Tumors of the rectum mainly originate from the mucosal epithelium, i.e., rectal cancer as described above, while tumors from mesenchymal tissues including fat, muscle, mucosa-associated lymphatic tissue, and vascular endothelium are rare. Our hospital summarized 12 cases of rectal mesenchymal tumors from January 1995 to June 2002, accounting for 2.1% of all rectal tumors, including malignant nerve sheath tumor, melanoma, smooth muscle tumor, smooth muscle sarcoma, and so on. Our further examination revealed that most of them were rectal mesenchymal tumors. The same problem was reported in the literature that tumors originally thought to be of smooth muscle or vascular origin were eventually confirmed to be mesenchymal tumors. The concept of mesenchymal tumor has been developed and accepted with some process and practical implications. The term “mesenchymal tumor”, which is generally referred to as gastrointestinal stromal tumors (GIST), refers to mesenchymal cell tumors that originate in the gastrointestinal tract and abdominal cavity, accounting for approximately 1% of all gastrointestinal tumors. Rectal mesenchymal tumors account for about 5% of GIST and about 80% of rectal mesenchymal tissue tumors. Originally, it was thought that rectal non-epithelial tumors are diverse and complex in origin, and their diagnosis and treatment mostly rely on direct surgery and lack of effective adjuvant therapy after surgery. In recent years, it was found that GIST, which accounts for the vast majority of rectal mesenchymal tissue tumors, characteristically exhibits positive CD117 (c-Kit) staining, and thus can be specifically bound by the tumor-targeting drug Gleevec to inhibit its function, and 80% of patients achieved good clinical results, which greatly changed its treatment strategy. I. Mechanisms of rectal mesenchymal tumor c-Kit is a tyrosine kinase transmembrane receptor protein encoded by the c-Kit proto-oncogene located on chromosome 4q11 to q12. The ligand is stem cell factor (SCF), which interacts with c-Kit outside of the cell membrane, causing the latter to become a dimer, while the intracellular tyrosine residues of c-Kit protein are phosphorylated by SCF, which activates c-Kit. this signal is transmitted downstream to phosphorylate cellular substrates, stimulating cell proliferation and enhancing cell survival. . Almost all GISTs express c-Kit protein, and mutated c-Kit genes also retain the property of expressing c-Kit protein. Clinical characteristics of rectal mesenchymal tumors The gender difference between male and female patients with rectal mesenchymal tumors in our data is not obvious, and the age can range from 27-69 years old, with a median age of 59.5 years. There is no specific clinical symptom, early manifestation is blood in stool, rectal swelling, change of stool habit, and change of stool shape after tumor enlargement, and even pain and discomfort in perineum after invasion of surrounding tissues. About 1/3 of the patients have no clinical symptoms, and most of them are found by routine physical examination, endoscopy and imaging examination. Since the mass is usually close to the anus (2-9 cm), it can be palpated during rectal examination. Therefore, rectal palpation is an examination method that should not be neglected. Rectal GIST is less invasive, so it can grow to a larger size. In one patient in our group, the mass was more than 12 cm in diameter, but still had an intact envelope with clear borders and did not invade the adjacent organs. The literature reports that 10% to 40% of the masses have infiltrated the surrounding tissues, thus the recurrence rate after resection is high, up to 40% to 80%. During resection of the tumor, the tumor envelope may rupture and cause implantation. Recurrence is mostly localized or in the liver, but also occurs in the abdomen, pelvis, lung, mediastinum, etc. Lymph node metastasis is rare. Lymph node metastasis is rarely seen. At present, there are many imaging methods for rectal mesenchymal tumor, including general X-ray (barium enema), ultrasound (conventional ultrasound, intracavitary ultrasound, ultrasonography), fiberoptic endoscopy, convergent endoscopy, CT, MRI, selective angiography (DSA) and even positron emission tomography (PET). It is necessary to choose according to the characteristics and sensitivity of various examination methods. 