Breast cancer is not a singular disease but a collection of different diseases, each with unique biological characteristics. These distinctions are based on what makes the cancer cells grow, such as specific hormones or proteins. Researchers classify these variations into molecular subtypes. Understanding these subtypes is fundamental for effective breast cancer management.
Significance of Molecular Subtypes
Identifying molecular subtypes has transformed breast cancer diagnosis and treatment, moving beyond older classifications that primarily focused on factors like tumor size or whether lymph nodes were involved. It helps predict how the disease might behave and guides the prognosis. This approach underpins the concept of personalized medicine in oncology, where treatments are specifically tailored to the individual patient’s cancer. This shift from a one-size-fits-all approach to highly specific interventions has improved outcomes for many patients.
How Subtypes Are Identified
Classifying breast cancer into its molecular subtypes relies on specific laboratory tests that analyze the biological makeup of cancer cells. Immunohistochemistry (IHC) is a common technique used to detect the presence and amount of certain proteins on the surface of cancer cells. For breast cancer, IHC typically assesses the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) proteins.
Another technique, Fluorescence In Situ Hybridization (FISH), is often employed to confirm HER2 status. While IHC can indicate HER2 protein overexpression, FISH directly detects amplification of the HER2 gene itself, providing a more definitive assessment when IHC results are ambiguous. Beyond these protein-based methods, gene expression profiling offers a more detailed look at the genes active within the tumor. This advanced testing can further refine subtype classification, helping to distinguish between subtypes like Luminal A and Luminal B, which share some protein expressions but differ in their underlying genetic activity.
Major Molecular Subtypes
Breast cancer is broadly categorized into four main molecular subtypes based on the presence or absence of specific receptors: Luminal A, Luminal B, HER2-enriched, and Triple-Negative.
Luminal A
Luminal A breast cancer is the most common subtype, characterized by being estrogen receptor-positive (ER+), progesterone receptor-positive (PR+), and HER2-negative (HER2-). These tumors typically have a low proliferation rate, often leading to a more favorable prognosis.
Luminal B
Luminal B breast cancer is also ER-positive and often PR-positive, but it can be either HER2-negative or HER2-positive. A distinguishing feature of Luminal B tumors is their higher proliferation rate compared to Luminal A.
HER2-enriched
HER2-enriched breast cancer is defined by the overexpression or amplification of the HER2 protein. These tumors are typically estrogen receptor-negative and progesterone receptor-negative. This subtype is known for its faster growth compared to luminal cancers.
Triple-Negative Breast Cancer (TNBC)
Triple-Negative Breast Cancer (TNBC) lacks expression of estrogen receptors, progesterone receptors, and HER2. This means the cancer cells do not have these common receptors that can be targeted by specific therapies. TNBC tends to be more aggressive.
Treatment Approaches Based on Subtype
The specific molecular subtype of breast cancer directly informs the chosen treatment strategy.
Luminal A and Luminal B Treatment
For Luminal A and Luminal B breast cancers, which are characterized by the presence of estrogen and/or progesterone receptors, hormone therapy is a primary treatment modality. Medications like tamoxifen or aromatase inhibitors are used to block the effects of hormones or reduce their levels, thereby slowing cancer growth. Chemotherapy may also be considered, particularly for Luminal B tumors due to their higher proliferation rates.
HER2-enriched Treatment
For HER2-enriched breast cancer, treatments specifically target the HER2 protein. These include HER2-targeted therapies such as trastuzumab or pertuzumab, which are often administered in combination with chemotherapy. These targeted drugs work by attaching to the HER2 protein on cancer cells, inhibiting their growth and division.
Triple-Negative Breast Cancer (TNBC) Treatment
Triple-Negative Breast Cancer (TNBC) presents a different challenge because it lacks the common receptors for hormone therapy or HER2-targeted treatments. Therefore, chemotherapy is the cornerstone of treatment for TNBC. Researchers are also exploring newer options like immunotherapy, which aims to boost the body’s own immune response against the cancer cells.