Radiotherapy is one of the effective measures for head and neck cancer, but besides achieving the expected anti-cancer effect, head and neck radiotherapy can also cause damage to normal tissues and a series of oral complications, such as: mucositis, oral dryness, radioactive caries, dental closure and radioactive osteonecrosis. These oral complications may lead to interruption of anti-cancer treatment and affect patients’ postoperative quality of life. 1.Mucositis Mucositis is also called stomatitis. Radiotherapy destroys rapidly dividing cells and causes short-term damage to oral mucosa in the oral cavity. After the first week of radiotherapy, patients often experience tenderness and swelling of oral tissues, mucosal erythema and edema, which gradually increase. By the 4th week, most of the oral tissues are swollen. Patients have painful, burning sensation in the mouth when eating especially spicy or rough food. The stability of the micro-ecological environment of the oral cavity is disrupted after radiotherapy, and mucositis can be aggravated if combined with microbial infection. Generally, mucositis lasts until 1~2 weeks after the end of radiotherapy. Infection, smoking and alcohol consumption will aggravate mucositis and prolong the healing time. Before radiotherapy, patients should undergo oral hygiene examination, polish sharp and rough tooth tips and fillings, and remove bad restorations. At the beginning of radiotherapy, patients can rinse their mouth with carbonate solution for 1 minute, at least 8~10 times a day. The solution dissolves mucus and loosens food debris. Severe mucosal inflammation should be seen at the stomatology department, and fungal infection can be detected by using mycophenolate or ketoconazole tablets to control the infection. If the patient is unable to dissolve the tablets due to dry mouth, the appropriate oral spray can be used. 2, oral dryness Oral dryness is the most common complication after head and neck radiotherapy. In the first week of radiotherapy, saliva flow can be reduced by nearly 50%, and as the course of treatment progresses, the flow can be reduced by 95%. In many cases, the decrease in saliva is irreversible. The main prevention and treatment measures for radiation dry mouth include precision radiotherapy using 3D conformal or intensity modulation techniques, and salivary gland transposition prior to radiotherapy to reduce the dose to the major salivary glands. Precision radiotherapy is now widely used in many large hospitals and has significantly reduced the degree of dry mouth after radiotherapy, but this complication cannot yet be completely avoided; surgical prophylaxis adds additional surgical risks that are unacceptable to many patients. For patients who have developed radiotherapy dry mouth, the means available include M choline receptor agonists, salivary substitutes and Chinese medicine treatment, etc. The representative drug of M choline receptor agonists is trigonelline, which can significantly increase the resting salivary flow and thus reduce the occurrence of radiation dry mouth, but does not help to improve oral mucositis and quality of life scores. The drug has a slow onset of action, requires long-term use for sustained efficacy, is expensive, and has certain side effects such as sweating, dysuria, asthma, and cardiac arrhythmias. The main components of artificial saliva include carboxymethyl cellulose, polyethylene oxide or animal mucin, etc. Several studies have confirmed that saliva substitutes represented by artificial saliva can significantly improve dry mouth symptoms in patients, but they need to be used very frequently throughout the day (including nighttime). Chinese medicine has a certain role in both prevention and treatment, but the effect is mild and slow, and it needs to be used consistently for a period of time. 3.Radioactive caries The main reasons for radioactive caries are: 1) the ionization radiation of head and neck destroys the function of salivary gland, the quality of saliva is changed, the removal ability of saliva to carbohydrate and acid, the buffering ability of acid is reduced, which makes the normal flora of oral cavity transform into the cariogenic flora of high risk degree and the destruction of remineralization ability of tooth structure; 2) the awareness of oral health care and self-cleaning ability of cancer patients are poor, and they are easy to choose soft food, carbohydrate-rich food, or use cariogenic saliva stimulants. And after radiotherapy, enamel structure produces physicochemical changes, solubility and demineralization depth are changed, caries sensitivity is enhanced. The clinical manifestation of radiographic caries is very characteristic: caries damage can occur on the cervical part, cusps, incisal edges and smooth surfaces of teeth, and it progresses rapidly and can involve most or all of the teeth in a few weeks. Pre-operative dental examination should be combined with X-ray film. The carious teeth that can be preserved should be treated timely, and the incurable teeth should be extracted as early as possible. Before radiotherapy patients should learn the correct method of brushing, how to use dental floss or soft wooden toothpicks to remove interdental plaque, etc. 4. Closure of teeth When the masticatory muscles or other soft tissues around the temporomandibular joint are located in the radiation field, the radiation causes local edema, cell destruction and fibrosis of the tissues, resulting in restricted mouth opening and reduced mobility of the affected muscle system. After the start of radiation therapy, these patients need to do daily opening exercises to maintain maximum mouth opening and jaw mobility. If a decrease in mouth opening is observed, a wedge-shaped hard rubber block of moderate thickness or a stepped wooden block can be used as an opener, and the thickness can be gradually increased to make the mouth opening gradually increase. Generally, mouth opening restriction (myogenic) can last until 3~6 months after the end of radiotherapy. Therefore, for patients who are at risk of dental closure, continue to strengthen the opening training during this period, supplemented by physical therapy. 5.Radiation osteonecrosis Radiation osteonecrosis is the most serious complication of head and neck radiotherapy, with an incidence of 5.4%, and many ORN patients have no obvious symptoms in the early stage. The main cause of radiation osteonecrosis is the vascular damage and bone damage caused by radical radiation of head and neck cancer, and the secondary infection caused by tooth extraction surgery, odontogenic infection or injury during or after radiotherapy often becomes its causative factor. The mandible is more susceptible to necrosis than the maxilla because of the dense bone and relatively less abundant blood supply. To prevent radiation osteonecrosis, routine periodontal cleaning and oral hygiene should be performed before radiotherapy; remove existing metal dentures in the oral cavity, and active dentures should be worn after radiotherapy has been terminated for a period of time to avoid mucosal damage.