Radiation therapy for rectal cancer
The incidence of rectal cancer is rising sharply in the past 10 years or so, and the causes may be related to the following factors.
1. high-fat, low-fiber diet.
2, elevated levels of nitrosamines in food.
3, malignant transformation of adenomas.
4, schistosome infection.
5, pelvis treated with radiation therapy.
6, chronic ulcerative proctitis.
I. Anatomical and pathological features
The rectum begins at the level of the 3rd sacral vertebra and is connected to the sigmoid colon, and migrates downward through the pelvic diaphragm to the anal canal and ends at the anus. The total length is about 12-15 cm, and the intestinal wall can be divided into mucosal layer, mucosal muscle layer, submucosal layer, intestinal wall muscle layer and plasma layer (there is no plasma layer in the rectum below the peritoneal reflex). Since there are no lymphatic vessels in the mucosal layer, in situ cancer confined to the mucosal layer has no lymphatic metastasis, while once the tumor invades the submucosa, the metastasis rate increases rapidly because of its rich lymphatic vessels and vascular network.
The ways of proliferation of rectal cancer.
1.Direct spread: Rectal cancer is easy to invade around the intestinal cavity such as prostate and bladder and along the axis of intestinal tube. However, the distance of rectal cancer invasion along the upper and lower end of the intestinal tube is not long, usually only 2-3cm.
2.Lymphatic metastasis: lymphatic metastasis is the main metastatic route of rectal cancer. Lymph of rectum above the dentate line mainly drains upward and is injected into the root lymph node of inferior mesenteric artery via superior rectal lymph node and pararectal lymph node. The lymphatic drainage of the rectum below the peritoneal reflex is not only upward, but also to the inferior rectal arteriovenous paravalvular lymph nodes on both sides, and then injected into the internal iliac lymph node route. Since it is difficult to completely clear the lymph nodes in the internal iliac vessels and closed holes of rectal cancer, extra attention should be paid to the scope and dose distribution of this area during radiation therapy.
3.Hematogenous metastasis: Since the mesenteric vessels drain to the portal vein, therefore, hematogenous metastasis of rectal cancer is commonly liver metastasis.
Pathological classification of rectal cancer
(1) Papillary adenocarcinoma;
(2) tubular adenocarcinoma;
(3) Mucinous adenocarcinoma;
(4) Indolent cell carcinoma;
(5) Undifferentiated carcinoma;
(6) Small cell carcinoma;
(7) Adenosquamous carcinoma;
(8) squamous cell carcinoma;
(9) Carcinoid tumor.
Diagnosis
1.Changes in stool habit and shape, such as constipation, diarrhea, urgency, blood in stool, mucus blood stool, thin stool, etc. Some patients can be misdiagnosed as “hemorrhoids” for various reasons.
2.Anal rectal finger examination: Anal finger examination is a simple and easy method which is easily accepted by patients, generally it can detect rectal tumor within 7-8cm from the anus, during finger examination, attention should be paid to the location, size, hardness, basal mobility of the tumor and whether it adheres to the surrounding tissues and organs.
3.Fiber colonoscopy: It is the safest and reliable examination method to diagnose rectal cancer, which can visually observe the size, location, color, ulcer and active bleeding of the tumor, and can take the material and send it for examination to obtain pathological results.
4.CT examination: it can show the degree of thickening of local intestinal wall and the involvement of surrounding tissues and organs such as prostate, bladder, uterus and metastasis of pelvic lymph nodes, which is beneficial to diagnosis and staging, and also provides a basis for choosing treatment plan.
III. Clinical staging
Staging criteria of colorectal cancer in 1984 Suzhou Pathology Conference.
1.The lesion is confined to the mucosa or involves the submucosa.
2.The lesion is infiltrated into the superficial muscular layer.
3.Lesion infiltrates deep muscle layer.
The lesion penetrates the deep muscular layer and infiltrates the plasma membrane layer, extra-plasma membrane or perirectal tissue.
The lesion has lymph node metastasis (including early colorectal cancer with lymph node metastasis.)
IV. Treatment principles
Radical surgery for rectal cancer is currently the preferred treatment for rectal cancer, but the overall 5-year survival rate has been hovering around 50%, and the main reason for failure of rectal cancer treatment is still local recurrence. In the past, the role of radiation therapy for rectal cancer was once neglected due to the rapid development of surgical technology and the perceived insensitivity of rectal cancer to radiation as well as the limitations of radiotherapy equipment and technology. In recent years, as the 5-year survival rate of rectal cancer treated by surgery is always around 50%, people have become more aware of the limitations of single-measure treatment of tumors. Meanwhile, with the update of radiotherapy equipment, the rapid development of radiation technology and radiobiology, people pay more attention to surgery, radiotherapy and radiotherapy. People pay more and more attention to the methods and efficacy of surgery, radiotherapy and chemotherapy in the integrated treatment of rectal cancer. At present, there are more studies on the integrated treatment of surgery and radiotherapy.
