Learn about the different ways that pathologists test for and diagnose breast cancer.

Diagnosing Breast Cancer

The treatment of breast cancer is a multidisciplinary team effort, and the role of the pathologist is to determine the correct diagnosis. The majority of breast cancers are first detected on abnormal screening imaging, such as mammography, magnetic resonance imaging (MRI), or ultrasound. The type of radiographic imaging used depends upon the patient's age and the characteristics of the breast tissue, such as how fibrous or fatty it is.

When a mass or a radiographic abnormality is detected, a tissue sample must be obtained to determine the correct diagnosis. The pathologist evaluates the tissue to determine if it is benign or malignant (cancerous). Not all masses that are detected by imaging are cancer; some breast masses are benign tumors such as fibroadenomas.

What are the steps taken to diagnose breast cancer?

Step 1: The Patient Undergoes a Breast Biopsy

Different methods may be used to get a sample of breast tissue to submit to the pathologists for evaluation. These methods include skin punch biopsy, fine needle aspiration (FNA), core needle biopsy, and excisional biopsy. The decision of which method to use is influenced by the characteristics of the mass as well as the patient's breast tissue.

Punch Biopsy   

A punch biopsy consists of using a circular blade attached to a handle; the size and shape of the circular blade resembles a hollow pencil eraser. The circular blade is used to take a full thickness sample of a lesion that involves the skin. This is used in the breast to evaluate abnormalities such as nodules or ulcerations present on the breast skin or nipple. Punch biopsies can be performed by dermatologists, surgeons, or other primary care physicians.

Fine Needle Aspiration   

An FNA consists of using a fine needle to pull out (aspirate) cells from the lesion.  A lesion which appears to be a simple benign cyst may be aspirated by FNA. Most FNAs are done in association with imaging, such as an ultrasound.

Core Needle Biopsy   

A core needle biopsy consists of using a small needle to take out a thin core of tissue from the abnormal area. This is often done under radiographic guidance by breast radiologists and can be done in association with a mammogram, an MRI or an ultrasound. A core needle biopsy is the most common type of specimen obtained for initial evaluation of a breast mass or radiographic abnormality.

Excisional Biopsy   

Excisional biopsies consist of removal of a larger amount of tissue than is removed with a core needle biopsy. For instance, the tissue may measure 2 x 2 cm. These are performed by surgeons in an operating room. If a mass is very deep in the breast, such as adjacent to the chest wall, it may be too deep to be reached by a core needle biopsy. In these cases, excisional biopsy may be the only way to sample the tissue.

Step 2: A Pathologist Makes a Diagnosis of Cancer

The tissue from the biopsy is processed in the pathology lab, and the tissue is cut into thin sections and transferred to glass slides. Special dyes are applied to the glass slides that stain the tissues pink and blue so that the cells are visible under the microscope. Pathologists then look at the slides under a microscope to make a diagnosis of whether the tissue sample is benign or malignant (cancer). 

Under the Microscope: Benign vs Malignant Breast Masses

  • Benign Fibroadenoma

    Benign fibroadenoma

    The most common breast mass in young women is a fibroadenoma, which is a benign neoplasm of both the stromal and epithelial components of the breast. Here, it is a well-circumscribed (non-infiltrative) lesion that lacks atypia.

  • Benign Fibrocystic Changes

    Benign fibrocystic changes

    Breast masses are often due to fibrocystic changes, consisting of cystically dilated ducts, some with benign usual hyperplasia and apocrine metaplasia of the cells. Fibrocystic changes can fluctuate with hormonal changes such as during the menstrual cycle.

  • Invasive Ductal Carcinoma

    Malignant ductal carcinoma

    Invasive ductal carcinomas often have irregular, infiltrative borders that are described as “spiculated” (“star-like”) on radiographic images. The cells show atypia and have the potential to spread or metastasize to the lymph nodes and distant body organs.

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Step 3: Biomarker Testing is Performed on Cancers

One important aspect of the role of pathologist’s in the evaluation of breast cancer is biomarker testing, specifically the accurate assessment of:

  • the estrogen receptor (ER)
  • the progesterone receptor (PR)
  • the HER-2 status of a patient's breast cancer

Biomarkers can be prognostic, predictive, or both. Prognostic biomarkers are independent measures of prognosis such that the presence or absence of the biomarker is associated with a patient's overall clinical outcome (i.e., risk of recurrence and mortality). Predictive biomarkers, in contrast, predict whether or not a patient will respond to a given therapy.

ER and PR   

Estrogen and progesterone are endocrine hormones that drive the growth of most breast cancers. Estrogen and progesterone stimulate cancer cells to grow by binding their specific receptors, the estrogen receptor (ER) and the progesterone receptor (PR). Approximately 70% of breast cancers express ER and PR, which means that estrogen and progesterone can stimulate these cancers to grow. ER and PR are weak prognostic biomarkers and strong predictive biomarkers. A tumor that expresses ER and PR is called "ER and PR positive,"  and these patients have a more favorable prognosis than if the tumor "ER and PR negative."  In addition, the ER and PR expression predicts that the patient will likely benefit from endocrine therapy that targets the hormone receptor, such as tamoxifen and others.


