Cancer: Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. They form a subset of neoplasms or tumor.
Tumor: – It is a group of cells that have undergone unregulated growth and will often form a mass cells called neoplasm or tumor.
Cancer or malignant neoplasm is a disease of uncontrolled proliferation of cells without any differentiation. It commonly originates in the tissues in which the cells are regularly replaced by mitosis. Cells divide uncontrollably and destroy body tissue.
Normal cells show a property called contact inhibition, Contact inhibition is a process of arresting cell growth when cells come in contact with each other. As a result, normal cells stop proliferating when they form a monolayer in a culture dish. Contact inhibition is a powerful anticancer mechanism.
But Cancer cells appear to have lost this property (contact inhibition). And thus divide and grow over each other in an uncontrolled manner even when in contact with neighboring cells. This results in the invasion of surrounding tissues, their metastasis to nearby organs, and eventually tumorigenesis (Tumorigenesis is the creation or formation of a mass of cells called tumor).
Cancer can spread from its original site by local spread, lymphatic spread to regional lymph nodes or by hematogenous spread via the blood to distant sites, known as metastasis. When cancer spreads by blood, it usually spreads all over the body.
Neoplasm or tumor is of two types:
1. Benign neoplasm or tumor – This is a solid neoplasm or tumor that stops growing by itself and does not invade other tissues and remain confined to a particular site. But benign tumors can be serious if they press on vital structures such as blood vessels or nerves.
2. Malignant neoplasm or tumor – In malignant tumors, neoplastic or tumor cells grow very rapidly, invading and damaging the surrounding normal tissues. Cells sloughed form such tumors reach distant sites through blood, and whenever they get lodge in the body, they start forming a new tumor there. This property called metastasis is the most feared property of malignant tumors.
Symptoms of cancer
When cancer begins it doesn’t produce any symptoms, symptoms appear as the mass grows. It also depends on cancer’s type and location and few symptoms are specific. Local symptoms may occur due to the mass of the tumor or its ulceration.
(i) Mass effects from lung cancer can block the bronchus resulting in cough or pneumonia.
(ii) Esophageal cancer can cause narrowing of the esophagus, making it difficult or painful to swallow.
(iii) Colorectal cancer may lead to narrowing or blockages in the bowel, affecting bowel habits.
(iv) Masses in breasts or testicles may produce observable lumps.
Some cancers may cause specific groups of systemic symptoms, termed paraneoplastic syndrome. Examples include the appearance of myasthenia gravis in thymoma and clubbing in lung cancer.
Causes of cancer
The majority of cancers, most of the cases, are due to genetic mutations from environmental and lifestyle factors. Some of the cancers are, are due to inherited genetics.
Common environmental factors that contribute to cancer death include tobacco (25–30%), diet and obesity, infections, radiation (both ionizing and non-ionizing), stress, lack of physical activity and pollution.
Cancer is generally not a transmissible disease.
Exposure to particular substances have been linked to specific types of cancer. These substances are called carcinogens.
(i) Overexposure to ionizing radiations like X-rays, UV rays, gamma rays, etc. which literally punches holes in the DNA, breaking the correct genetic sequences lead to mutation.
(ii) Tobacco smoke causes 90% of lung cancer. It also causes cancer in the larynx, head, neck, stomach, bladder, kidney, esophagus and pancreas. Tobacco smoke contains over fifty known carcinogens, including nitrosamines and polycyclic aromatic hydrocarbons.
(iii) Chemicals like nicotine, caffeine, steroids and arsenic air pollutants cause cancers of lungs, brain, breast or blood.
Oncoviruses (viruses that can cause cancer) include:
(i) Human papillomavirus (cervical cancer)
(ii) Epstein–Barr virus (B-cell lymphoproliferative disease and nasopharyngeal carcinoma)
(iii) Kaposi’s sarcoma herpesvirus (Kaposi’s sarcoma and primary effusion lymphomas)
(iv) Hepatitis B and hepatitis C viruses (hepatocellular carcinoma- liver cancer)
(v) Human T-cell leukemia virus-1 (T-cell leukemias).
Bacterial infection may also increase the risk of cancer, as seen in Helicobacter pylori-induced gastric carcinoma.
Parasitic infections associated with cancer include Schistosoma haematobium (squamous cell carcinoma of the bladder).
The liver flukes, Opisthorchis viverrini and Clonorchis sinensis (cholangiocarcinoma).
A cancer syndrome or family cancer syndrome is a genetic disorder in which inherited genetic mutations in one or more genes predispose the affected individuals to the development of cancers and may also cause the early onset of these cancers.
Many of these syndromes are caused by mutations in tumor suppressor genes, genes that are involved in protecting the cell from turning cancerous. Other genes that may be affected are DNA repair genes, oncogenes, and genes involved in the production of blood vessels (angiogenesis). Common examples of inherited cancer syndromes are hereditary breast-ovarian cancer syndrome and hereditary non-polyposis colon cancer (Lynch syndrome).
The vast majority of cancers are non-hereditary. Hereditary cancers are primarily caused by an inherited genetic defect. Hereditary cancer syndromes underlie 5 to 10% of all cancers and there are over 50 identifiable hereditary forms of cancer.
Most cancers are initially recognized by the appearance of signs or symptoms or through screening. People with suspected cancer are investigated with medical tests.
These commonly include blood tests, X-rays, (contrast) CT scans and endoscopy.
The tissue diagnosis from the biopsy indicates the type of cell that is proliferating, its histological grade, genetic abnormalities, and other features. Together, this information is useful to evaluate the prognosis and to choose the best treatment.
Cytogenetics and immunohistochemistry are other types of tissue tests. These tests provide information about molecular changes (such as mutations, fusion genes, and numerical chromosome changes) and may thus also indicate the prognosis and best treatment.
Types of cancer
Cancers are classified by the type of cell that the tumor cells resemble. These types include:
(i) Carcinoma: Carcinoma is the most common type of cancer. It begins in the epithelial cells. This group includes many of the most common cancers and include nearly all those in the breast, prostate, lung, pancreas, and colon.
Carcinomas occur when the DNA of a cell is damaged or altered and the cell begins to grow uncontrollably and become malignant.
(ii) Sarcoma: Sarcoma is a rare type of cancer in the human. It arises from transformed cells of connective tissue (i.e. bone, cartilage, fat, nerve), each of which develops from cells originating in mesenchymal cells outside the bone marrow.
(iii) Lymphoma: Lymphoma is a group of blood cancers that develop from lymphocytes. The enlarged lymph nodes are usually painless. Symptoms may include enlarged lymph nodes, fever, drenching sweats, unintended weight loss, itching, and constantly feeling tired.
(iv) Leukemia: Leukemia is a group of cancers that usually begin in the bone marrow and result in high numbers of abnormal white blood cells. These white blood cells are not fully developed and are called blasts or leukemia cells. Symptoms may include bleeding and bruising problems, feeling tired, fever, and an increased risk of infections.
(v) Blastoma: Blastoma is a type of cancer, more common in children, that is caused by malignancies in precursor cells. Examples are nephroblastoma, medulloblastoma, and retinoblastoma.