Radiation therapy also has a great weight in the treatment of malignant tumors. Radiotherapy is mostly used as an adjuvant treatment for cancer, but for some cancers, radiation therapy is its main treatment. The common radiation therapy for cancer involves the following issues. 1.What are the commonly used radiation therapy for tumors? Radiation therapy is a commonly used method to treat malignant tumors at present. Due to the progress of radiation equipment, improvement of technology and accumulation of experience, the efficacy has been significantly improved. 65~75% of malignant tumors including preoperative, postoperative and intraoperative comprehensive treatment need to use radiotherapy, and about 40% of patients need radical radiation treatment. At present, there are four main types of radiation therapy for tumors in China: (1) X-rays produced by ordinary X-ray therapy machines. Due to the low energy of the radiation, it is only used for the treatment of superficial tumors, and now it is mostly replaced by the electron ray generated by gas pedal, so the ordinary X-ray treatment machine has been gradually eliminated. (2) Gamma (γ) rays produced by the artificial radioisotope cobalt-60 in the decay process. At present, these two radiotherapy equipment in China has been basically eliminated. (3) by the medical linear gas pedal generated by the high-energy X-rays and electron beam. The above three radiation therapy equipment is mainly used for external irradiation. (4) Inter-tissue and intracavitary radiotherapy. It is a radiation source (mainly γ-rays produced by iridium-192) sealed directly into the tissue being treated or into the natural cavity of the human body, such as the treatment of cervical cancer, nasopharynx and esophagus cancer. Another kind of inter-tissue radiation source is radioactive iodine-125 permanently implanted into the tumor site of human body through the sender or sender catheter. Gamma rays generated by iodine-125, irradiated with high dose, can effectively kill tumor cells, shrink lesions, reduce recurrence, bring the disease under control or cure, prolong survival, and relieve or alleviate pain and improve the quality of life of patients. It has the advantages of less damage and faster recovery. This kind of permanent implantation of radioactive particles between tissues is an emerging internal radiotherapy modality in recent years, also known as in vivo gamma knife therapy or particle knife therapy. 2.What is precision radiotherapy? What treatment modalities are included? The rapid development of three-dimensional stereotactic radiotherapy based on linear gas pedal in the late 1990s has brought radiotherapy into a brand new stage, which is called precision radiotherapy stage. Precision radiotherapy refers to: (1) the maximum irradiation dose within a certain range of the target area (tumor); (2) the smallest irradiation dose to the surrounding normal tissues outside the target area (tumor); (3) the most accurate positioning and irradiation of the target area (tumor); and (4) the most uniform dose distribution within the target area (tumor). Precision radiotherapy has the advantages of high precision, high dose, high efficacy and low damage, and is the development direction of radiation oncology in the 21st century. Compared with conventional external radiation therapy, it has the following advantages: (1) minimizing the irradiation of normal tissues and organs around the tumor; (2) significantly increasing the total dose of irradiation to the tumor target area; (3) reducing the recent or late complications of normal tissues. The emergence of conformal radiotherapy is a major contribution of radiation physics to radiotherapy, thus making precision radiotherapy a reality. Precision radiotherapy includes: stereotactic radiosurgery, also known as gamma knife; stereotactic radiotherapy, also known as X-ray knife; three-dimensional conformal radiotherapy and intensity-controlled conformal radiotherapy. (1) gamma (γ) knife (full name stereotactic radiosurgery treatment planning system) with cobalt-60 γ-ray as the radiation source, using focused radiation to make high dose concentrated in the target area, the target area outside the dose reduction is very steep, the normal tissue and vital organs outside the tumor target area can be protected, so it has the characteristics of a knife, destroy the tumor lesion as a knife cut, similar to the effect of surgery to remove the tumor. (2) Stereotactic radiotherapy is also called X-ray knife. Using linear gas pedal X-rays as the radiation source, it adopts advanced three-dimensional positioning technology, treatment planning system and three-dimensional reconstruction system, using the principle of geometric three-dimensional focusing, with multiple small fields or rotation and other central irradiation techniques, so that the tumor target area gets the maximum irradiation dose, while the normal tissue around the lesion is only irradiated by low dose, which can be considered as a kind of conformal radiotherapy. The biggest difference with γ-knife is that it can be used in a fractionated treatment mode, not only for the treatment of small volume