What Is Invasive Micropapillary Breast Cancer? Differences from Classic Breast Cancer and Treatment Approaches

Micropapillary Breast Cancer: A Rare Subtype That Tends to Reach the Lymph Nodes, and What Matters in Treatment

Patients who see the words "invasive micropapillary carcinoma" or "micropapillary component" on their pathology report often wonder how different this is from classic breast cancer and how it affects treatment. Invasive micropapillary carcinoma is a rare subtype that makes up about 1–2% of all breast cancers. Its most notable feature is a tendency to spread into the lymphatic vessels and the axillary (underarm) lymph nodes. Yet when it is compared with classic breast cancer of the same stage, survival has generally been shown to be similar. In this article, based on a recent review of the topic, we look at the features, diagnosis and treatment approach of micropapillary breast cancer. We have previously explained the Ki-67 value, HER2 positivity and common pathology terms on pathology reports in separate articles.

Why does this matter?

Most breast cancers are classified as "invasive carcinoma of no special type (NST)" (formerly called invasive ductal carcinoma); in this article we refer to it as "classic breast cancer." The rest are "special types," such as lobular, mucinous, tubular, medullary and micropapillary. Some of these special types behave more favorably than the classic type, while others follow a different course (breast cancer symptoms, stages and treatment).

The micropapillary type calls for special attention because of its tendency to spread into the lymphatic vessels and lymph nodes. Seeing this term on a pathology report can be worrying for patients; yet with accurate staging and treatment, outcomes are similar to those of classic breast cancer of the same stage. That is why understanding what the micropapillary type really means is important, both to avoid unnecessary fear and to know which points should not be overlooked in the treatment plan.

What is invasive micropapillary breast cancer?

Invasive micropapillary breast cancer is a rare type of breast cancer in which the tumor cells are arranged in small, papilla-like clusters within empty-looking spaces, with the secretory surface of the cells facing outward. It makes up about 1–2% of breast cancers. It has a greater tendency to spread into the lymphatic vessels and axillary lymph nodes; its treatment follows the same principles as classic breast cancer and is planned according to hormone receptor status, HER2 status and stage.

How does it differ from classic breast cancer?

Rates are compiled from different case series and vary between studies.

These findings explain why the micropapillary type is often described as "more aggressive": at diagnosis, the disease has more often already spread to the axillary lymph nodes, and the risk of the cancer coming back in the breast or chest wall (locoregional recurrence) is higher. However, once stage and lymph node status are taken into account, survival turns out to be no different from classic breast cancer. In an analysis of the National Cancer Database (USA), involvement of 4 or more lymph nodes was linked to shorter survival, whereas involvement of 1–3 lymph nodes was not. This shows that stage is the main factor shaping the outcome (what is metastasis).

Molecular features: which subtype is it usually?

About 70–80% of micropapillary breast cancers are hormone receptor–positive (luminal); HER2-positive and triple-negative types together make up about 10–20%. A substantial share of these tumors have a high Ki-67 and a high nuclear grade. Genetic studies frequently show gains in chromosome regions such as 8q (where the MYC gene is located), 17q and 20q.

How is it diagnosed?

• Imaging: Mammography usually shows an irregular mass with spiculated (spiky) margins; ultrasound shows an irregular mass that casts a shadow behind it. There may be enlarged axillary lymph nodes and more than one focus in the breast; MRI is helpful for assessing how widespread the disease is.
• Biopsy and pathology: The diagnosis is made with a needle biopsy (what is a biopsy). The pathologist assesses the proportion of the micropapillary pattern and the "inside-out" feature using special stains such as EMA. The micropapillary component seen on the biopsy may turn out to be a different proportion in the surgical specimen.
• Differential diagnosis: Serous carcinoma of the ovary that has spread to the breast, and extensive lymphatic vessel involvement, can look similar under the microscope; special stains are used to tell them apart.
• Other parts of the report: ER, PR, HER2, Ki-67, tumor size, lymphovascular invasion and lymph node status form the basis of the treatment plan.

Treatment approach

There is no treatment guideline or targeted drug specific to micropapillary breast cancer. As with classic breast cancer, treatment is planned according to the tumor's biological subtype (hormone receptor and HER2 status) and stage. Even so, the way this type spreads means that some points need extra attention.

