Breast cancer is the second most common type of cancer in Canadian women, surpassed only by non-melanoma skin cancer.
Breast cancer accounts for approximately 25 percent of new cases of cancer and 13 percent of all cancer deaths in Canadian women. It is expected that one in every eight women in Canada will develop breast cancer in her lifetime, and one in 33 will die of it. On average, 78 Canadian women are diagnosed with breast cancer each day, and 15 die from it. Ductal carcinoma is the most common type of breast cancer, accounting for about 90 percent of all in situ breast carcinomas and 70 percent of all invasive breast carcinomas.
Part of the challenge breast cancer presents is that there are so many different types, and they are treated differently. Some breast cancer cells have characteristics that make them easier to treat. For example, newly diagnosed breast cancer cells are tested for the presence of hormone receptors, the proteins that fuel cancer growth. Breast cancers with cells that have hormone receptors are classified as hormone-receptor-positive (HR+).
Approximately two-thirds of breast cancers have at least one type of hormone receptor. Breast tumours may be positive for estrogen receptors (ER+), for progesterone receptors (PR+), or for both estrogen and progesterone receptors(ER/PR+). About 80 percent of all HR+ breast cancers are ER+ or ER/PR+/.
Two other types of breast cancer are HER2-positive and HER2-negative. HER2 stands for human epidermal growth factor receptor 2, a protein that normally helps control the growth and repair of healthy breast tissue. Sometimes, though, the HER2 gene becomes faulty and makes too many copies of itself, overproducing HER2 proteins on the surface of breast cells. This can result in HER2-positive breast cancer.
In about 20 percent of breast cancers, the cancer cells have extra copies of the gene that makes HER2 protein. HER2-positive breast cancers tend to be more aggressive than other types of breast cancer; however, treatments that target HER2 specifically are very effective.
HER2-negative breast cancer is generally divided into two types, HR+ breast cancer and triple-negative breast cancer, based on the presence or absence of hormone receptors on the surface of the cancer cells.
HER2-Low Breast Cancer
About half of patients with metastatic breast cancer have tumours with low levels of the HER2 protein. Until now, these patients have had limited treatment options. But the results of recent clinical trials could open the door to more effective treatment for these patients.
Treatment with an antibody conjugate (a combination of a monoclonal antibody with a chemotherapy agent) that is delivered via infusion into a vein shows promising results. The monoclonal antibody acts as a homing device that helps deliver the chemotherapy drug directly to the tumour, where it is released and kills the cells.
Patients in one recent trial employing this technique who were treated with trastuzumab deruxtecan (Enhertu®) lived nearly twice as long without their cancer growing and lived six months longer overall as those who received standard chemotherapy. The trial findings are expected to change how metastatic HER2-low breast cancer is treated.
As research expands beyond traditional research and adds incremental steps to standard therapy, physicians will be able to help more breast cancer patients. That provides a ray of hope for the nearly 28,600 women who are predicted to be diagnosed with breast cancer this year, and all who will follow in the coming years.
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