lesions, but also for the treatment of larger volume malignant tumors. (3) Three-dimensional conformal radiotherapy. It is the new hot spot of current radiation therapy technology. Its technical feature is to use 3D treatment planning system to design multiple non-coplanar irregular fields for fractionated irradiation, and the shape of the irradiated field is consistent with the shape of the lesion projection in the direction of beam axis view. Conformal radiotherapy has great advantages and can be used to treat intracranial lesions as well as body lesions, which is more versatile. (4) Intensity-modulated conformal radiotherapy. Compared with conventional radiotherapy, the advantages of intensity-conformal radiotherapy include: ① The use of precise positioning and body fixation technology greatly improves positioning and irradiation accuracy. ②The use of precise treatment planning, thus achieving automatic optimization of treatment. The segmented dose and total dose of radiotherapy are improved. ③The precise irradiation is used so that the shape of the target area (tumor) and the shape of the high dose distribution are consistent with the actual shape of the target area in the three-dimensional direction, and thus its dose distribution is more conformal, thus allowing a larger increase in tumor dose and/or a reduction in the amount of normal tissue received, and minimizing the amount of radiation to normal tissue around the target area; ④Simultaneous large-field irradiation and small-field It can also avoid over-irradiation of sensitive tissues in the target area, shorten the course of treatment, and can irradiate several independent lesions at the same time, such as multiple lung metastases and brain metastases. 3.Why can radiation treat cancer? The key target of radiation to kill cancer cells is DNA (deoxyribonucleic acid) in the cell nucleus. The division, proliferation and growth of tumors are all determined by DNA replication. Because tumor cells are more sensitive to radiation than normal tissue cells, tumor cells lose their ability to regenerate and proliferate until they are killed by a certain dose of radiation therapy, while normal cells can be completely restored by a sub-lethal dose of radiation, which is a reversible change. Clinically, this kind of normal cell population has low sensitivity to radiation, high tolerance and high repair ability, while tumor tissue is the opposite, so that the radiation has a certain selective effect between normal cell population and tumor cell population, which is one of the important factors for the effect of radiation therapy for malignant tumor. 4. Can all malignant tumors be killed by radiation? The answer is no. Radiation is insensitive to the following four kinds of tumor cells, which becomes a difficult point of radiation therapy. (1) Tumor cells genetically have low radiosensitivity factors: about 1/3 of all tumors belong to low radiosensitivity tumors, and it is difficult to achieve results with radiation therapy alone, such as osteosarcoma, fibrosarcoma, liposarcoma, rhabdomyosarcoma, etc. (2) Oxygen-deprived cells: tumor radiation therapy must be carried out in the oxygenated state, which is called oxygen effect. About 5-10% or more of the cells in the tumor masses seen clinically have low oxygen content, which are oxygen-depleted cells with low radiosensitivity. (3) Non-proliferative phase cells (G0 phase): There are resting phase cells in the non-proliferative phase in tumor tissues, accounting for about 20%-50% of the whole tumor mass, and such cells are hypersensitive to radiation. (4) S-phase cells in the proliferative phase: i.e. cells in the DNA synthesis phase, which are also difficult to kill with radiation and are hypersensitive cells. These four types of tumor cells are the basis for affecting the effect of tumor radiation therapy and the recurrence of some tumors after treatment, and are the key to hinder radiation therapy. Some targeted measures have been studied for the solution and countermeasures of these problems. 5.What are the differences in radiosensitivity of different types of tumor tissues? There are many factors affecting the radiosensitivity of tumors. From the viewpoint of tissue origin and pathological types, there are obvious differences in the inherent radiosensitivity of different tumor cells, which are manifested in different efficacy when receiving radiotherapy. Generally speaking, the more poorly differentiated, high growth rate and fast developing tumors are more sensitive to radiation, but their malignancy is also high and often have poor prognosis. Tumors can be divided into the following three categories according to the different radiosensitivity of tumor tissues. (1) Radiation-sensitive tumors. The so-called radiosensitive tumor means that the dose of radiation to destroy the tumor is much lower than the tolerance of normal tissue to radiation. For example, malignant lymphoma, germ cell tumor (seminoma, anaplastic cell tumor), nephroblastoma, medulloblastoma, retinoblastoma, etc. Although the intrinsic sensitivity of tumor cells plays an obvious role in the outcome of treatment, radiosensitive tumors tend to be highly