1.Conventional X-ray examination Mainly barium enema examination. Rectal mesenchymal tumors tend to grow distensibly and protrude into and out of the lumen, manifesting as mucosal elevation, sometimes with mucosal destruction and ulcer formation, and the appearance of sinus tracts in cases of necrosis. The main imaging manifestations are mucosal compression, narrowing of the intestinal lumen, or filling defect, mucosal destruction or ulcer formation on the surface, and occasionally liquid air flat. 2.CT examination CT scan reveals that the tumor is mostly round or round-like, and a few are irregular in shape. Benign tumors are mostly small in size, uniform in density and sharp in edge. It rarely invades adjacent organs and may have calcification. Malignant tumors are more than 5 cm in size, and the boundary is sometimes unclear and may adhere to adjacent organs. Sometimes the tumor is lobulated, with heterogeneous density. The tumor is prone to necrosis, cystic degeneration and hemorrhage, and has a mixed density (Figure 4). Enhancement is mostly uniform to moderate or obvious, and more obvious in venous stage. The larger tumors with necrosis and cystic changes often show obvious strengthening of the tumor peripheral entities. 3.MRI examination The MRI performance of mesenchymal tumor is relatively more complicated. The T1-weighted signal of small benign tumors can be similar to that of muscles, and the T2-weighted signal is uniform or slightly high, with clear borders; for larger tumors, necrosis, cystic degeneration and hemorrhage within the tumor can be uneven in T1 or T2-weighted images. It needs to be analyzed according to the situation. 4.Selective angiography When the tumor is small, the angiography shows: the boundary of the tumor is clear, the blood supply artery is slightly thickened, and the blood vessels around the tumor are in the shape of holding balls. When the tumor is enlarged or malignant, the blood supplying arteries are obviously thickened, the blood vessels are increased and disordered, interrupted, and may be in the shape of disorderly hair or spider web, and the edges of some blood vessels are blurred. Sometimes, the central contrast pool of the tumor is obvious, and sometimes the blood vessels are reduced or missing, which may be due to the central necrotic liquefaction of the tumor. 5.PET is currently the best method to detect tumor at the functional level. It can show the degree of tumor activity and is especially promising in identifying benign and malignant tumors. At present, it is more expensive, but promising. In conclusion, the above imaging methods are not specific for mesenchymal tumors. It should be noted that CT, MRI, and ultrasound are better to judge the possibility of resection before surgical resection, while CT, MRI, and ultrasound, especially PET, are better to observe the efficacy before and after treatment, in order to understand the change of tumor load on the one hand, and the loss or reduction of tumor activity on the other. The cell morphology of rectal mesenchymal tumor is divided into spindle cell type, epithelial-like cell type and mixed type. The spindle cell type is the most common, accounting for more than 90% of the cases. However, it is impossible to distinguish rectal GIST from smooth muscle-derived tumors and nerve sheath tumors purely from morphology. In isolated cases, epithelioid cell type mesenchymal tumors can be confused with hypofractionated adenocarcinoma. To establish the diagnosis, immunohistochemical testing is necessary. The most diagnostic value is a positive CD117 expression. positive CD117 expression and cellular features on light microscopy are the most important basis for the diagnosis of GIST. positive CD34 expression is very useful for the diagnosis. Only a very small number of tumors with negative CD117 expression can be diagnosed as GIST when there is a mutation in the c-Kit gene or an alteration in the PDGFRα gene. The vast majority of GISTs express Vimentin and CD34, but some have myogenic or neurogenic expression, as evidenced by positive expression of SMA and S-100. nestin can be positive, but the positive rate is only 8-12%, while smooth muscle tumors and nerve sheath tumors are negative. desmin is rarely expressed. The positive rates of Vimentin and CD34 in our group were 100% and 83.3% (5/6), respectively. 