Understanding HER2-Positive Breast Cancer: Key Signs and Diagnosis

A HER2-positive result can feel like a technical label, but it carries practical meaning: it helps explain tumor biology and guides which therapies are most likely to work. Because symptoms can be subtle or overlap with non-cancer conditions, clinicians rely on imaging and tissue testing to confirm what is happening and to measure HER2 accurately.

This article is for informational purposes only and should not be considered medical advice. Please consult a qualified healthcare professional for personalized guidance and treatment.

What Exactly is HER2-Positive Breast Cancer?

HER2 (human epidermal growth factor receptor 2) is a protein involved in signaling pathways that regulate cell growth. In HER2-positive breast cancer, tumor cells typically show HER2 overexpression (too much HER2 protein on the cell surface) and/or HER2 gene amplification (extra copies of the HER2 gene). This matters because HER2 signaling can drive cancer cell proliferation.

HER2 status is not the same as cancer stage. Stage describes how extensive a cancer is (tumor size, lymph node involvement, and spread), while HER2 status describes a biologic feature of the tumor. HER2 is also evaluated alongside estrogen receptor (ER) and progesterone receptor (PR) status, because these markers together shape prognosis discussions and influence treatment planning.

Early Clues: The Common Signs of Breast Cancer

Many breast cancers are found on screening mammography before noticeable symptoms appear. When signs do occur, they can include a new lump or thickening in the breast or underarm, changes in breast size or shape, or skin changes such as dimpling. Nipple changes can also be relevant, including new inversion, persistent pain in one spot, or discharge that is bloody or occurs without squeezing.

It is also possible to notice redness, swelling, or warmth. These symptoms can have non-cancer causes (for example, infection or benign cysts), which is why persistence and progression matter. A useful rule of thumb is that any new breast change that does not resolve or that clearly worsens should be evaluated clinically, even if discomfort is mild.

How Doctors Diagnose and Confirm HER2 Status

Diagnosis usually starts with clinical evaluation and imaging. In the U.S., this commonly includes diagnostic mammography and targeted breast ultrasound; breast MRI may be added in certain situations, such as dense breasts, evaluating disease extent, or clarifying unclear findings. Imaging helps determine where to sample tissue and whether lymph nodes look suspicious.

A biopsy is the step that confirms cancer and provides material for receptor testing. Most often, a core needle biopsy is performed, guided by ultrasound, mammography, or MRI depending on lesion visibility. The pathology report typically includes tumor type and grade, along with ER/PR and HER2 results.

HER2 is most often assessed by immunohistochemistry (IHC), which scores HER2 protein expression (commonly reported as 0, 1+, 2+, or 3+). If IHC is 2+ (equivocal), additional testing is usually done with in situ hybridization (ISH, often FISH) to check for HER2 gene amplification. Because treatment decisions can hinge on these results, laboratories follow standardized guidelines for scoring and quality control, and clinicians may repeat testing if results conflict with the clinical picture or if there is concern about sample adequacy.

What a HER2-Positive Diagnosis Means for Treatment

A HER2-positive diagnosis can expand treatment options because targeted anti-HER2 therapies are designed to block HER2-driven signaling. Treatment planning typically considers multiple factors at once: cancer stage, tumor grade, lymph node involvement, ER/PR status, overall health, and patient preferences. For earlier-stage disease, the plan may include surgery (lumpectomy or mastectomy) and often radiation depending on surgical approach and pathology findings.

Systemic therapy is frequently recommended, and it may be given before surgery (neoadjuvant) or after surgery (adjuvant). Chemotherapy is commonly combined with HER2-targeted therapy in many standard approaches. Examples of HER2-targeted medications used in different settings include trastuzumab and pertuzumab, as well as antibody-drug conjugates such as ado-trastuzumab emtansine (T-DM1) or fam-trastuzumab deruxtecan in specific clinical contexts. Certain tyrosine kinase inhibitors (such as tucatinib, neratinib, or lapatinib) may be considered in select situations. Because some HER2 therapies can affect heart function, clinicians often monitor cardiac function during treatment. If a cancer is also hormone receptor-positive, endocrine therapy may be part of the overall plan as well.

In practice, HER2 status helps your care team discuss why specific drugs are being recommended, what the goals of each treatment phase are, and how response is assessed over time through pathology results, imaging, and follow-up care.

Knowing whether a tumor is HER2-positive is a central part of modern breast cancer diagnosis because it clarifies tumor biology and informs therapy selection. While the signs that lead to evaluation can be nonspecific, imaging and biopsy-based testing provide the confirmation needed to plan care. Interpreting HER2 results together with stage and hormone receptor status can make the overall diagnosis feel more understandable and structured.