Tumor markers in breast cancer: a complete guide for patients (CA 15-3, HER2, ER/PR receptors)

Have you been diagnosed with breast cancer and your test report includes terms like “CA 15-3”, “HER2 positive”, “ER+”, “PR+” or “Ki-67”? These acronyms may seem intimidating, but behind them lies the exact “key” that allows your doctor to choose the right treatment for you. The most important news: breast cancer markers are not used for simple diagnosis, but especially for personalizing therapy and monitoring treatment response. This guide explains, in a way that everyone can understand, what each marker means and how it influences medical decisions.

1. Two categories of markers: blood and tumor

It is essential to understand from the outset that in breast cancer there are two completely different types of markers , with distinct roles:

• Blood (serum) markers: CA 15-3 and CEA. These are measured through a blood test and are mainly used to monitor the progression of advanced disease.
• Tissue (tumor) markers: hormone receptors (ER, PR), HER2 protein, and Ki-67 index. These are determined on the biopsy fragment by immunohistochemical analysis (IHC) and dictate what kind of treatment you will receive.

In other words, the simple biopsy tells you if it is cancer, and the IHC profile tells you how it should be treated.

2. CA 15-3: the “barometer” in the blood

CA 15-3 is a protein (mucin-derived antigen) produced by breast and cancerous cells. The normal value is below 30 U/mL .

Here is how the values are interpreted:

CA value 15-3	Possible meaning
<30 U/mL	Normal
30–100 U/mL	Benign breast conditions or incipient disease
>100 U/mL	Advanced breast cancer, large tumor volume

A fundamental point to remember: CA 15-3 is NOT used for screening or early diagnosis in healthy women. It is especially useful in metastatic disease , where it is elevated in 70–80% of patients.

Why is it so valuable? Because it works as a “barometer” of treatment:

• A decrease of more than 25% in CA 15-3 during chemotherapy indicates a good response to treatment and a favorable prognosis.
• An increase of more than 25% from baseline signals disease progression or relapse, often 2–6 months before imaging .

Be careful, however: approximately 20–30% of patients with metastatic disease have normal CA 15-3, especially when the tumor volume is small. Also, slightly elevated values can occur during pregnancy, breastfeeding, liver disease, or benign breast disease — that’s why a single isolated value doesn’t say anything, the trend over time matters .

3. Hormone receptors (ER and PR): the key to hormone therapy

This is where we get into the area that really changes treatment. Estrogen (ER) and Progesterone (PR) receptors are “antennae” on the surface of tumor cells. If your tumor is ER-positive or PR-positive , it means that the natural hormones in your body are “feeding” the cancer.

The good news? These tumors respond well to hormone therapy — pills like tamoxifen or aromatase inhibitors, which block the action of these hormones.

Some essential data:

• ER-positive breast cancer accounts for 60–70% of all invasive cases.
• ER+/PR+ (double positive) tumors have a better prognosis than ER+/PR- ones.
• ER-negative tumors do not benefit from hormonal therapy — it would be useless.

An important, less well-known detail: receptor status can change between the primary tumor and metastases. In 10–20% of patients, the ER or PR status changes at relapse. Therefore, the doctor may recommend a new biopsy of the metastatic lesion if the result would change the treatment.

4. HER2: critical marker for targeted therapy

HER2 protein (Human Epidermal Growth Factor Receptor 2) is a growth “accelerator” present on the surface of cells. In 15–20% of breast cancers, this protein is overexpressed, making the tumor more aggressive.

How is HER2 measured? By immunohistochemical (IHC) test, with a score from 0 to 3+:

• Score 0 or 1+: HER2-negative
• Score 2+: ambiguous result — requires confirmation by FISH test
• Score 3+ (or FISH positive): HER2-positive

The groundbreaking news: Although HER2-positive tumors were historically considered more aggressive, today they can be fought extremely effectively with modern targeted therapies — monoclonal antibodies such as trastuzumab (Herceptin). Adding trastuzumab to chemotherapy significantly improves both disease-free survival and overall survival. Basically, a diagnosis that was once frightening, today opens the door to a very effective treatment.

“HER2-low” emerged — tumors with an IHC score of 1+ or 2+/FISH negative. These patients can now benefit from next-generation targeted therapies (trastuzumab deruxtecan), an option that did not exist a few years ago.

5. Ki-67: how fast do cells divide?

Ki-67 marker is a “proliferation index” expressed as a percentage, which shows exactly how many cells in the tumor are actively dividing at that moment:

• Low Ki-67 (below 15–20%): slow-growing, less aggressive tumor.
• High Ki-67 (above 30–40%): rapidly dividing, more aggressive tumor, but often responds better to chemotherapy .

Ki-67 helps the doctor decide whether a patient needs chemotherapy or whether hormone therapy alone is sufficient.