HER-2/neu is the name of a gene that encodes the HER-2 protein, which acts as a “tyrosine kinase receptor” on the cell surface of the cancer cell. When a cancer has an abnormally increased number of HER-2 genes and an abnormally high amount of HER-2 protein on its cell surface, the cancer cell can divide and grow more. This feature makes the HER-2 gene called an “oncogene,” because when active or increased, it promotes the “oncogenesis” or cancer growth of the cell. HER-2 overexpression is seen in approximately 15-20% of invasive breast cancers, and it is an example of both a strong prognostic and predictive biomarker. HER-2 expression is associated with a diminished prognosis (e.g., higher risk of recurrence); however, it also predicts that a patient will more likely benefit from directed therapies that target HER-2 (such as trastuzamab and others).


Pathologists may also perform additional tests on breast cancer tissue to give more information about the cancer’s potential behavior. One such marker is Ki-67, which is a marker of cell proliferation. Studies have shown that Ki67 is a prognostic biomarker, such that higher Ki-67 (reflecting higher cancer cell proliferation) is associated with a diminished prognosis.

The Pathologist's Assessment of Biomarkers

Many factors influence the accuracy of detection of ER, PR, and HER-2 in the laboratory. These include pre-analytical factors (including prompt fixation of appropriate duration), analytic factors (including utilization of validated procedures with ongoing proficiency testing), and post-analytic factors (including appropriate reporting). Importantly, the results of these tests alone are the key determinants of what treatment is selected. The goal is to provide the right treatment to the right patient. The role of the pathologist is to accurately assess these biomarkers, which as indicated above is quite challenging and involved. The role of the oncologist is to treat the patient with one of several standardized therapies, such as endocrine therapy for ER/PR positive carcinomas, or trastuzumab for HER-2 positive carcinomas.

Under the Microscope: Biomarker Assessment in Breast Cancers

An ER positive carcinoma (immunostain)   

ER Positive Carcinoma

Testing for expression of the estrogen receptor (ER) on in situ (left) and invasive (right) ductal carcinoma is done by immunohistochemistry, using an immunostain for ER in the pathology laboratory. The immunostain binds to the ER protein in the cancer nucleus and is detected by a positive brown color. This in situ and invasive ductal carcinoma is diffusely positive for ER, with 100%, strong labeling of the carcinoma nuclei. Testing for the expression of progesterone receptor (PR) is done in the same way.

A HER-2 positive carcinoma (immunostain)   

Human Epidermal Growth Factor Receptor 2

Testing for the overexpression of the human epidermal growth factor 2 (HER-2) protein can be done by several methods. In this example, immunohistochemistry, using an immunostain for HER-2, is done in the pathology laboratory. Immunohistochemistry detected the HER-2 protein. The immunostain binds to the HER-2 protein on the surface of the cancer cell and is detected by a positive brown color. The HER-2 labeling is scored on a scale from 0-3, as described below. This invasive ductal carcinoma (right) shows 3+ labeling, which is positive for HER-2 protein overexpression. The normal breast ducts and lobules (left) are negative for the HER-2 protein overexpression.

A HER-2 positive carcinoma (FISH)   

Fish HER2

Testing for the overexpression of the human epidermal growth factor 2 (HER-2) protein can be done by several methods. In this example, fluorescence in situ hybridization (FISH) is performed in the pathology laboratory. FISH detects the HER-2 gene. In this example, a red color probe attaches to the HER-2 gene, and a green color probe attaches to a separate part of the same chromosome. In this case, there is much more red signal than the green signal; this shows that there are more copies (“amplification”) of the HER-2 gene, and the cancer is considered HER-2 positive or HER-2 amplified.

A Summary of the Key Points of Biomarker Testing

  • All primary invasive breast carcinomas should be tested for ER, PR, and HER-2. Since the ER, PR, and HER-2 status can change in a small percentage of breast recurrences or metastases, these markers should also be re-tested in subsequent recurrences and metastases.
  • Endocrine therapy is highly effective for ER and PR positive carcinomas. One would never want to deny a patient the potential benefits of endocrine therapy. As little as 1% labeling of invasive carcinoma nuclei predicts some clinical benefit from endocrine therapy (Harvey et al. JCO 1999;17: 1474-1481), so pathologists carefully examine these immunostains so as not to miss potentially significant focal labeling.
  • PR labeling adds predictive information to ER labeling. PR is downstream of ER in the signalling cascade, so assessing the expression of both ER and PR can provide useful information about a cancer’s potential responsiveness to endocrine therapy.
  • HER-2 status can be assessed by immunohistochemistry or in situ hybridization. In many pathology labs, including ours, HER-2 status is initially assessed by immunohistochemistry, which measures protein overexpression. The immunostain results are scored on a scale from 0-3; a score of 0 or 1+ is negative, a score of 2+ is equivocal, and a score of 3+ is positive. Cases which are equivocal for protein overexpression are assessed by in situ hybridization, either by fluorescence in situ hybridization (FISH) or chromogenic in situ hybridization (CISH), both of which look for HER-2 gene amplification. In some pathology labs, the FISH or CISH is performed first, with immunohistochemistry done subsequently to resolve any FISH or CISH equivocal cases.
  • Additional molecular testing may be performed on certain tumors. The decision to order these tests is made by an oncologist or surgeon.