malignant and can have distant metastases at an early stage. Therefore, high radiosensitivity does not mean high curability. (2) Tumors with moderate sensitivity to radiation. The so-called moderate sensitivity means that the lethal dose of tumor and the tolerated dose of normal tissue are close to each other, so the treatment rate is low. Such tumors include various epithelial cancers and certain adenocarcinomas, such as cervical cancer, laryngeal cancer, thyroid cancer, breast cancer, skin cancer, etc., which are quite important in the practical application of radiation therapy and should be regarded as indications for radiation therapy. Patients will have better results if they can get correct and sufficient surgical treatment with them. (3) Radiation insensitive tumors. As mentioned above tumor cells genetically have low radiosensitivity factors. These tumors require high irradiation dose to make the tumor regress, but will cause irreversible damage to normal tissues, so they are also called radioresistant tumors. 6.What is the significance of preoperative radiotherapy for tumors? (1) Pre-operative radiotherapy can make the endothelial cells of blood vessels and lymphatic vessels proliferate, thus narrowing the lumen and forming occlusive vasculitis, blocking the lymphatic spread of cancer cells, and reducing the chance of blood dissemination of cancer cells. (2) Pre-operative radiotherapy can make some cancer cells die or reduce the living function and proliferation ability of cancer cells to different degrees, thus reducing the chance of metastasis or implantation due to surgery, meanwhile, the infiltration and local clinical metastases in the marginal part of tumor can often not be helped by surgery, and pre-operative radiotherapy can best play its killing role, because large cancer body contains lack of oxygen cells which are difficult to be destroyed by radiation, and the recurrence rate is high after simple radiotherapy. In this case, surgical excision of the cancer body can not only reduce the amount of conventional radiation but also eliminate the problem of local recurrence. (3) Preoperative radiotherapy can make the primary foci recede, the inflammation around the tumor subside, and connective tissue wrapping is formed around it, which can achieve the purpose of improving the surgical resection rate and expanding the indications for surgery. (4) It is observed that the detection rate of cancer cells in blood increases at the time of surgery, suggesting the possibility of medical proliferation, and preoperative radiation can reduce or turn negative the detection rate of positive cancer cells in blood. 7.What is the significance of postoperative and intraoperative radiotherapy for tumor? The main indications of postoperative radiotherapy are: local residual or suspected residual cancer foci, including tumor adhesions, peri- or extra-peri-peri-peri invasion, tumor rupture, and regional lymph node invasion. Postoperative radiotherapy is used to kill macroscopic residual tumors or subclinical lesions that have not been removed by surgery and to reduce recurrence. For example, glioma, head and neck tumors often have unclear boundaries and poor differentiation due to the characteristics of anatomical site and tumor itself, and the recurrence rate of surgical resection is high, so postoperative radiotherapy is advocated to obtain better results. Intraoperative radiation therapy is valuable for some tumors. When tumors are removed surgically, there are invisible cancer cells or residual tumor lesions that are difficult to be removed surgically, which are the root cause of local recurrence or metastasis. Intraoperative radiotherapy mostly adopts electronic wires, which can accurately and safely irradiate high doses of residual cancer or subclinical lesions without affecting normal tissues. Intraoperative radiotherapy has definite superiority for locally advanced gastric cancer cases. Other cases include pancreatic cancer, biliary tract cancer, superficial bladder cancer, prostate cancer, lung and mediastinal tumors, recurrent brain tumors and soft tissue sarcomas, etc. Intraoperative radiotherapy can achieve significant palliative effect or eradicate microscopic lesions to reduce recurrence rate. 8.What is internal irradiation? Intracorporeal irradiation, also known as intra-tissue irradiation, is the permanent implantation of radioactive particles (125I or 103pd) into tumor tissue to cause tumor necrosis. The indications for radioactive particle implantation therapy are: the nature of the lesion is clear and the patient refuses surgical resection; the tumor cannot be removed surgically; after partial resection of the tumor, the residual tumor cannot be operated again; the tumor is located in an important functional area and the risk of surgery is too great; the effect of intracranial tumor after extracorporeal irradiation is not good and particle implantation is used as a supplementary treatment; brain metastases, no more than 3; the lesion is limited in scope, usually 5cm is appropriate, or The lesion shrinks after extracorporeal irradiation, and particle implantation is used to supplement the amount.