V. Radiotherapy
1.Pre-operative radiotherapy
(1) Pre-operative radiotherapy can reduce the volume of tumor, reduce the infiltration of tumor to surrounding tissues and organs, and make the tumor that cannot be removed by original diagnosis possible to be removed, so as to improve the surgical resection rate.
(2) Preoperative radiotherapy can reduce the positive rate of pelvic lymph nodes and decrease the percentage of advanced cases, thus achieving the purpose of stage reduction.
(3) Preoperative radiotherapy can both shrink the primary tumor and kill the surrounding subclinical lesions, thus reducing the rate of distant metastasis.
(4) Preoperative radiotherapy can reduce the local recurrence rate and improve the survival rate. Most scholars believe that preoperative radiotherapy can reduce the local recurrence rate by about 10-15%, while the cure rate of postoperative recurrence followed by radiotherapy or surgery is extremely low. Most of the literature reports that preoperative radiotherapy can improve the overall 5-year survival rate by 10-15%, while some reports suggest that there is no significant difference between the 5-year survival rate of preoperative radiotherapy and surgery alone, which may be related to the following three factors.
(1): Some early cases can be cured by surgical resection alone, and the administration of preoperative radiotherapy does not work for them.
②: Some cases are not accurately diagnosed and metastases of distant subclinical foci have already occurred at the time of treatment.
③: Some patients with rectal cancer die from other secondary diseases rather than from the tumor, and the presence of this factor may also have an impact on survival rates. Therefore, most scholars now believe that: preoperative radiotherapy should not be routinely performed for rectal cancer, but should be studied in randomized groups by stage with precise diagnostic staging. It has been reported that preoperative radiotherapy for rectal cancer at advanced stage (T3, T4) can significantly improve the 5-year survival rate.
2.Intraoperative radiotherapy: Its purpose is to increase the irradiation dose of tumor and reduce the unnecessary irradiation of normal tissues. Because this method needs to be implemented in collaboration with surgery, anesthesiology and radiotherapy departments at the same time, and the operation is complicated, although it has achieved certain efficacy, it is not promoted in clinical practice.
3.Postoperative radiotherapy: It is mainly for cases with incomplete surgical resection and lymph node dissection, cancer residual tumor at the cut edge and tumor that has broken through the plasma membrane layer with high possibility of recurrence. Post-operative radiotherapy can improve the survival rate and reduce the local recurrence rate.
4.Simple radiotherapy: including simple intracavitary treatment, simple external irradiation, combined intracavitary and extracorporeal radiotherapy. It is generally used for radical radiotherapy in some early cases (which must be carefully selected), but it is not often used also for palliative radiotherapy for patients who cannot be operated or relapse after surgery for various reasons and for patients with advanced rectal cancer.
5.Introduction of external irradiation techniques
External irradiation techniques for rectal cancer include four-field cassette technique, three-field technique and two-field technique. According to the principle of radiation therapy dosimetry and long-term clinical practice, the irradiation field should include the primary tumor and regional lymph nodes according to the invasion and lymph node metastasis of rectal cancer. At present, four-field cassette irradiation technique is commonly used.
(1) Anterior and posterior fields: upper boundary: the level of the 5th lumbar vertebra; lower boundary: 1.5cm below the lowest perineum; left and right boundaries: 2cm outside the largest transverse diameter of the true pelvis.
(2) Bilateral field: upper and lower borders are the same as anterior and posterior fields; posterior border: including 0.5cm outside the sacrum; anterior border: 2-3cm outside the anterior edge of the 5th lumbar vertebral body. when the dose of the target area is about 50GY, the lead file should be considered to be applied to protect the femoral head and sacral nerve. Preoperative radiotherapy is generally used 45GY/20-25 times/4.5-5 weeks. Rest 4-5 weeks before surgery treatment plan. Postoperative radiotherapy is generally required to start radiotherapy as soon as the wound heals, and it is routinely required to start with 45GY/20-25 times/4-5 weeks for the whole pelvic cavity, and then to make up the dose according to CT examination and surgical resection for the residual lesion reduction.