2 cases showed focal positive expression of S-100, 4 cases were positive for NSE, and no positive SMA expression was seen. There are different criteria for determining the biological behavior of GIST. However, most scholars believe that GIST is not truly benign and all have the possibility of metastasis. However, they can be distinguished as highly malignant, moderate, low-grade malignant and very low-grade malignant. Those with a previous tumor diameter of more than 5 cm, fresh tumor necrosis and extensive hemorrhage caused by non-surgical procedures, abundant cellular components, marked cellular anisotropy, and a high prevalence of splitting images are considered to be highly malignant. Recently, Miettinen suggested that only the tumor size and the number of mitotic images are valuable, while tumor necrosis, porosity, and tumor vascular distribution are not of much value, nor is the intensity of CD117 staining. However, if there are c-Kit mutations in the tumor surrounding tissues, the possibility of high malignancy is high. V. Treatment of rectal mesenchymal tumor 1. Surgery Surgery is still the most effective treatment for rectal mesenchymal tissue tumors including GIST. The majority of patients in this group received surgical treatment. 5 patients had tumor resection through the anus, 1 case had tumor resection through the sacral approach, 2 cases received combined abdominoperineal resection, 1 case underwent bilateral oophorectomy due to ovarian metastasis; 2 cases underwent Dixon’s operation, respectively, oophorectomy and vaginal wall resection at the same time, and 1 case underwent modified Bacon’s operation. The study found that the median survival of those with positive margins or unresectable was only 9 to 12 months. Those with large tumors and high malignancy were prone to recurrence even after complete resection. The median time to tumor recurrence is about 7 months to 2 years, and recurrence is unavoidable after a second surgery. Previously, there was no effective treatment for inoperable resectable and post-surgical recurrent metastatic tumors. In contrast, the current use of Gleevec in locally progressive stromal tumors often results in tumor shrinkage and facilitates complete surgical resection. We have a young patient with a tumor 3 cm from the anal verge and a tumor with a diameter of 5 cm. This patient had a significant reduction in tumor diameter to 2.5 cm after 3 months of oral Gleevec, which was excised locally and completely via the anus, with no recurrence at 1 year and 6 months of follow-up. The literature reports that complete remission is rare in patients taking Gleevec. Even when effective, the tumor remains somewhat active. Therefore, the main objective of neoadjuvant chemotherapy with Gleevec for mesenchymal tumors is still to improve the surgical resection rate. 2. Drug therapy The recently developed oral drug Gleevec is a tyrosine receptor inhibitor that specifically binds c-Kit. clinical studies have found it to have better efficacy against rectal GIST. In a phase II clinical trial, the efficiency rate was over 80% (40.1% PR, 41.5% SD). Shen Lin et al. from the School of Clinical Oncology, Peking University reported that the efficiency of 30 patients with gastrointestinal mesenchymal tumors treated with Gleevec (200-600 mg/day) was 58.3%. There is no uniform dose standard for Gleevec treatment, but 400 mg/day is safer and more effective. Some studies have found that increasing the dose of the drug in patients with ineffective or progressive disease at these doses can still be effective. Clinical studies are currently underway in China, and the doses are smaller than those in European studies, but the efficacy is positive. Common side effects of Gleevec are edema, nausea and vomiting, abdominal pain, weakness, rash or skin flushing, and bleeding. Most of them are mild or moderate. Overall, the drug has a good safety profile. 3.Other treatments After liver metastases appear in rectal mesenchymal tumor, selective angiography can be considered, and liver metastases with rich blood supply can be embolized. Interventional treatment of the primary tumor has not been reported. Other treatments such as radiation therapy and immunotherapy have not been proven to be effective and are generally not used clinically. Prognosis of rectal mesenchymal tumor Because of the low incidence of rectal mesenchymal tumor itself, there are few studies on prognosis. The median survival time of 6 patients in our group with 66 months of follow-up was 23 months. However, the number is too small. Mostly, the survival of the whole gastrointestinal mesenchymal tumor is evaluated and estimated. Previous literature reported a median survival of 31 months. Prognostic factors regarding mesenchymal tumors are currently considered to include stage of disease at diagnosis, tumor size, nuclear split image count, surgery, genetic mutations in c-Kit and PDGFA, use of Gleevec, and method of use.