Cancer refers to a set of diseases that arise due to the uncontrolled multiplication of an abnormal cell. Normally, a cell divides in response to the chemical signals it receives from hormones and other messenger molecules of the body. In addition, the number of times a cell can divide is controlled through several intricate mechanisms that induce cell death after a certain number of cycles.
In certain cases, a normal body cell undergoes an alteration, mostly genetic mutation or DNA damage, such that it either becomes independent of these growth signals or grows resistant to the cell death. The balance between cell multiplication and cell death gets disrupted. As a result, the cell continues to divide, even in the absence of signals for cell division. This cancerous cell multiplies and gives rise to several more, and the cycle continues, leading to manifestation of symptoms and progression of the disease.
How does cancer grow?
A cancer cell arises due to disruption in the balance between cell division and cell death. The number of such cancerous cells gradually increases, giving rise to a mass of cells called a malignant tumour. This tumour is called primary tumour or primary cancer at stage 0.
The growth rate of this malignant tumour depends on the rate at which cells divide. This enlarging tumour then distorts the structural integrity of the respective tissue. In addition to this, the cells undergo several other mutations, as a result of which, they gain properties like resistance to drugs as well as synthesis and secretion of regulatory molecules. Some of the regulatory molecules secreted by cancer cells activate angiogenesis, the process of formation of new blood vessels from the existing ones.
From the diagnostic point of view, the cancer is now classified as stage I or II depending on the extent of tumour growth. This enables the tumour to gain access to vital nutrients required for its growth and cellular processes, as well as helps to get rid of the metabolic wastes. A small mass of cancerous cells thus transforms into huge clumps, and eventually some cells escape from this mass, spreading to nearby tissues and later on to distant organs.
How does cancer spread to other parts of the body?
The growing tumour invades the surrounding tissues as well as enters the surrounding blood vessels and lymphatic nodes. When the growing cancer has invaded and damaged the nearby tissue, it is classified as stage III cancer.
Once inside the blood vessels and lymph nodes, cancerous cells migrate to distant organs and form new tumours. This process is called metastasis. Angiogenesis, followed by metastasis, is the basic route for the spread of cancer from one body part to another.
The first step in metastasis is the entry of cancer cells into the blood vessel by a process called intravasation. Here, a cancerous cell attaches to the cells lining the blood vessels and pushes itself into the lumen of the blood vessel. It then circulates through the blood vessel along with other blood cells, and reaches the narrow capillaries present in the peripheral tissues.
Movement of the cancerous cell gets arrested in the capillaries, and it exits from the circulatory system by extravasation. It squeezes through two adjoining cells lining the capillary, and lodges itself in the tissue around the capillary. In this distant tissue, the cell may lie dormant or divide, and give rise to a new tumour called micrometastases. These tumours grow and initiate angiogenesis, and the cycle continues. These are termed as secondary tumours or secondary cancers. This stage where the cancer has spread to tissues away from the primary site is termed Stage IV.
Successful metastasis to a distant site not only depends on the precise features of the cancer cell but also on the surrounding non-cancerous cells, molecular environment around the tissue, activity of the immune cells present around the tumour and in the blood vessels.
What are the common sites?
Although the site to which cancer spreads depends on the primary tissue/organ of origin, bone, liver, and lungs are the most common ones. Given below is a list of the major cancers and the respective sites of metastasis.
- Bladder: Bone, liver, lung
- Breast: Bone, brain, liver, lung
- Colorectal: Liver, lung, peritoneum
- Kidney: Adrenal gland, bone, brain, liver, lung
- Lung: Adrenal gland, bone, brain, liver, other lung tissues
- Melanoma: Bone, brain, liver, lung, skin/muscle
- Ovary: Liver, lung, peritoneum
- Pancreas: Liver, lung, peritoneum
- Prostate: Adrenal gland, bone, liver, lung
- Stomach: Liver, lung, peritoneum
- Thyroid: Bone, liver, lung
- Uterus: Bone, liver, lung, peritoneum, vagina
Cancerous growths in the body comprise cancer cells as well as non-cancerous cells, and it is the interaction between these two entities that determines the rate of growth and spread of cancer
to other parts of the body.