Radiotherapy is one of the basic treatments for malignant tumors, but before the 1990s, radiotherapy was rarely given to patients with HCC because of its poor effectiveness and high damage to the liver. Since the mid-1990s, modern precision radiotherapy techniques, including 3-dimensional conformal radiation therapy (3DCRT), intensity modulated radiation therapy (IMRT) and stereotactic radiotherapy (SBRT), have become increasingly mature and widely used, providing new opportunities for the treatment of hepatocellular carcinoma by radiotherapy. Domestic and foreign scholars have reported clinical practice and research on the use of modern precision radiotherapy for the treatment of HCC that cannot be surgically resected, and for selected HCC patients, the 3-year survival rate after radiotherapy can reach 25%-30%. It is generally accepted that radiotherapy can be considered for patients with hepatocellular carcinoma whose tumors are confined and cannot be surgically resected due to poor liver function, or whose tumors are located in important anatomical structures that cannot be technically resected, or whose patients refuse surgery. In addition, for patients with distant metastases, palliative treatment is sometimes available to control pain or relieve compression, etc. (a) Indications for radiotherapy for hepatocellular carcinoma. 1.It is mainly applicable to: ①Good general condition, such as KPS≥70, liver function Child-Pugh grade A, single lesion; ②Patients with residual lesions after surgery; ③Local tumor treatment of liver is needed, otherwise serious complications will arise, such as obstruction of liver portal, tumor embolism of portal vein and hepatic vein; ④Palliative treatment of distant metastases, such as lymph node metastasis, adrenal metastasis and bone metastasis. It can alleviate the patient’s symptoms and improve the quality of life. 2. Indications for radiotherapy as an important means of comprehensive treatment for hepatocellular carcinoma: ① HCC confined to the liver: radiotherapy combined with hepatic artery intervention can significantly improve the efficiency and survival rate; ② HCC with cancer embolism: radiotherapy can target cancer embolism that appears after surgical or interventional treatment and cancer embolism in the primary focus (including inferior vena cava cancer embolism), which can prolong the survival of patients at grade C; ③ HCC with lymph node metastasis: radiotherapy HCC with lymph node metastasis: radiotherapy can significantly improve the survival of HCC patients with lymph node metastasis; ④HCC with adrenal metastasis: radiotherapy can relieve the symptoms of adrenal metastasis, but there is no evidence that radiotherapy can prolong survival; ⑤HCC with bone metastasis: radiotherapy aims to relieve the symptoms and thus improve the quality of patient survival, but there is no evidence that it can prolong patient survival; ⑥ICC: radiotherapy can prolong the survival of patients with positive margins after resection and unresectable ICC and unresectable ICC patients. Most of the above radiotherapy for hepatocellular carcinoma is palliative in nature, with poor efficacy, and even if it can prolong survival, it is relatively short and cannot replace conventional treatment for hepatocellular carcinoma; however, other therapies for the above clinical conditions have not shown better efficacy and stronger evidence-based medical evidence. (B) Techniques of radiotherapy for hepatocellular carcinoma. 1.Division of radiotherapy dose: The existing clinical experience shows that large division of radiation, such as 5Gy per time, 3 times per week, total dose of 50Gy, has strong tumor killing effect, but also has great radiation damage to the normal liver. Conventional segmentation irradiation, such as 2Gy/time, once a day, 5 times a week with a total dose of 50-62 Gy, is well tolerated by the normal liver and has significant tumor suppression. Which segmentation method is better needs further clinical practice and research comparison; however, for patients who need to relieve clinical symptoms in the short term, large segmentation radiotherapy is more suitable, because the tumor regression is faster and the symptoms improve significantly. 