Radiotherapy, also called radiation therapy, is a treatment that uses high-energy rays to kill tumor cells or shrink them. When to use radiotherapy? 1.Before surgery (neoadjuvant therapy): If the tumor is large, using radiotherapy before surgery can shrink the tumor and also reduce local recurrence of rectal cancer. Radiotherapy is often used in combination with chemotherapy, which is called radiotherapy. Radiotherapy can improve the effect of radiotherapy, but at the same time, the side effects are increased. 2.After surgery (adjuvant therapy): radiotherapy is used after surgery to kill residual tumor cells that are not visible to the naked eye. The purpose is also to prevent local recurrence. 3.When surgery is not possible: If patients cannot bear surgery because of physical reasons, radiotherapy can be used to control tumor progression. 4.Progressive tumors: Radiotherapy can be used to slow down the symptoms of progressive tumors, such as intestinal obstruction, bleeding, or pain. It can also be used to treat colon cancer that has spread to the bones or brain. What are the types of radiotherapy? 1. External radiation is the most commonly used. The machine emits radiation outside the body and irradiates the tumor in the body through the skin. It is often used in combination with chemotherapy before surgery, which can shrink the tumor, thus making it easier to remove the tumor and also significantly reducing the local recurrence rate. The doctor will also determine the course of radiation therapy based on the location and stage of the tumor. External radiation can also be used after surgery to prevent local recurrence (the tumor is removed and grows back in the original site). It is less commonly used for colon cancer because colon cancer usually metastasizes to the liver. External irradiation of the pelvis or abdomen can be used to treat colorectal cancer that is not resectable (unresectable). It can also be used to treat colon cancer that has spread to the bone or brain. Extracorporeal irradiation can be done on an outpatient basis, five days a week for five to six weeks. Extracorporeal irradiation begins with a plan or simulation, which means that the body is marked and the length is measured in order to achieve an accurate exposure. The patient lies flat and the rays are irradiated to the pelvis from different directions. Near the end of the treatment, the doctor will again make a plan or simulation to allow the irradiation to be more focused on the areas where the tumor is likely to recur. The final 3-5 days may be spent focusing irradiation on these areas. 2. Brachytherapy This method involves putting radioactive particles (radioisotopes) into a tube or some kind of container and then placing them near the rectal cancer or inside the tumor. Brachytherapy is usually a high dose rate, that is: a single high dose is given in a very short period of time. The container is removed at the end of the treatment. The irradiation can be repeated within a few days. There are fewer side effects and fewer long-term complications associated with brachytherapy. However, more clinical studies are needed to confirm its efficacy and it is not recommended for routine use. 3. Intracorporeal irradiation It can also be used to irradiate rectal tumors at close range through the anus with a special device. Not every hospital has this device, and it is not the standard radiotherapy method for rectal cancer. Side effects of radiotherapy Patients usually have no discomfort at the beginning of radiotherapy. As the course of treatment increases, side effects gradually accumulate and can include weakness, nausea, vomiting, loose stools and diarrhea are common, and diarrhea will worsen when combined with chemotherapy. The frequency of urination may increase. Some patients may also experience loss of pubic hair, skin changes at the radiotherapy site, and sexual dysfunction. The side effects can slowly disappear or lessen over time after radiation therapy is completed. However, symptoms of rectal irritation or bladder irritation may persist. If these symptoms appear, you should tell your doctor and receive treatment in time. Indications for radiotherapy 1. Radiotherapy is not recommended for stage I rectal cancer. However, radical surgery is recommended after local resection with one of the following factors; if refusal or inoperability to operate, postoperative radiotherapy is recommended. (1) postoperative pathological stage of T2; (2) tumor maximum diameter >4cm; (3) tumor occupying >1/3 of the bowel circumference; (4) hypofractionated adenocarcinoma; (5) nerve invasion or vascular aneurysm embolus; (6) positive cut margin or tumor <3mm from the cut margin. 2.clinical diagnosis of stage II/III rectal cancer, preoperative radiotherapy or preoperative simultaneous radiotherapy is recommended. 3.Pathologically diagnosed as stage II/III rectal cancer after radical surgery, if preoperative radiotherapy is not performed, postoperative synchronous radiotherapy must be performed. 4.For locally advanced inoperable rectal cancer (T4), preoperative simultaneous radiotherapy must be performed, and after radiotherapy, re-evaluated for radical surgery. 5.Stage IV rectal cancer: for resectable or potentially resectable stage IV rectal cancer, chemotherapy ± radiotherapy of the primary lesion is recommended, and reassessment of resectability after treatment; palliative reduction radiotherapy is performed for metastases if necessary. 6.Local recurrence of rectal cancer: for patients with resectable local recurrence, surgical resection is recommended before considering whether to perform postoperative radiotherapy. For patients with unresectable local recurrence, if they have not received pelvic radiotherapy in the past, it is recommended to perform preoperative simultaneous radiotherapy, reassess after radiotherapy, and strive for surgical resection. What is the extent of radiotherapy exposure (target area)? Radiotherapy for rectal cancer must include: 1. High-risk recurrence area of primary tumor including tumor/tumor bed, rectal mesenteric area and presacral area, and target area of low to medium rectal cancer should include sciatic rectal fossa. 2.The regional lymphatic drainage area includes the lymphatic drainage area of the common iliac vessels in the true pelvis, the rectal mesenteric area, the lymphatic drainage area of the internal iliac vessels and the closed lymph node area. 3.If there is tumor and/or residual, whole pelvic irradiation followed by local reduction of field for additional irradiation. 4.Radiotherapy for recurrent pelvic lesions: if there is no previous history of radiotherapy, radiotherapy for recurrent tumors and high-risk recurrent areas is recommended, and local additive radiotherapy for tumors can be considered. If there is a previous history of radiotherapy, the decision of radiotherapy will be based on the situation. Irradiation dose Whether using conventional irradiation techniques or new techniques such as 3D conformal radiotherapy or intensity-modulated radiotherapy, there must be a clear definition of irradiation dose. For 3D conformal and intensity-modulated radiotherapy, a volumetric dose definition must be applied, and for conventional irradiation, an isocentric dose definition model should be applied. 1. DT 45-50.4 Gy at 1.8-2.0 Gy per session for 25-28 sessions is recommended for areas of high risk of recurrence of the primary tumor and regional lymphatic drainage areas. Conventional split irradiation is recommended for locally advanced inoperable rectal cancer. For preoperative radiotherapy with 25 Gy/5 times/1 week or other dose splitting, the effective biological dose must be ≥30 Gy. 2. For those with tumor and/or residuals, whole pelvic irradiation followed by local field reduction with additional DT 10-20 Gy. Chemotherapy regimen and sequence for synchronous radiotherapy 1. 5-FU or capecitabine is recommended as the base regimen. 2. Sequence of postoperative radiotherapy and adjuvant chemotherapy. After radical surgery for stage II-III rectal cancer, the sandwich treatment mode of synchronous radiotherapy followed by adjuvant chemotherapy or 1-2 cycles of adjuvant chemotherapy, synchronous radiotherapy and then adjuvant chemotherapy is recommended.