symptom is a lump in the breast, which may be felt or seen on imaging. Other possible symptoms include changes in breast shape or texture, skin dimpling, and nipple discharge. The tumor is sometimes discovered on a screening mammogram before symptoms develop. These symptoms are not unique to apocrine carcinoma and can occur with any breast cancer type.

How is the diagnosis made?

The diagnosis is made after a tissue sample is examined under the microscope by a pathologist. The sample is obtained during a biopsy — typically a core needle biopsy — or at the time of surgical excision.

Under the microscope, apocrine carcinoma cells are large with abundant, granular, bright pink cytoplasm and prominent central nucleoli inside large, round nuclei. The cells invade the breast tissue in a way similar to invasive ductal carcinoma. To confirm the diagnosis and distinguish apocrine carcinoma from other breast cancer types — and from benign apocrine changes in the breast — pathologists use immunohistochemistry. The key pattern is strong positivity for the androgen receptor (AR), combined with the absence of estrogen receptor (ER) and progesterone receptor (PR).

Nottingham histologic grade

The Nottingham histologic grade assesses how aggressive the tumor is likely to be 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 tubular structures. More tubules = score of 1; very few = score of 3. Apocrine carcinomas typically form a few tubules, so this category usually scores higher.
2. Nuclear pleomorphism — How variable and abnormal the nuclei look compared to normal cells. Apocrine carcinoma cells typically have large nuclei with prominent nucleoli, often scoring 2 or 3.
3. Mitotic count — How many cells are actively dividing (mitotic figures) in a defined area. Fewer dividing cells = score of 1; more = 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. Cells closely resemble normal tissue and grow relatively slowly.
• Grade 2 (intermediate grade) — Total score 6–7: moderate abnormality and growth rate.
• Grade 3 (high grade) — Total score 8–9. Cells look markedly abnormal and tend to grow more rapidly. Most invasive apocrine carcinomas are grade 2 or 3, reflecting the typically large and pleomorphic nuclei of apocrine cells.

Tumor size

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

Tumor extension

Invasive apocrine 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, raising the pathologic tumor stage to pT4.

Lymphovascular invasion

Lymphovascular invasion means cancer cells have entered small blood vessels or lymphatic channels near the tumor. Once inside these channels, cancer cells can travel to lymph nodes or reach distant organs through the bloodstream. Your report will state whether lymphovascular invasion is “present” or “absent.” When present, it is associated with a higher risk of spread and recurrence and may influence recommendations about additional treatment such as chemotherapy or radiation therapy.

Surgical margins

A margin is the edge of tissue removed during surgery. The pathologist examines the cut surfaces to determine whether cancer cells are present at the edge.

• Negative margin — No cancer cells at the cut edge. This suggests the tumor was completely removed and is associated with a lower risk of local recurrence.
• Positive margin — Cancer cells are 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 the distance from the nearest tumor cells to the cut edge — a wider negative margin is generally associated with a lower risk of recurrence. Margins are only assessed in surgical excision specimens, not in biopsy samples.

Lymph nodes

Lymph nodes are small immune organs that can trap cancer cells as they spread through the lymphatic system. When breast cancer spreads, it typically reaches the axillary (underarm) lymph nodes first. During surgery, a sentinel lymph node biopsy or axillary lymph node dissection is performed to check for spread. Your report will include the total number of lymph nodes examined, the number containing cancer, and the size of any deposits.

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 leads to more intensive treatment recommendations.

Your report may also mention extranodal extension — when cancer breaks through the outer wall of a lymph node into surrounding tissue — which is associated with a higher recurrence risk.

Biomarker and molecular testing

Biomarker testing is essential for understanding invasive apocrine carcinoma and guiding treatment. The biomarker profile of apocrine carcinoma differs from most other breast cancers in important ways.

Estrogen receptor (ER) and progesterone receptor (PR)

Invasive apocrine carcinoma is almost universally negative for estrogen receptor (ER) and progesterone receptor (PR). This is one of its defining features. ER and PR negativity means the tumor does not rely on estrogen or progesterone to grow. It therefore does not respond to standard hormone-blocking therapies such as tamoxifen or aromatase inhibitors.

Testing is performed by immunohistochemistry (IHC). Results report the percentage of positive cells and staining intensity. ER is considered positive if at least 1% of cells stain positive. If your report shows ER or PR positivity, it may indicate either a mixed tumor with non-apocrine areas or a tumor requiring re-evaluation of its classification.

HER2

HER2 is amplified in a meaningful minority of invasive apocrine carcinomas — estimates range from approximately 20–50% in some series, higher than in most other breast cancer subtypes. HER2 testing is therefore particularly important in this tumor type. 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.
• Fluorescence in situ hybridization (FISH) — performed when IHC is 2+, counting HER2 gene copies to confirm amplification status.

