Causes of cancer pain
Cancer pain can be caused by pressure on, or chemical stimulation of, specialised pain-signalling nerve endings called nociceptors (nociceptive pain), or by damage or illness affecting nerve fibers themselves (neuropathic pain). Infection. Infection of a tumor or its surrounding tissue can cause rapidly escalating pain, but 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. Tumor-related. Tumors can cause pain by crushing or infiltrating tissue, or by releasing chemicals that make nociceptors responsive to stimuli that are normally non-painful ("cf." allodynia). Nervous system. Between 40 and 80 percent of patients with cancer pain experience neuropathic pain. Brain Meninges Spinal cord compression Nerve infiltration or compression Dorsal root ganglion inflammation Brachial plexopathy Bone. Invasion of bone by cancer is the most common source of cancer pain. About 70 percent of breast and prostate cancer patients, and 40 percent of those with lung, kidney and thyroid cancers develop bone  metastases. It is commonly felt as tenderness, with constant background pain and instances of spontaneous or movement-related exacerbation, and is frequently described as severe. Tumors in the marrow instigate a vigorous immune response which enhances pain sensitivity, and they release chemicals that stimulate nociceptors. As they grow, tumors compress, consume, infiltrate or cut off blood supply to body tissues, which can cause pain. Fracture Skull Pelvis. Pain produced by cancer within the pelvis varies depending on the affected tissue, but it frequently radiates diffusely to the upper thigh, and may refer to the lumbar region. Lumbosacral plexopathy is often  caused by recurrence of cancer in the presacral space, and may refer to the external genitalia or perineum. Local recurrence of cancer attached to the side of the pelvic wall may cause pain in one of the iliac fossae. Pain on walking that confines the patient to bed indicates possible cancer adherence to or invasion of the iliacus muscle. Pain in the hypogastrium (between the navel and pubic bone) is often found in cancers of the uterus and bladder, and sometimes in colorectal cancer especially if infiltrating or attached to either uterus or bladder. Viscera. Visceral pain is diffuse and  difficult to locate, and is often referred to more distant, usually superficial, sites. Liver Kidneys and spleen Abdominal and urogenital hollow organs Gastrointestinal Respiratory system Pancreas Rectum Serous mucosa. Carcinosis of the peritoneum may cause pain through inflammation, disordered visceral motility, or pressure of the metastases on nerves. Once a tumor has penetrated or perforated hollow viscera, acute inflammation of the peritoneum appears, inducing severe abdominal pain. Pleural carcinomatosis is normally painless. Soft tissue. Invasion of soft tissue by a tumor can cause pain by inflammatory or mechanical stimulation of nociceptors, or destruction of mobile structures such as ligaments, tendons  and skeletal muscles. Diagnostic procedures. Some diagnostic procedures, such as venipuncture, paracentesis, and thoracentesis can be painful. Lumbar puncture Post-dural-puncture headache Treatment-related. 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. Chemotherapy. Chemotherapy may cause mucositis, muscle pain, joint pain, abdominal pain caused by diarrhea or constipation, and peripheral neuropathy Chemotherapy-induced peripheral neuropathy Mucositis Muscle and joint pain Radiotherapy. Radiotherapy may affect the connective tissue surrounding nerves, and may damage or kill white or gray matter in the brain or spinal cord. Fibrosis around the brachial or lumbosacral plexus Spinal cord damage