Common biomarkers used in breast cancer diagnosis and monitoring

Breast cancer is one of the most common cancers diagnosed in women worldwide. It has various subtypes and molecular characteristics that differ from patient to patient, and no two treatment plans are the same. To aid in its diagnosis and monitoring, healthcare providers and patients rely on the analysis of specific biomarkers to guide and optimize decision-making. 

Common biomarkers used in breast cancer diagnosis and monitoring are hormone receptors such as estrogen and progesterone receptors, human epidermal growth factor receptor 2 (HER2), Ki-67, and circulating tumor markers such as CA15-3, CA 27.29, and Thymidine Kinase 1 (TK1). In this article, we will review each of them, explaining what they are and how they can be used to diagnose cancer and improve treatment planning.
Hormone receptors (HR) 

Hormone receptors (HR) include both estrogen receptors (ERs) and progesterone receptors (PRs). They are proteins found on the surface of breast cancer cells. HR-positive breast cancers contain receptors that bind to the hormone estrogen which promotes cancer growth.  

During diagnosis, HRstatus must be determined, as it can help clinicians decide on the best course of hormone therapy. This can be done through a tissue biopsy, where a tissue sample from the breast tumor is collected and tested via immunohistochemistry (IHC).
Human epidermal growth factor receptor 2 (HER2) 

HER2 is a protein that promotes cell growth. In some breast cancers, it is overexpressed, and HER2-positive cancers are associated with more aggressive tumor behavior. Regular testing of HER2 expression can lead to a more accurate and personalized treatment plan.
Ki-67 

Ki-67 is a protein that is associated with cell proliferation, and it is measured using IHC. Measuring Ki-67 expression can provide doctors with information on the growth rate of breast cancer cells, as well as determine tumor aggressiveness.

Circulating tumor markers

Blood tests can measure specific proteins or tumor markers and their activity levels in circulation, such as CA15-3 (Cancer Antigen 15-3), CA27.29 (Cancer Antigen 27.29), and Thymidine Kinase 1 (TK1). These proteins (or substances) are produced by cancer cells or as a byproduct of cancer cell growth, and they can provide valuable information on the presence and behavior of the disease. 

CA15-3 is overexpressed in some breast cancer cells, and in this context, it can be monitored over time to assess cancer treatment response and detect the potential for recurrence of the disease.

CA27.29 is another circulating tumor marker, and in the context of breast cancer, its levels may be elevated. Testing for CA27.29 levels can provide clinicians with insights into the effectiveness of cancer treatment – increasing levels may indicate ineffective therapies, while decreasing or stable levels may indicate a positive treatment response. 

Finally, Thymidine Kinase 1 is an enzyme that is involved in DNA synthesis and cell proliferation. In the context of breast cancer, TK1 levels and activity can be measured with a biomarker test to give clinicians insight into the status of cancer cell proliferation. Elevated TK1 levels have also been associated with more aggressive tumor characters, such as a bigger tumor, a tumor of higher grade, and lymph node involvement. 

Other biomarkers – and a word of caution 

In addition to the above widely used biomarkers, other ones such as BRCA1 and BRCA2 gene mutations (associated with hereditary breast cancer) may also be monitored. These markers help identify individuals who have a higher risk of developing cancer. 

A word of caution that accompanies biomarker testing is that some of these tumor markers may also indicate signs of other diseases or situations, such as the elevated presence of CA15-3 in the blood of a liver cancer patient. Therefore, more testing is needed to supplement biomarker testing results for accurate results. 

Biomarker testing should be used in conjunction with other tools 

As mentioned, despite biomarkers being a good indicator for the presence and levels of tumor activity and disease progression, they should not be the only indicator. Clinicians can and do use other tools to diagnose and monitor the cancer. These include clinical exams, imaging solutions, histopathological analyses, genetic testing, and more, to gain a comprehensive understanding of disease development. 

The importance of regular testing and scans 

It is crucial to undergo regular testing and scans for those undergoing breast cancer treatment. Biomarker testing, alongside other monitoring methods, allows healthcare providers to accurately evaluate the response to treatment over time, which can then access the effectiveness of the therapies and medicines administered.  

Undergoing regular testing can also help monitor the impact of treatments on the body. Certain treatments, such as chemotherapy, can produce negative side effects on various organs because of their strength. Regular biomarker testing can help identify the effectiveness of these treatments early on, and if they are not producing positive results, doctors can minimize or reduce treatment dosage or frequency, or even eliminate them completely. This can undoubtedly improve the patient’s quality of life.

Of course, it must not go unsaid that the frequency and timing of testing should be determined by one’s healthcare provider, based on individual patient characteristics, treatment accessibility and protocols, and clinical guidelines. It is essential that patients and carers keep lines of communication with clinicians clear and open to optimize treatment management.

This is a guest blog entry.