Tumors scoring IHC 1+ or 2+/FISH-negative are classified as HER2-low, which may be eligible for trastuzumab-deruxtecan in the metastatic setting. Tumors scoring IHC 3+ or FISH-amplified are HER2-positive and typically treated with HER2-targeted therapies such as trastuzumab, pertuzumab, and trastuzumab-deruxtecan — often combined with chemotherapy.

Androgen receptor (AR)

Testing for the androgen receptor (AR) is a key part of evaluating invasive apocrine carcinoma. AR is a protein inside cells that allows them to respond to androgens — male hormones such as testosterone that are present in both men and women at varying levels. In normal breast tissue, apocrine cells express AR, and most invasive apocrine carcinomas retain strong, widespread AR expression. AR positivity is therefore one of the features that help the pathologist confirm the tumor’s apocrine nature.

AR testing is performed by immunohistochemistry. Your report will state the percentage of cells showing AR staining and the staining intensity (weak, moderate, or strong). Invasive apocrine carcinomas typically show AR positivity in more than 70–90% of cells with strong intensity.

AR positivity is clinically important because it identifies apocrine carcinoma as a potential target for androgen-blocking therapies. Drugs such as bicalutamide and enzalutamide — which block androgen receptor signaling — are being actively investigated in clinical trials for AR-positive, ER-negative breast cancers, including apocrine carcinoma. While not yet an established standard of care for breast cancer, early clinical trial results are promising, and your oncologist may discuss whether a clinical trial is an option for your situation.

Genomic testing

Genomic tests such as the 21-gene recurrence score (Oncotype DX) are validated primarily for hormone receptor-positive, HER2-negative breast cancer. Since most invasive apocrine carcinomas are ER/PR-negative, these tests are generally not applicable. In the uncommon cases where apocrine carcinoma shows some ER positivity and is HER2-negative, genomic testing may occasionally be considered, but its performance in this setting is not well established. Your oncologist will advise whether any genomic testing is appropriate for your specific case.

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 targeted 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 lymph node involvement into a single score:

• RCB-0 (pathologic complete response) — No residual invasive cancer in the breast or lymph nodes. 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; higher recurrence risk.

Because invasive apocrine carcinoma is ER/PR-negative, it does not respond to neoadjuvant hormone therapy. Neoadjuvant chemotherapy may be used for higher-stage tumors. For HER2-positive apocrine carcinoma, neoadjuvant therapy typically combines chemotherapy with HER2-targeted agents, and pathologic complete response rates in HER2-positive disease are generally favorable.

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 apocrine carcinoma?

The prognosis for invasive apocrine carcinoma is generally similar to that of other invasive breast cancers of comparable stage and grade. It is determined by the same major prognostic factors: tumor size, lymph node involvement, margin status, grade, and the presence or absence of distant spread. Stage-for-stage, outcomes appear broadly comparable to invasive ductal carcinoma.

Several features of apocrine carcinoma are relevant to prognosis:

• Grade — Most invasive apocrine carcinomas are grade 2 or 3 due to the inherently large, pleomorphic nuclei of apocrine cells. A higher grade is associated with a higher risk of recurrence.
• ER/PR negativity — The absence of hormone receptors means standard endocrine therapy is not applicable, which limits one of the most effective and well-tolerated treatment options available for other breast cancer subtypes.
• HER2 status — When HER2-positive, patients benefit significantly from HER2-targeted therapies, which have substantially improved outcomes in HER2-positive breast cancer overall. HER2-positive apocrine carcinoma is treated with the same effective regimens used for HER2-positive invasive ductal carcinoma.
• AR positivity — The high rate of AR expression opens a potential treatment avenue through androgen receptor-targeted therapies, which are currently under active investigation in clinical trials. Early results suggest some patients with AR-positive, ER-negative breast cancer benefit from anti-androgen treatment.
• Triple-negative subgroup — Cases that are ER-negative, PR-negative, and HER2-negative are treated with chemotherapy as the primary systemic option. Immunotherapy (pembrolizumab) may be added for PD-L1-positive tumors in the neoadjuvant or metastatic setting, following the same criteria used for triple-negative invasive ductal carcinoma.

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.

• 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 were the ER and PR results — is my tumor hormone receptor-negative?
• What is the HER2 status — is my tumor HER2-negative, HER2-low, or HER2-positive?
• What was the androgen receptor (AR) result, and how strongly positive is the tumor?
• Is my tumor triple-negative (ER-negative, PR-negative, HER2-negative), and does that affect treatment?
• Are there clinical trials for AR-positive, ER-negative breast cancer that I should consider?
• What systemic treatment is recommended — chemotherapy, HER2-targeted therapy, anti-androgen therapy, or a combination?
• Was PD-L1 testing performed, and could I benefit from immunotherapy?
• If I received neoadjuvant therapy, what was my residual cancer burden score?
• What follow-up imaging and appointments will I need?