Tumor staging is a way to classify the size and spread of a cancer. Staging primarily focuses on the size of the tumor and if it has spread to the nearby lymph nodes. Stages range from 0 to IV, with 0 being the least severe and IV being the most severe.[1] Stages are also often further subdivided by adding a letter after the number, such as Stage IIB.[2] To determine a cancer’s stage, the TNM system is most commonly used. The T refers to the size of the tumor, the N refers to the number of lymph nodes involved, and the M refers to metastasis or spread to distant sites.1 Lymph node involvement is an important measure of cancer spread because cancer cells that have broken off from the primary tumor either travel through the bloodstream or the lymph system. For the cancer cells that travel through the lymph system, they often end up in the lymph nodes. If cancer is found in the lymph nodes, it is more likely to continue spreading to other locations.[3] The general classifications for each tumor stage can be found at the end of this article.

The general staging classification does not take into account factors unique to breast cancer such as HER2 (human epidermal growth factor receptor 2) status and hormone status. HER2, a protein encoded by the HER2 gene, promotes growth; therefore, breast cancers with an excess of this protein tend to grow and spread quickly. Estrogen and progesterone are hormones that can increase cancer growth if the tumor has receptors for them. When the breast tumor has hormone receptors, the hormones in the body can bind to these receptors and promote cancer cell growth.4 Tumor grade is another factor that can be considered beyond the general staging classification. Tumor grade refers to how abnormal the cancer cells look with grade 1 (low grade) meaning the cancer cells look similar to normal cells and grade 3 (high grade) meaning the cancer cells look very different. High grade tumors tend to grow and spread more quickly because they function very differently from normal cells.[5]

Considering these additional factors, there are exceptions to the general staging classifications provided. For the cases described for Stage IB cancers, it is typically classified as Stage IA regardless of lymph node involvement if the cancer is hormone receptor-positive. For the case described for Stage IIA where the tumor is 2-5 cm and has not spread to the axillary lymph nodes, it is typically classified as Stage I if it is HER2-negative. If the cancer is also estrogen receptor-positive, progesterone receptor-negative, and has a low risk of recurrence according to gene expression testing, it will usually be classified as Stage IA. For the case described for Stage IIB where the tumor is 2-5 cm and has spread to 1 to 3 of the axillary lymph nodes, it is typically classified as Stage I if it is HER2-positive and estrogen and progesterone receptor-positive. For the case described for Stage IIIA where the tumor is larger than 5 cm and has spread to 4 to 9 of the axillary lymph nodes, it is typically classified as Stage IB if it is HER2-positive, estrogen and progesterone receptor-positive, and grade 2. For the case described for Stage IIIB where the tumor is larger than 5 cm and has spread to 4 to 9 of the axillary lymph nodes, it is typically classified as Stage IIA if it is HER2-positive, estrogen and progesterone receptor-positive, and grade 3. For the case described for Stage IIIC cancers, it is typically classified as Stage IIIA if the cancer is also estrogen and progesterone receptor-positive and grade 2.[2]

References

1.     Cancer Staging https://www.cancer.gov/about-cancer/diagnosis-staging/staging (accessed Apr 27, 2021).

2.     Breast Cancer Stages: 0 Through IV https://www.breastcancer.org/symptoms/diagnosis/staging (accessed Apr 27, 2021).  

3.     Lymph Nodes and Cancer https://www.cancer.org/cancer/cancer-basics/lymph-nodes-and-cancer.html (accessed May 4, 2021).

4.     Types of Breast Cancer: Triple Negative, ER-Positive, HER2-Positive https://www.webmd.com/breast-cancer/breast-cancer-types-er-positive-her2-positive (accessed Apr 30, 2021).  

5.     Tumor Grade Fact Sheet https://www.cancer.gov/about-cancer/diagnosis-staging/prognosis/tumor-grade-fact-sheet (accessed Apr 30, 2021).