It 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.
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 patients 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.
Tumors cause pain by crushing or infiltrating tissue, triggering infection or inflammation, or releasing chemicals that make normally non-painful stimuli painful. Invasion of bone by cancer is the most common source of cancer pain. It is usually felt as tenderness, with constant background pain and instances of spontaneous or movement-related exacerbation and is frequently described as severe. Rib fractures are also common in breast, prostate and other cancers with rib metastases(spread).
Pain from cancer of the organs, such as the stomach or liver (visceral pain), is diffuse and difficult to locate, and is often referred to more distant, usually superficial, sites. Invasion of soft tissue by a tumor can cause pain by inflammatory or mechanical stimulation of pain sensors, or destruction of mobile structures such as ligaments, tendons and skeletal muscles
Some diagnostic procedures, such as lumbar puncture, venipuncture, paracentesis, and thoracentesis can be painful.
Potentially painful cancer treatments include immunotherapy which may produce joint or muscle pain; radiotherapy, which can cause skin reactions, enteritis, fibrosis, myelopathy, bone necrosis, neuropathy or plexopathy; chemotherapy, often associated with mucositis, joint pain, muscle pain, peripheral neuropathy and abdominal pain due to diarrhea or constipation; hormone therapy, which sometimes causes pain flares; targeted therapies, such as trastuzumab and rituximab, which can cause muscle, joint or chest pain; angiogenesis inhibitors like bevacizumab, known to sometimes cause bone pain; and surgery, which may produce post-operative pain, post-amputation pain or pelvic floor myalgia.
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 cancer patients referred for pain relief. Another report described seven patients whose previously well-controlled pain escalated significantly over several days. Antibiotic treatment produced pain relief in all patients within three days.