What is Breast Cancer?

The breast is an organ in both the male and female bodies. However, characteristics such as the composition of breast tissue and the hormone profile mean that only female breasts are capable of producing and storing milk to feed newborns and babies. 

Anatomy of the breast

The region of the breast extends from the underarm to the centre of the sternum (flat bone on the chest located between the two breasts), and up to the clavicle. The breast is made up of different tissues that make it possible to produce and secrete maternal milk.

• Mammary gland: mammary glands contain all the tissues responsible for producing and secreting milk during breastfeeding. Each breast contains a mammary gland made up of 15 to 25 compartments known as lobes and ducts. Each lobe is made up of lobules, which are in turn made up of alveoli.  
• Lobules: small structural units that produce milk when stimulated by prolactin, a hormone secreted during pregnancy.  
• Alveoli: maternal milk production unit. 
• Excretory ducts: channels that carry milk from the lobules to the nipple. 
• Nipple: orifice in the middle of the breast’s areola through which milk is expelled. 
• Areola: the brown or pinkish part around the nipple. It contains areolar glands that produce secretions to lubricate and disinfect the nipple and areola. The areola also has a muscle that erects the nipple. 
• Adipose tissue: fatty tissue that protects the breast. The amount varies from one woman to the next and contributes to breast volume and shape.  
• Muscle: the breasts sit on top of the chest muscle known as the large pectoral.  
• Connective tissue: structural tissue such as ligaments that support the breasts. 

The lymphatic system protects us from infection and diseases. It carries lymph, a fluid containing immune-system cells called lymphocytes that eliminate foreign bodies, bacteria and viruses. This system is made up of lymphatic vessels that collect and transport lymph away from the breast to small bean-shaped masses of lymphatic cells, called lymph nodes. The lymph nodes filter foreign bodies and cancer cells out of the lymph.

The breast contains numerous vessels and groups of lymph nodes:

• Lymph nodes around the clavicle (subclavicular and supraclavicular). 
• Underarm lymph nodes (auxiliary) 
• Parasternal lymph nodes (internal)  

Throughout our lives, our breasts are influenced by sex hormones, primarily estrogen and progesterone. As the concentration of these hormones varies over a woman’s lifetime (puberty, menstruation, pregnancy, breastfeeding, menopause), the function and composition of the breasts evolves with the rhythm of these fluctuations.

Breast cancer is the uncontrolled growth of abnormal cells in the breast tissue. This cellular proliferation causes the formation of a mass known as a cancerous tumour. A cancerous tumour is a group of cells that can invade and destroy the tissue in which they are located.

Breast cancer is a multi-factor disease. Learn more about the risk factors. The disease can take several months or even several years to develop. 

In general, genetic mutations are what make cancer cells defective. The body has a number of protection mechanisms to recognize, correct and even destroy abnormal mutated cells and prevent the development of cancer.

Genetic mutations become problematic when they occur in the genes essential for cellular proliferation or survival and the body does not correct them. Genetic mutations can increase the activity of a gene promoting cellular proliferation or inhibit a protective gene. These specialized genes are essential because they make it possible, among other things, to prevent the division of abnormal cells and to repair damage to cell DNA.  

When these genes are deregulated in abnormal cells, they are able to bypass or deactivate the body’s protection and proliferation control mechanisms. They are then considered uncontrolled, as they become able to proliferate independently, without signals sent by the body. By escaping cell division control, they multiply and are then the starting point of breast cancer. 

Breast cancer is an extremely heterogeneous disease. Learn more about the different types of breast cancer.  

Cancer cells exhibit different behaviours. They might stay in their tissue of origin. These are known as in situ cancers. But cancerous cells can also acquire the ability to spread. These are known as invasive cancers.  

Cells can then invade the surrounding tissues in the breast or spread to other distant organs. This spread of cancer cells to other organs contributes to the formation of new tumours in more distant places from the primary tumour. These new tumours are known as secondary tumours or metastases and are responsible for most breast cancer-related deaths.

Breast cancer affects us all

Breast cancer is the most common cancer in women in Canada and the second-leading cause of cancer death in Canadian women. Although breast cancer affects mainly women, approximately 1% of breast cancer cases are in men. 

Breast cancer incidence and mortality

In Quebec in 2021:

• 8,045 women were diagnosed with breast cancer, and 1,383 Quebec women died from the disease.
• Women aged 50 to 69 have the highest rate of breast cancer.
• 68 men were diagnosed with breast cancer, and 16 Quebec men died from the disease. Learn more about male breast cancer.

These statistics indicate that if these trends continue in Quebec:

• 1 in 8 women will develop breast cancer in her lifetime.
• 1 in 36 women will die of breast cancer.

Survival of breast cancer patients

Early detection ⁽¹⁾ improves the chances of survival, particularly by providing access to more effective treatments. 

Mortality rates due to breast cancer have been steadily decreasing since the mid-1980s. In fact, the age-standardized mortality rate ⁽²⁾ has fallen by 46% since 1986. The five-year net survival rate ⁽³⁾ is currently 89% for women and 76% for men. 

This reduction can be explained, among other things, by earlier diagnosis ⁽¹⁾, in particular through the Québec Breast Cancer Screening Program (PQDCS). Moreover, treatments for breast cancer are getting better and better because of research programs. 

Definitions

⁽¹⁾ Early detection: means that the breast cancer is detected in the first stages of the disease, i.e., when the tumour is still under a certain size and the cancer cells have invaded little or no nearby tissues. 
⁽²⁾ Age-standardized mortality rate: corresponds to the number of cancer-related deaths per 100,000 people, adjusted by distribution by age of the Canadian population in 1991, to take into account changes in the age distribution over time. 
⁽³⁾ Five-year net survival rate:net survival makes it possible to estimate the percentage of people who will survive their cancer. We often use the five-year net survival rate to analyze the survival of people with cancer. For example, in Canada, the five-year net survival rate for breast cancer in women is 89% (considering all stages). This means that approximately 89% of women diagnosed with breast cancer will live for at least five years after diagnosis (after adjustment for other causes of death).

References

• Canadian Cancer Statistics Advisory Committee in collaboration with the Canadian Cancer Society, Statistics Canada and the Public Health Agency of Canada. Canadian Cancer Statistics 2023. Toronto, ON: Canadian Cancer Society;2023. Available at: cancer.ca/Canadian-Cancer-Statistics-2023-EN (accessed august 2024).
• Canadian Cancer Statistics Dashboard. https://cancerstats.ca (accessed august 2024).
• Statistics from the Quebec Cancer Registry of the Ministère de la Santé et des Services sociaux (accessed august 2024).