Pain has been defined by the International Association for the Study of Pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage.
Not all cancer patients experience pain, but it is one of the most common symptoms associated with cancer. Approximately one quarter of newly diagnosed cancer patients have some degree of pain, as do one third of patients undergoing cancer treatment and three quarters of patients with advanced disease.
Pain is a highly subjective experience. The same physical stimulus may cause greater or lesser 
amounts of pain in different people and even in the same person in different situations. Nonphysical stressors, such as emotional, financial, and spiritual difficulties, can influence the patient's experience of pain.
Over the past 25 years, scientific advances in molecular biology, neuroscience, pharmacology, and anesthesiology have led to a better understanding of the anatomy and physiology of pain. Pain receptors and mediators have been identified and duplicated in the laboratory, and medications and methods of treatment have been improved.
Types of Cancer Pain
Accurate diagnosis of the cause and type of pain is key to finding the most effective treatment. Neuropathic and nociceptive pain, for example, respond very differently to pain medications such as nonsteroidal anti-inflammatory drugs (NSAIDs), opioids, antidepressants, and anticonvulsants.
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Acute, Chronic & Breakthrough Pain
Cancer pain may be acute (relatively short-term) from tumor growth, injury, and certain treatments and procedures; or chronic (continuing over several weeks or months). Both types can be of varying severity. Breakthrough pain is a sudden temporary flare in pain intensity. Breakthrough pain can occur even when a patient takes the proper pain medication, and a doctor may prescribe a special dose or treatment to use when pain breaks through normal pain medication.
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Neuropathic Pain
Cancer pain can be caused by injury to or compression of nerves or other components of the nervous system, in which case it is termed neuropathic.
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Nociceptive Pain
Pain may also be caused by an inflammatory response to ongoing nerve-tissue damage. This pain, called nociceptive, can be defined further by site of origin:
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Visceral Pain
Visceral pain is caused by injury to an internal organ such as the liver. Such pain is often hard to pinpoint and can be throbbing, aching, or sharp.
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Somatic Pain
Somatic pain primarily involves bone. It can usually be pinpointed and can also be throbbing, aching, or sharp.
