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.
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.
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.
Nociceptors are nerve fibers that detect stimuli that could potentially cause damage to the body such as extreme heat, pressure, or contact with caustic chemicals. When the nociceptors detect a stimulus, the pain pathway is initiated. The pain pathway is composed of four parts: transduction, transmission, perception, and modulation. Transduction is when the thermal, mechanical, or chemical energy from the pain-causing stimuli is converted into electrical energy so it can be transmitted through the nervous system. Transmission occurs when the energy has been converted and the nerve impulses travel along the nerve fibers into the spinal cord and to the brain. The target structure is the thalamus, which acts as a control panel and forwards the information to the appropriate section(s) of the brain. When the information reaches the brain, perception occurs. This is the point in the pain pathway that the person becomes aware of pain. Based on the information in the nerve impulses, the brain is able to identify the location and intensity of the pain and what kind of reaction it warrants. When the brain reacts this is known as modulation. The muscles contract to withdraw from the source of pain and the brain releases inhibitory chemicals to decrease the transmission and provide analgesic relief.
The chemical changes associated with infection of a tumor or its surrounding tissue can cause rapidly escalating pain, but infection is sometimes overlooked as a possible cause. One study found that infection was the cause of pain in four percent of nearly 300 people with cancer who were referred for pain relief. Another report described seven people with cancer, whose previously well-controlled pain escalated significantly over several days. Antibiotic treatment produced pain relief in all of them within three days.
Some people in pain tend to focus on and exaggerate the pain's threatening meaning, and estimate their own ability to deal with pain as poor. This tendency is termed "catastrophizing". The few studies so far conducted into catastrophizing in cancer pain have suggested that it is associated with higher levels of pain and psychological distress. People with cancer pain who accept that pain will persist and nevertheless are able to engage in a meaningful life were less susceptible to catastrophizing and depression in one study. People with cancer pain who have clear goals, and the motivation and means to achieve those goals, were found in two studies to experience much lower levels of pain, fatigue and depression.
Oral analgesia is the cheapest and simplest mode of delivery. Other delivery routes such as sublingual, topical, transdermal, parenteral, rectal or spinal should be considered if the need is urgent, or in case of vomiting, impaired swallow, obstruction of the gastrointestinal tract, poor absorption or coma. Current evidence for the effectiveness of fentanyl transdermal patches in controlling chronic cancer pain is weak but they may reduce complaints of constipation compared with oral morphine.
Due to the poor quality of most studies of complementary and alternative medicine in the treatment of cancer pain, it is not possible to recommend integration of these therapies into the management of cancer pain. There is weak evidence for a modest benefit from hypnosis; studies of massage therapy produced mixed results and none found pain relief after 4 weeks; Reiki, and touch therapy results were inconclusive; acupuncture, the most studied such treatment, has demonstrated no benefit as an adjunct analgesic in cancer pain; the evidence for music therapy is equivocal; and some herbal interventions such as PC-SPES, mistletoe, and saw palmetto are known to be toxic to some people with cancer. The most promising evidence, though still weak, is for mind-body interventions such as biofeedback and relaxation techniques.