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A neoplasm or tumor is a group of cells that have undergone unregulated growth and will often form a mass or lump, but may be distributed diffusely. According to Global Cancer Observatory or GLOBOCAN, 18.1 million new cancer cases was reported on 2018.

According to World Health Organization, cancer is the second leading cause of death globally, accounting for an estimated 9.6 million deaths, or one in six deaths, in 2018. Approximately 70% of deaths from cancer occur in low- and middle-income countries.

Lung, prostate, colorectal, stomach, and liver cancer are the most common types of cancer in men. While breast, colorectal, lung, cervical and thyroid cancer are the most common among women.

All tumor cells show the ten hallmarks of cancer. These characteristics are required to produce a malignant tumor.
They include:
Genome instability and mutation
Resisting cell death,
Deregulating cellular energetics
Sustaining proliferative signalling
Evading growth suppressors
Avoiding immune destruction
Enabling replicative immortality
Tumor-promoting inflammation
Activation invasion and metastasis
And inducing angiogenesis

#How the immune system recognises and kills a cancer cell?
Another important role of the immune system is to identify and eliminate tumors. This is called immune surveillance.

Normal cells exposed to carcinogen, radiation, viral infections, or inherited genetic mutations may change to transformed cells.
These processes can be reversible if the tumor suppression mechanism such as DNA repair and apoptosis is active.
The transformed cells of tumors express antigens that are not found on normal cells, what we called tumor specific antigens.
To the immune system, these antigens appear foreign, and their presence causes immune cells to attack the transformed tumor cells.

Immunosurveillance is a term used, to describe the processes by which cells of the immune system look for and recognise foreign pathogens, such as bacteria and viruses, or pre-cancerous and cancerous cells in the body.

The first step of immunosurveillance is elimination.
When tumor arise in a tissue, a number of immune cells can recognize and eliminate them. The macrophage, dendritic cell, gamma delta T cell, NK cell, CD4 and CD8 T cell, and NKT cell recognize and destroy potential tumor cells.

If the elimination is not completely successful, what follows is an equilibrium phase. Tumor cells undergo changes or mutation that aid their survival. T cells, IL-12, and IFN-gamma are required to maintain tumor cells in a state of immune-mediated dormancy.
NK cells and molecules that participate in the recognition or effector function of cells of innate immunity are not required.
During this phase, a process known as cancer immunoediting continuously shapes the properties of the tumor cells.

Tumors that cannot be recognized and eradicated by the immune system in the elimination or equilibrium phases progress into the escape phase.
Tumor is capable to create a immunosuppressive microenvironment to evade immune attack;
Immunosuppressive cytokines such as TGF-β suppress the inflammatory T-cell response & CTL for controlling tumor growth.
PDL-1 programmed death ligand-1, a B7 family member, directly inhibit immune response, And IDO or indoleamine 2,3-dioxygenase catabolizes tryptophan which make tumor cells can produce physical barrier against cells of the immune system.
In the end, These tumor cells emerge to cause clinically apparent disease.