6. “Triple-negative” and PD-L1: when immunotherapy intervenes

When a tumor is ER-negative, PR-negative AND HER2-negative , it is called triple-negative breast cancer (TNBC) — a form that does not respond to either hormonal therapy or anti-HER2 therapies. This has historically been the most difficult to treat.

This is where a new marker comes in: PD-L1 . It assesses whether the tumor is “hiding” from the immune system. In triple-negative cancer, PD-L1 is positive in 40–60% of cases. A high value (CPS ≥10) indicates that the patient is an ideal candidate for next-generation immunotherapy (atezolizumab or pembrolizumab combined with chemotherapy).

7. Genomic testing and BRCA mutations

In addition to classic markers, modern medicine also uses genomic tests that analyze the activity of multiple genes simultaneously, to decide whether chemotherapy is necessary:

• Oncotype DX (recurrence score based on 21 genes): a low score (0–10) indicates low risk, and a high score (≥26) indicates high risk of recurrence.
• MammaPrint (70-gene signature) and PAM50/Prosigna (50 genes) provide similar risk classifications.

Last but not least, BRCA1/BRCA2 mutation testing is recommended for all patients diagnosed before the age of 50, with triple-negative tumors or with a family history. The presence of a BRCA mutation opens access to a special class of drugs called PARP inhibitors .

Marker	📊 Value / Normal score	🧪 What does it evaluate?	🔎 Therapeutic implication
CA 15-3	<30 U/mL	Blood barometer (advanced disease)	30–100 = benign conditions; >100 = advanced cancer; useful in monitoring recurrence
ER/PR receptors	Negative / Positive (IHC)	If hormones “feed” the tumor	ER+/PR+ → respond to hormonal therapy (tamoxifen, aromatase inhibitors)
HER2	Score 0–3+ (IHC); FISH if 2+	Growth “accelerator”	3+ (positive) → targeted therapy (trastuzumab/Herceptin)
Ki-67	Low <15–20%	Cell division rate	>30–40% = aggressive tumor, but responds better to chemotherapy
PD-L1	Negative / Positive (CPS)	If the tumor “hides” from immunity	Positive → candidate for immunotherapy (especially in triple-negative)

💡 The 4 subtypes of breast cancer resulting from these markers: Luminal A (ER+/HER2-/low Ki-67), Luminal B (ER+/high Ki-67), HER2-positive and Triple-negative (ER-/PR-/HER2-)

How Artificial Intelligence helps you correctly interpret breast markers

As you have seen, a single breast cancer diagnosis hides a complex combination of variables: CA 15-3, ER, PR, HER2, Ki-67, PD-L1, genomic tests and BRCA status. Each marker has its own thresholds and therapeutic implications, and international protocols are updated from one month to the next.

Oncoexpertai platform comes in . Unlike a simple reading of the analyses, our advanced Artificial Intelligence algorithms instantly scan your entire immunohistochemical profile (hormone receptors, HER2, Ki-67) and correlate it with thousands of clinical studies and international oncology guidelines updated in real time (NCCN / ESMO). The technology does not replace the doctor, but gives him the certainty that the proposed regimen — be it hormonal therapy, anti-HER2 targeted therapy or immunotherapy — is the most effective for your specific profile.

If you just want a completely free opinion without uploading any medical documents, use one of the 2 orange boxes “Talk to Dr. Onisim” or green “Message to Dr. Onisim”

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Disclaimer: The information in this article is for informational and educational purposes only. It is not a substitute for the consultation, diagnosis, or treatment provided by a qualified medical professional. For any decision regarding your health, always consult a medical professional.

Bibliography

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4. Modi S, Jacot W, Yamashita T, et al. Trastuzumab Deruxtecan in Previously Treated HER2-Low Advanced Breast Cancer. New England Journal of Medicine. 2022;387(1):9–20. doi: 10.1056/NEJMoa2203690
5. Cortes J, Rugo HS, Cescon DW, et al. Pembrolizumab plus Chemotherapy in Advanced Triple-Negative Breast Cancer. New England Journal of Medicine. 2022;387(3):217–226. doi: 10.1056/NEJMoa2202809
6. Tutt ANJ, Garber JE, Kaufman B, et al. Adjuvant Olaparib for Patients with BRCA1- or BRCA2-Mutated Breast Cancer. New England Journal of Medicine. 2021;384(25):2394–2405. doi: 10.1056/NEJMoa2105215
7. Allison KH, Hammond MEH, Dowsett M, et al. Estrogen and Progesterone Receptor Testing in Breast Cancer: ASCO/CAP Guideline Update. Journal of Clinical Oncology. 2020;38(12):1346–1366. doi: 10.1200/JCO.19.02309

Dr. Onisim Florin Senior Medical Oncologist | Founder of OncoExpertAI

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