The causes of cancer pain are multiple: 1. mechanical pain: due to rapid growth of tumor and compression of peripheral nerves, resulting in pain; 2. high intratumoral pressure: proliferation of tumor cells can generate high pressure in the tumor, causing tension pain; 3. microfracture: osteoclast activation factor stimulates osteoclast resorption of bone during bone metastasis, resulting in progressive osteolysis destruction, causing microfracture of local lesions and Pain appears; 4. Movement: movement of joints can also stimulate nerve endings to appear pain; 5. Biological pain: it refers to the release of cytokines and chemical mediators from tumor cells to stimulate surrounding tissues and nerve endings to appear pain. In summary, the clinical causes of cancer pain can be broadly classified into four types: pain directly caused by cancer, cancer-related pain, pain caused by cancer treatment and pain caused by concomitant diseases. About 87% to 92.5% of pain is related to tumor, 17% to 20.8% is caused by treatment, and some patients have both factors. (a) Pain directly caused by cancer Tumor cells with swelling or infiltrative growth tend to form masses and compress surrounding tissues or block various ducts, such as lymphatic vessels, blood vessels, intestinal ducts, etc., causing pain. Infiltrative growth may also invade nerves, blood vessels, lymphatic vessels and thoracic and peritoneal membranes and cause pain. 1.Pain caused by local invasion of nerve, blood vessel and periosteum by tumor. For example, primary breast cancer invades ribs, intercostal nerve and pleura, and axillary lymph node metastasis invades brachial plexus nerve, which can cause pain. Supraglottic sulcus tumor invades peripheral nerves and thorax, causing pain. Head and facial tumors invade the nerves, oral mucosa and periosteum, etc., causing pain. 2.Local compression of tumor. (1) The volume of diseased tissues increases, while the tissue envelope and bony volume remain unchanged, resulting in distension and pain in local tissues. For example, primary hepatocellular carcinoma shows distension and pain in the liver area; prostate cancer shows local pain; primary intracranial tumor or metastatic carcinoma causes distension and pain due to increased intracranial pressure. (2) Obstruction in the lumen causes obstruction and spasm, resulting in pain. For example, colon cancer causes intestinal obstruction, resulting in abdominal pain; bronchopulmonary cancer causes obstructive pneumonia and chest pain due to poor drainage. (3) Invasion of lymphatic vessels, blood vessels and soft meninges, resulting in obstruction of lymphatic fluid, blood and cerebrospinal fluid reflux, leading to local edema and pain. For example, breast cancer axillary lymph node metastasis compresses the axillary vein, blocking the return of axillary vein, leading to edema of upper limbs and causing pain. Superior vena cava obstruction syndrome causes edema of the head, face and upper limbs, resulting in swelling and pain. 3.Pain caused by pathological fracture. It is common in lung cancer, breast cancer and other metastases to the rib cage and lumbar spine, which cause pain when pathological fracture occurs, and pain is reduced after secondary paraplegia. For example, osteoarthrosis syndrome (pestle and mortar finger, osteoarthralgia, periosteal hyperplasia, etc.), myasthenia gravis, polymuscular neuralgia, etc., commonly found in lung cancer, thymoma, etc. Tumor cells can also secrete cytokines such as interleukin-8 (IL-8), tumor necrosis factor (TNF), inflammatory factors, growth factors and enzymes, which are also related to pain. 2.The high metabolism of tumor cells and lack of oxygen easily cause the increase of tissue metabolites, especially the increase of some pain-causing substances such as hydrogen ions, which cause pain. 3.Patients with advanced cancer suffer from a series of pathophysiological changes due to overconsumption and malnutrition of the body, such as pain caused by bedsores, constipation and muscle spasm. (3) Pain related to cancer treatment 1. Surgical treatment: while removing tumors, nerves, blood vessels and lymphatic vessels are inevitably damaged, and poor local drainage, incision infection, non-healing and scar formation after surgery can cause pain. For example, the incidence of postoperative pain in breast cancer is 6%, which is mostly related to intercostal nerve injury, upper limb edema, scarring and incision non-healing. Intercostal nerve pain caused by chest surgery, etc. 2, chemotherapy: the pain caused by the side effects of chemotherapy drugs is complex and diverse. Neurotoxic drugs (vincristine) and paclitaxel drugs are common for peripheral neuralgia, often accompanied by numbness of the extremities, sometimes manifested as abdominal pain and burning pain in the hands and feet, which can disappear after stopping the drugs. Chemotherapeutic drugs can cause phlebitis, which can cause sterile inflammation when extravasated, such as adriamycin and mitomycin. Some of the drugs themselves are foaming agents, which can cause severe burning-like pain when they spill out of the blood vessels, often making it difficult for patients to sleep at night. Certain drugs (such as isocyclophosphamide) are metabolized in the body and discharged to the bladder via the ureter, stimulating the bladder and ureter and causing pain. 3, radiation therapy: can cause radioactive dermatitis and even radioactive skin ulcers, which can produce different degrees of pain. After radiation therapy, it can cause local fibrous tissue proliferation and compression, and cause lymphatic vessels obstruction and edema, which can also cause pain. After high-dose radiation therapy, bone tumor can reduce bone density and even fracture and cause pain. Radiation therapy can also cause pain due to nerve damage. (4) Other factors: Patients with advanced tumors have low immune function and are prone to herpes zoster and other painful conditions. (4) Pain not related to cancer Pain caused by cancer patients’ previous diseases, such as gout, arthritis and phlebitis, etc.