2.Radiotherapy plan. (1) Radiotherapy technique: The results of dosimetric comparison show that IMRT radiotherapy has better dose conformability in the target area and reduced irradiated dose to the normal liver compared with 3DCRT. Therefore, the 3DCRT technique is generally used first; if the dosimetric requirements are not met, then IMRT is used, which is more suitable for patients with large hepatocellular carcinoma that exposes the normal liver to larger doses, or patients with severe cirrhosis that cannot tolerate large doses of radiation. (2) Respiratory control: It is recommended to use respiratory control techniques such as active breath coordinator (ABC) to limit the movement of the tumor during radiotherapy and thus reduce the radiation dose to the normal liver. (3) Target area localization: CT and MRI image fusion techniques combined with iodine oil deposition after TACE are recommended to determine the bulk tumor extent (GTV) of hepatocellular carcinoma. The clinical tumor volume (CTV) is GTV plus 5 mm-10 mm, and the planned volume (PTV) is CTV plus 6 mm under the condition of using ABC device, which should be determined according to the patient’s respiration when ABC is not used. Currently, some authors advocate 2 TACE sessions prior to radiation therapy, with an interval of 3-6 weeks before reassessing the need for further radiation therapy. This protocol may have the following benefits: (i) small hepatocellular carcinoma lesions can be detected and treated; (ii) it facilitates the identification of the tumor target area; (iii) it facilitates the verification of the radiotherapy plan before its implementation; and (iv) it has the potential to delay the local dissemination within the liver and delay the appearance of dissemination within the liver. (iii) Complications of radiotherapy. Complications of radiotherapy include toxic side effects in the acute phase (during radiotherapy) and liver injury in the late phase of radiotherapy (within 4 months). 1, acute phase (during radiotherapy) toxic side effects: ① anorexia, nausea, vomiting, more serious with upper gastrointestinal bleeding, especially in patients whose radiation field involves larger volume of duodenum, jejunum and stomach; ② acute liver function damage: manifested as rising bilirubin and rising serum ALT; ③ bone marrow suppression, especially in patients whose liver is illuminated in large volume, or with hypersplenism. 2.Late injury of radiotherapy: mainly radiation induced liver disease (RILD), its clinical manifestations and diagnostic criteria: ① have received high-dose radiotherapy to the liver; ② occur after the end of radiotherapy; ③ clinical manifestations are of 2 kinds: typical RILD: rapid onset, the patient rapidly develops a large amount of ascites and liver enlargement in a short period of time ③There are two types of clinical manifestations: typical RILD: rapid onset, with rapid onset of massive ascites and liver enlargement within a short period of time, accompanied by an increase in AKP to >2 times the normal value or an increase in ALT to >5 times the normal value; atypical RILD: impairment of liver function only: AKP to >2 times the normal value or an increase in ALT to >5 times the normal value, without liver enlargement and ascites; ④clinical symptoms and impairment of liver function caused by the development of liver tumor can be excluded. RILD is a serious radiological complication, and once it occurs, more than 70% of patients can die from liver failure within a short period of time; the main treatment is symptomatic, including the use of adrenal glucocorticoids and diuretics, along with aggressive hepatoprotective drugs and supportive therapy. The key to avoiding RILD is to limit the dose to the normal liver to a tolerable level when designing the radiotherapy plan. The radiation tolerated dose to the liver of patients with liver cancer in China is significantly lower than that reported abroad, because most liver cancers in China have an underlying cirrhosis. According to the domestic data. The tolerated dose (whole liver average dose) for liver is 23 Gy for Chlild-Pugh class A patients and probably 6 Gy for Chlild-Pugh class B patients. More caution should be taken for patients who are prone to RILD, including poor pre-existing liver function, such as liver function of Child-Pugh class B; normal liver with large irradiated volume and high dose; patients with concomitant cancerous emboli in vessels, such as those in the portal and inferior vena cava. If TACE was used concurrently, the interval between TACE and liver radiotherapy was shorter than 1 month. In addition, patients who develop acute hepatic impairment during radiotherapy, such as ≥RTOG grade II liver injury, have up to 60% chance of developing RILD later if radiotherapy is continued. Therefore, radiotherapy should be discontinued in such patients to avoid the development of RILD after treatment. In conclusion, acute liver injury is often reversible and easily repaired; whereas late liver injury is often irreversible and is a serious radiation injury with a mortality rate of up to 80% once it occurs. The main causative factors include heavy underlying liver lesions (Child class B or C), excessive volume of irradiated normal liver tissue, and excessive dose. Prevention is the key, and the irradiation dose is limited to the tolerable range (generally considered to be 22 Gy for the national population).