Cancer pain  Pain in cancer may arise from a tumor compressing or infiltrating nearby body parts; from treatments and diagnostic procedures; or from skin, nerve and other changes caused by a hormone imbalance or immune response. Most chronic (long-lasting) pain is caused by the illness and most acute (short-term) pain is caused by treatment or diagnostic procedures. However, radiotherapy, surgery and chemotherapy may produce painful conditions that persist long after treatment has ended. The presence of pain depends mainly on the location of the cancer and the stage of the disease. At any given time, about half of all people diagnosed with malignant cancer are experiencing pain, and two thirds of those with advanced cancer experience pain of such intensity that it adversely affects their sleep, mood, social relations and activities of daily living. With competent management, cancer pain can be eliminated or well controlled in 80% to 90% of cases, but nearly 50% of cancer patients in the developed world receive less than optimal care. Worldwide, nearly 80% of people with cancer receive little or no pain medication. Cancer pain in children is also reported as being under-treated. Guidelines for the use of drugs in the management of cancer pain have been published by the World Health Organization (WHO) and others. Healthcare professionals have an ethical obligation to ensure that, whenever possible, the patient or patient's guardian is well-informed about the risks and benefits associated with their pain management options. Adequate pain management may sometimes slightly shorten a dying person's life. Pain is classed as acute (short term) or chronic (long term). Chronic pain may be continuous with occasional sharp rises in intensity (flares), or intermittent: periods of painlessness interspersed with periods of pain. Despite pain being well controlled by long-acting drugs or other treatment, flares may occasionally be felt; this is called breakthrough pain, and is treated with quick-acting analgesics. The majority of people with chronic pain notice memory and attention difficulties. Objective psychological testing has found problems with memory, attention, verbal ability, mental flexibility and thinking speed. Pain is also associated with increased depression, anxiety, fear, and anger. Persistent pain reduces function and overall quality of life, and is demoralizing and debilitating for the person experiencing pain and for those who care for them. Pain's intensity is distinct from its unpleasantness. For example, it is possible through psychosurgery and some drug treatments, or by suggestion (as in hypnosis and placebo), to reduce or eliminate the unpleasantness of pain without affecting its intensity.