I. Fast neutron radiotherapy Fast neutrons are high linear energy transfer (LET) rays, which are not affected by the cell cycle due to their high relative biology effect (RBE) and low oxygen enhancement ratio (OER). It is suitable for the treatment of many retroperitoneal tumors because of its high relative biological effect (RBE), low oxygen enhancement ratio (OER), independent of cell cycle, and low sublethal and potentially lethal damage repair. Fast neutron therapy can also be divided into preoperative, postoperative radiotherapy or preoperative + postoperative radiotherapy. The dose of preoperative fast neutron irradiation is generally 10-15 Gy, and the dose of postoperative radiotherapy is generally 10-20 Gy. A mixture of fast neutron and photon radiotherapy can be used to reduce the complications caused by fast neutron irradiation alone. For example, Hamburg et al. treated 13 cases of postoperative chordoma with residual tumor diameter of 2 to 400 px. 11 cases were treated with fast neutrons and 2 cases with fast neutrons + photons. The radiation dose was 11-16.2 Gy of fast neutrons, with a mean follow-up of 27 months, 8/13 without local progression, 50% local control rate and 45% 4-year survival rate, and none of them had acute or late radiation injury. As early as the 1950s, Takahashi proposed the concept of conformal radiation therapy, i.e., the shape of the irradiation field is suitable for the shape of the irradiated lesion (target area). Conformal irradiation solved the problem of conformal shape of the irradiation field and avoided or reduced the irradiation of normal tissues and organs, but did not solve the problem of uneven dose required due to the inconsistent thickness of the tumor. Further research has led to the development of intensity modulated radiation therapy, in which the output dose rate at each point in the irradiation field in each direction can be automatically adjusted according to the shape, thickness and density of the tumor in the target area, so that the dose at each point in the target area can be uniformly distributed to meet the therapeutic requirements. The combination of conformal irradiation and intensity modulated irradiation is called intensity modulated conformal radiotherapy. For retroperitoneal tumors, especially for partially resected or unresectable tumors, the purpose of applying intensity modulated conformal radiotherapy is to focus the irradiation dose to the tumor tissue to the greatest extent while minimizing or avoiding irradiation to the surrounding normal tissues and organs as much as possible, and to make the whole tumor receive uniform dose, so as to improve the control rate of local tumors, reduce the recurrence rate, and also improve the survival quality after treatment. Radiation injury and complications of retroperitoneal tumor The sensitivity of retroperitoneal tumor to radiotherapy is generally low, and the required radiotherapy dose is large and time is long, plus there are many blocking organs in front and behind the tumor, which are easily damaged by radiotherapy, so the incidence of complications is high. The systemic reactions caused by radiation therapy for retroperitoneal tumor mainly include nausea, vomiting, weakness, leukocyte and platelet reduction, etc. The local reactions are mainly related to the irradiated normal tissues and organs, such as intestinal irradiation can cause radioactive enteritis, abdominal distension and diarrhea in the acute stage, and intestinal stenosis, intestinal perforation and intestinal necrosis in the late stage. Irradiation of other organs can cause different degrees of damage, such as radioactive hepatitis, nephritis, cystitis, myelitis, paraplegia, etc., mainly related to the tumor and radiotherapy site, the sensitivity of the irradiated organ to radiation and the irradiation dose. Glean et al. explored the treatment of retroperitoneal sarcoma with the combination of multiple methods of radiotherapy and chemotherapy. A group of 37 patients with highly malignant retroperitoneal tumors continued to receive adjuvant therapy after complete resection. All 37 patients received postoperative radiotherapy and 21 received chemotherapy, with a 3-year actual survival rate of 43%, which appeared to be unaffected by chemotherapy. Poor tolerance of radiotherapy and chemotherapy was significantly associated with long-term toxicity in these patients, all of whom were treated in the course of surgical recovery. Therefore, attention has begun to shift to preoperative multimodal treatment of retroperitoneal sarcoma, with cytotoxic chemotherapy and radiation sensitizers, which have been successfully applied during external irradiation prior to resection. Preoperative treatment is of great benefit because it is given when the patient is most tolerant, minimizing the risk of failure of resection or making an otherwise unresectable tumor completely resectable. Also at the forefront of research in recent years is the use of intraperitoneal chemotherapy early after complete or partial tumor resection. However, none of these approaches has yet become the standard of care for retroperitoneal tumors.