• Surgery: Breast-conserving surgery or mastectomy is chosen based on the size and extent of the tumor and the patient's preference. The micropapillary type alone is not a reason for mastectomy; however, clear surgical margins are especially important.
• The axilla: Because lymph node involvement is common, a careful assessment of the axilla with ultrasound, plus a biopsy if needed, is recommended before surgery. If there is no clinical sign of involvement, sentinel lymph node biopsy is the standard approach (axillary lymph node biopsy).
• Radiotherapy: Because the risk of locoregional recurrence is higher, decisions about radiotherapy are made carefully, especially in patients with lymph node involvement. In observational studies, radiotherapy was linked to fewer recurrences and longer survival; however, these studies do not firmly prove cause and effect.
• Hormone receptor–positive, HER2-negative: Hormone therapy is the backbone of treatment. In patients with lymph node involvement and other risk features, CDK4/6 inhibitors (abemaciclib, ribociclib) and, if needed, chemotherapy come into consideration (treatment of hormone receptor–positive breast cancer, abemaciclib with 1–3 positive lymph nodes, ribociclib). Genomic tests can help with the chemotherapy decision (genomic tests).
• HER2-positive: HER2-targeted therapies such as trastuzumab and pertuzumab are given together with chemotherapy; trastuzumab emtansine is used for residual disease after surgery (treatment after surgery in HER2-positive breast cancer, Kadcyla).
• Triple-negative: As in classic triple-negative breast cancer, chemotherapy and, in suitable patients, immunotherapy; olaparib if a BRCA mutation is present (treatment sequence in triple-negative breast cancer, olaparib).

Response to treatment before surgery

Some studies suggest that tumors with a micropapillary component may respond less well to treatment given before surgery. In a study from China that included 244 patients with HER2-positive disease, the pathological complete response rate with preoperative treatment was 21.6% in tumors with a micropapillary component and 47.1% in those without. The researchers showed that these tumors contained fewer immune cells and that a different signaling pathway (beta-catenin) was active. However, the study was retrospective, the micropapillary group was small (38 patients), and the results need to be confirmed by other studies.

Can it occur in other organs?

Yes. A micropapillary pattern can also be seen in cancers of the bladder, lung, colon, salivary glands and ovary, and in these organs, too, it is usually linked to a tendency toward lymphatic vessel and lymph node involvement. For example, the micropapillary type of bladder cancer can significantly affect treatment decisions (bladder cancer). The information in this article applies specifically to breast cancer.

Questions you may want to ask your doctor

• Is my tumor pure micropapillary, or does it contain a micropapillary component? What is the proportion?
• What are my hormone receptor and HER2 status? If the HER2 result was borderline, was a gene-level test (FISH) done?
• How were my axillary lymph nodes assessed? Was an ultrasound, and a biopsy if needed, done?
• Do I need radiotherapy? How is my risk of local recurrence being assessed?
• Is treatment before surgery suitable for me? How will my response to treatment be monitored?

During and after treatment, regular exercise and a healthy lifestyle support adherence to treatment and quality of life (exercise during breast cancer treatment). For recent advances in breast cancer treatment, see our articles what has changed since 2020 and CDK4/6 inhibitors; for other special types, see our article on lobular breast cancer.

Frequently asked questions

What is invasive micropapillary breast cancer?

It is a rare type of breast cancer in which the tumor cells are arranged in small clusters within clear spaces and the secretory surface of the cells faces outward. It makes up about 1–2% of breast cancers. Compared with classic breast cancer, it has a greater tendency to spread into the lymphatic vessels and axillary lymph nodes.

Is micropapillary breast cancer more dangerous than classic breast cancer?

Lymph node involvement at diagnosis is more common, and the risk of locoregional recurrence is higher. However, in meta-analyses, once stage and lymph node status were taken into account, survival was not found to differ from classic breast cancer. The most important factors shaping the outcome are stage, lymph node involvement and the tumor's biological subtype.

How is micropapillary breast cancer treated?

There is no treatment specific to this type; as with classic breast cancer, treatment is planned according to hormone receptor status, HER2 status and stage. Surgery, radiotherapy if needed, hormone therapy, chemotherapy and HER2-targeted therapies are used. Careful assessment of the axillary lymph nodes and local control are especially important.

Does it matter if the micropapillary component is small?

Yes. Studies suggest that it is the presence of micropapillary features, more than their proportion, that is linked to lymph node involvement and recurrence risk. This is why even a small micropapillary component should be noted in the pathology report and taken into account in the treatment plan.

Glossary

Invasive carcinoma

Breast cancer that has grown through the wall of a milk duct or lobule into the surrounding tissue.

Invasive carcinoma of no special type (NST)

The most common type of breast cancer; formerly called invasive ductal carcinoma.

Micropapillary

Tumor cells forming small, papilla-like clusters without supporting tissue.

"Inside-out" pattern (reverse polarity)

The secretory surface of the cells, which normally faces inward, turning outward toward the surrounding tissue.

Lymphovascular invasion

Tumor cells seen inside the lymphatic vessels; linked to the risk of spread.

Locoregional recurrence

The disease coming back in the breast, the chest wall or the regional lymph nodes.

Pathological complete response

No living tumor cells remaining in the surgical specimen after treatment given before surgery.

FISH

A gene-level test that measures the number of copies of the HER2 gene.