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 Cancer is a group of diseases characterized by uncontrolled cell division that leads to abnormal tissue growth. Many a times, people often use the terms cancer and tumor synonymously. But all tumours are not cancerous.

There are two main types of tumors: malignant tumours and benign tumours:-

Malignant tumours: These are the cancerous cells that can invade and destroy surrounding healthy tissue, including organs. The cancer can spread to distant parts of the body through the lymphatic system or bloodstream. Not all tumours are cancerous.

Benign tumours:  These types of tumours do not grow uncontrollably or do not invade neighboring tissues. They do not spread throughout the body.

There are over 200 different known cancers that affect humans. Most of the cancers are named from where they start. For example, lung cancer starts in the lung, and breast cancer starts in the breast. The spread of cancer from one part of the body to another is called metastasis. Symptoms and treatment depend on the cancer type and how advanced it is. Most treatment plans may include surgery, radiation and/or chemotherapy.

Various causes of cancer are:-

Genetic mutations 
Harmful rays due to exposure to sun/ radiation
Diet and physical activity
Environmental factors
Symptoms of cancer, are usually caused by the effect of cancer on the part of body where it is growing. Although symptoms of cancer may appear in the form of general debility including weight loss or tiredness, yet it is generally advised that if anyone is experiencing unusual symptoms for more than a few weeks than he/she should seek medical attention.

Local symptoms: Local symptoms generally occur due to the mass of tumour or its ulceration. For example:-

Esophageal cancer can cause narrowing of the esophagus, making it difficult or painful to swallow, Colorectal cancer may lead to narrowing or blockages in the bowel, resulting in changes in bowel habits.

Systemic symptoms: General symptoms occur due to distant effects of the cancer that are not related to direct or metastatic spread. These may include:-

Unintentional weight loss
Getting extremely tired easily (Fatigue)
Changes in the skin color/ appearance
Remedial treatment: Remedial treatment refers to treatment which seeks to make the patient feel better and may or may not be combined with an attempt to attack the cancer. Treatment includes action to reduce the physical, emotional, spiritual, and psycho-social agony experienced by people with cancer.

Surgery: Surgery is the fundamental method of treatment of most isolated cancers and may play a role in remedial measure and prolongation of survival. Biopsy is normally required. It is typically an important part of establishing the definitive diagnosis and staging the tumour required. Localized cancer surgery typically attempts to remove the entire mass along with the lymph nodes in the region (in some cases).

Radiation: Radiation therapy involves the use of ionizing radiation in an attempt to either cure or ameliorate the symptoms of cancer. It is used in about half of the cases and can be either from internal sources in the form of brachytherapy or external therapy.

Chemotherapy: Chemotherapy in addition to surgery has proven to be useful in a number of different cancer types including: breast cancer, colorectal cancer, pancreatic cancer, osteogenic sarcoma, testicular cancer, ovarian cancer, and some lung cancers.

Stem cell transplants for treating cancer: Sometimes very high doses of chemotherapy are used, often with radiation therapy, in order to destroy the cancer cells. This treatment also kills the stem cells in the bone marrow. Soon after treatment, stem cells are replanted to replace those that were destroyed. These stem cells are given into a vein, much like a blood transfusion. Over time they settle in the bone marrow and begin to grow and make healthy blood cells. This process is called engraftment.
Cancers can generally be recognized by appearance of signs and symptoms through screening.

Screening: Screening tests help in detecting cancer at an early stage (before the symptoms appear). When abnormal tissue or cancer is found early, it is easy to treat or cure. By the time symptoms appear, the cancer might have grown and spread. This can make the cancer harder to treat or cure. It is important to remember that when a doctor suggests any screening test, it does not always mean  that there is a  cancer. People with suspected cancer are investigated with medical tests. These tests include Blood tests, X-rays, MRIs, Biopsy, Pap-smears, CT scans and Endoscopy and many more.
Cancers are classified based on the nature of cells that the tumor cells resemble and are therefore classified as:-

Carcinoma: Cancers that are derived from epithelial cells are known as carcinomas. This group of cancer is most common, especially in the older people, and includes nearly all cancers developing in the breast, prostate, lung, pancreas, and colon.

Sarcoma: These cancers are derived from connective tissues (i.e. bone, cartilage, fat), each of which develop from cells originating in mesenchymal cells outside the bone marrow.

Lymphoma and leukemia: These two classes of cancer arise from hematopoietic (blood-forming) cells that leave the marrow and tend to mature in the lymph nodes and blood, respectively. Leukemia is the most common type of cancer in children.

Germ cell tumor: Cancers derived from pluripotent cells ( refers to a stem cell that has the potential to differentiate into any of the three germ layers, endoderm (interior stomach lining, gastrointestinal tract, lungs), mesoderm (muscle, bone, blood, urogenital), and ectoderm (epidermal tissues and nervous system), most often presenting in the testicle or the ovary (seminoma).

Blastoma: These types of cancers are derived from immature "precursor" cells or embryonic tissue. Blastomas are more common in children than in older adults.
National Institute of Cancer  Prevention and Research (NICPR)