WHAT IS CANCER?

Cancer, which causes about 13% of all deaths worldwide, is a group of diseases in which cells are:
1. aggressive - grow and divide without respect to normal limits,
2. invasive - invade and destroy adjacent tissues, and
3. sometimes metastatic - spread to other locations in the body.

These three malignant properties of cancers differentiate them from benign tumors, which are self-limited in their growth and do not invade or metastasize (although some benign tumor types are capable of becoming malignant).

CAUSES OF CANCER

Cancer may affect people at all ages, even fetuses, but risk for the more common varieties tends to increase with age. Cancer is a diverse class of diseases which differ widely in their causes and biology. The common thread in all known cancers is the acquisition of abnormalities in the genetic material of the cancer cell and its progeny. Cancer may be caused by:

• Chemical carcinogens such as tobacco smoke and alcohol
• Ionizing radiation such as radon gas and UV rays from the sun
• Ifectious diseases associated with viruses like  human papillomavirus, hepatitis B and hepatitis C virus, Epstein-Barr virus, and human T-lymphotropic virus
• Hormonal imbalances
• Immune system dysfunction like HIV
• Heredity

Other cancer-promoting genetic abnormalities may be randomly acquired through errors in DNA replication, or are inherited, and thus present in all cells from birth. Complex interactions between carcinogens and the host genome may explain why only some develop cancer after exposure to a known carcinogen. New aspects of the genetics of cancer pathogenesis, such as DNA methylation, and microRNAs are increasingly being recognized as important.

Research into the pathogenesis of cancer can be divided into three broad areas of focus.
1. The first area of research focuses on the agents and events which cause or facilitate genetic changes in cells destined to become cancer.
2. Second, it is important to uncover the precise nature of the genetic damage, and the genes which are affected by it.
3. The third focus is on the consequences of those genetic changes on the biology of the cell, both in generating the defining properties of a cancer cell, and in facilitating additional genetic events, leading to further progression of the cancer.

Genetic abnormalities found in cancer typically affect two general classes of genes. Cancer-promoting oncogenes are often activated in cancer cells, giving those cells new properties, such as hyperactive growth and division, protection against programmed cell death, loss of respect for normal tissue boundaries, and the ability to become established in diverse tissue environments. Tumor suppressor genes are often inactivated in cancer cells, resulting in the loss of normal functions in those cells, such as accurate DNA replication, control over the cell cycle, orientation and adhesion within tissues, and interaction with protective cells of the immune system.

Cancer is usually classified according to the tissue from which the cancerous cells originate, as well as the normal cell type they most resemble. These are location and histology, respectively. A definitive diagnosis usually requires the histologic examination of a tissue biopsy specimen by a pathologist, although the initial indication of malignancy can be symptoms or radiographic imaging abnormalities. Most cancers can be treated and some cured, depending on the specific type, location, and stage.

SIGNS AND SYMPTOMS

Roughly, cancer symptoms can be divided into three groups:
• Local symptoms: unusual lumps or swelling (tumor), hemorrhage (bleeding), pain and/or ulceration. Compression of surrounding tissues may cause symptoms such as jaundice.
• Symptoms of metastasis (spreading): enlarged lymph nodes, cough and hemoptysis, hepatomegaly (enlarged liver), bone pain, fracture of affected bones and neurological symptoms. Although advanced cancer may cause pain, it is often not the first symptom.
• Systemic symptoms: weight loss, poor appetite and cachexia (wasting), excessive sweating (night sweats), anemia and specific paraneoplastic phenomena, i.e. specific conditions that are due to an active cancer, such as thrombosis or hormonal changes.

Every symptom in the above list can be caused by a variety of conditions (a list of which is referred to as the differential diagnosis). Cancer may be a common or uncommon cause of each item.

TREATMENT

Once diagnosed, cancer is usually treated with a combination of surgery, chemotherapy and radiotherapy. As research develops, treatments are becoming more specific for different varieties of cancer. There has been significant progress in the development of targeted therapy drugs that act specifically on detectable molecular abnormalities in certain tumors, and which minimize damage to normal cells. Cancer may be treated by:

• Surgery
• Radiation therapy
• Chemotherapy
• Targeted therapies
• Immunotherapy
• Hormonal therapy
• Symptom control

The choice of therapy depends upon the location and grade of the tumor and the stage of the disease, as well as the general state of the patient (performance status). A number of experimental cancer treatments are also under development.

Complete removal of the cancer without damage to the rest of the body is the goal of treatment. Sometimes this can be accomplished by surgery, but the propensity of cancers to invade adjacent tissue or to spread to distant sites by microscopic metastasis often limits its effectiveness. The effectiveness of chemotherapy is often limited by toxicity to other tissues in the body. Radiation can also cause damage to normal tissue.

Because "cancer" refers to a class of diseases, it is unlikely that there will ever be a single "cure for cancer" any more than there will be a single treatment for all infectious diseases.

PROGNOSIS

Cancer has a reputation for being a deadly disease. While this certainly applies to certain particular types, this is increasingly being overturned by advances in medical care. Some types of cancer have a prognosis that is substantially better than nonmalignant diseases such as heart failure and stroke.

The prognosis of cancer patients is most influenced by the type of cancer, as well as the stage, or extent of the disease. In addition, histologic grading and the presence of specific molecular markers can also be useful in establishing prognosis, as well as in determining individual treatments.

Progressive and disseminated malignant disease has a substantial impact on a cancer patient's quality of life, and many cancer treatments (such as chemotherapy) may have severe side-effects. In the advanced stages of cancer, many patients need extensive care, affecting family members and friends. Palliative care solutions may include permanent or "respite" hospice nursing.

Cancer patients, for the first time in the history of oncology, are visibly returning to the athletic arena and workplace. Patients are living longer with either quiescent persistent disease or even complete, durable remissions.