What is Cancer?

Cancer is a broad term for a group of diseases characterized by the uncontrolled growth of cells within their native tissue to form a mass, from which, in certain instances, cells may disseminate to various other regions of the body. Among the primary causes of cancer are genetic alterations that disrupt the regulated programs of cell division and differentiation inherent in healthy tissues, thereby inducing uncontrolled cellular activity. As a result of these alterations, factors promoting the proliferation and expansion of cell groups increase, while concurrently, the mechanisms responsible for the elimination of pathological cells via natural processes and the immune system diminish. Consequently, cancer cells generally become insensitive to the external regulatory signals that govern tissue physiology.

Although certain genetic factors conducive to cancer may, in rare circumstances, be hereditarily transmitted to subsequent generations, other genetic modifications can manifest de novo as individual-specific somatic mutations within the affected cells.

Unlike benign tumours, malignant metastatic tumours—a term frequently used in the characterization of cancers—possess the capability to extensively invade their surrounding areas, spread to regional lymph nodes, and metastasize to distant tissues and organs within the body. They exhibit invasive properties, spreading into tissues that may be difficult to reach through surgical procedures or pharmacological interventions. The cells that constitute cancerous tissues exhibit highly heterogeneous characteristics.

Clinically, even when cancers manifest in similar tissues across different individuals or within the same individual, they are composed of cells with diverse structural features and distinct molecular abnormalities. This phenomenon is defined as cancer cell heterogeneity.

Şekil 1 – Fig. 1: The migration of a tumour cell to different organs via the circulatory system (metastasis). Tumour cells can advance through the connective tissue and enter blood vessels by means of the enzymes they secrete. Through this mechanism, for example, a cancer originating in the gastric tissue can spread to the pulmonary tissue.

1.1 What are the differences between cancer cells and normal cells?

Cancer cells differ from normal cells in several respects. For instance, cancerous cells can multiply and grow even in the absence of the molecular signals that trigger their growth, whereas normal cells cannot grow and proliferate unless they receive such signals.

Fig. 2: Characteristics of cancer cells

Cancerous cells ignore the molecular signals that initiate the cessation of division or the "cell death" process, also known as programmed cell death or apoptosis,

• Cancerous cells invade adjacent tissues and can spread to tissues in other parts of the body (unlike cancer, normal cells cease their growth when they come into contact with other cells in the tissue, and in adult individuals, most normal cells do not migrate within the body),
• Cancerous cells contain factors that stimulate the direction of blood vessels toward tumours in order to obtain nourishment. This process is also referred to as angiogenesis. These blood vessels sustain the life of the tumorous tissue by supplying oxygen and nutrients to the tumours and removing waste products from them.
• Cancerous cells evade the immune system. The immune system normally eliminates damaged or abnormal cells; however, in certain instances, it becomes desensitized to cancerous cells.
• Cancerous cells can deceive the immune system by mimicking normal cells to assist in their survival and growth. Certain cancer cells can redirect immune cells to protect the tumour rather than attack it.
• Cancerous cells accumulate multiple unusual molecular alterations resulting in the deletion or amplification of various regions within their chromosomes. Consequently, even the chromosome numbers of some cancer cells can exhibit significant variations compared to normal cells (aneuploidy).
• Cancerous cells rely on different types of nutrients than normal cells. Additionally, some cancer cells generate energy from nutrients in a manner distinct from most normal cells. This enables cancer cells to grow more rapidly.

Cancer Cell Angiogenesis

Şekil 3 - Fig. 3: Cancer cell angiogenesis. The increased nutritional demand of the growing tumour tissue is met by newly developing blood vessels.

Frequently, cancer cells rely so heavily on these abnormal behaviours that they cannot survive without them. Researchers capitalize on this dependency by developing therapies that target the abnormal characteristics of cancer cells. For instance, certain cancer treatments inhibit the growth of blood vessels toward tumours, essentially depriving the tumour of essential nutrient sources.

The fact that the millions of cells comprising cancerous tissue are distinct from one another, and thus possess a heterogeneous structure, constitutes one of the primary obstacles to cancer treatment being a straightforward and rapid process. Consequently, if not treated in a timely manner, malignant cancers can generally lead to severe health complications that may prove fatal to the individual, due to the alterations they induce in healthy tissues, the physical pressure exerted by their growth, and the subsequent tissue dysfunction.

1.2 How do cancerous tissues emerge and develop in a healthy individual?

Although the exact cause of cancer in individuals is not definitively known, there are two distinct groups of risk factors that trigger cancer. These risk factors are categorized as modifiable and non-modifiable factors. While non-modifiable factors include age, gender, family history, and similar elements, modifiable factors consist of environmental agents. These include:

• Smoking and alcohol consumption
• Poor dietary habits
• Food additives
• Certain chemical substances (tar, gasoline, dyes, asbestos, etc.)
• Exposure to radiation
• Exposure to excessive doses of Xrays
• Prolonged exposure to sunlight
• Air pollution

Being exposed to one or more of these risk factors does not guarantee the definite development of cancer in individuals; however, it increases the likelihood of developing cancer.