The patient is an 86-year-old male who was admitted to the hospital with the main reason of “swelling in the left thigh for 1 year, aggravated for 1 month”. The patient had a history of lumbar spine tuberculosis 30 years ago and was treated with medication. The patient had no progressive wasting, weakness and night sweats. Physical examination: swelling in the upper middle part of the left thigh, subcutaneous veins were visible, local skin temperature was higher than the opposite side, tension was high, and there was no pressure pain. There was no obvious abnormality in the activity of the left lower limb, and mild depressed edema in front of the left tibia. Laboratory tests: albumin 18.20g/l, sedimentation 113mm/h, C-reactive protein 26.7mg/l, tumor markers, etc. did not show any significant abnormalities. X-Ray and CT of femur both showed soft tissue swelling shadow in left thigh, no bone invasion was seen. No primary lesions or metastatic lesions were found in the ultrasound of the heart and CT of the chest. Ultrasound and MRI of the left femur suggested that a cystic predominantly cystic solid mass was seen in the soft tissue of the left mid-thigh, and the lesion was enveloped. Consider: mucinous fibrosarcoma? Mucinous liposarcoma? Intraoperatively, the patient saw a compartmentalized cystic mass between the quadriceps muscle of the left thigh, measuring about 25.5*16.5*18.6, weighing about 5.0 Kg, with an intact envelope and a descending branch of the lateral spinous femoral artery penetrating into the mass. In this case, no enlarged resection was performed, and a large amount of yellowish fluid was seen after dissection, with grayish-white filthy-like material inside the swelling. The patient recovered well after surgery, and the pathological diagnosis: malignant fibrous histiocytoma. Discussion Malignant fibrous histiocytoma of soft tissue is one of the common malignant tumors in middle-aged and elderly people. It is a malignant tumor with fibroblasts and histiocytes as the basic cells originating from the primitive mesenchymal cells of soft tissue, with complex composition and variable morphology, and high morbidity and mortality. The disease is mostly seen in men aged 50 to 70 years old, and it is a malignant tumor that occurs in the limbs and the latter of the peritoneum. About 90% of the lesions are deep, mostly subfascial, and usually without painful symptoms. The time from the onset of swelling to the establishment of the diagnosis usually varies from several months to several years. Currently, it is generally accepted that surgical resection is the most effective treatment for primary malignant fibrous histiocytoma. Early and complete surgical resection is often used, and because malignant fibrous histiocytoma is highly malignant and lymph node metastatic, extensive regional lymph node dissection should be performed whenever possible [2]. The situation of surgical margins is an important prognostic factor affecting survival, and the ideal surgery should be performed with extensive resection including 2 to 125 px outside the margins, striving to achieve a residual negative microscopic tumor margin (R0) to achieve a better local control rate. The extent of surgical resection for malignant fibrous histiocytoma of the extremities is controversial. Limb-preserving surgery has shown a trend toward higher local recurrence compared with amputation. However, the 5-year disease-free survival and overall survival rates are approximately the same. In practice, however, the extent of resection 50px beyond the incisional margin is often limited by the adjacent vital tissues such as neurovascular vessels. In this case, the mass was adjacent to the sciatic nerve and femoral artery, and limb-sparing surgery was not possible to perform 50px beyond the incisional margin. Malignant fibrous histiocytoma is highly malignant, prone to recurrence and metastasis, and has a poor prognosis. The literature reports a 5-year survival rate of 50% [4]. Many clinical studies at home and abroad have shown that the pathological type, tumor size and stage, and treatment modality are the main factors affecting the prognosis of patients. Zheng et al [5] analyzed the main factors affecting the prognosis of 113 cases of soft tissue sarcoma after surgery; Cox multifactorial analysis also suggested that the 5-year survival rate was higher when the tumor was located in the extremities than in those with a non-extremity origin; univariate analysis showed that the prognosis of those >60 years old was better than those ≤60 years old. Tumors in superficial sites have a higher recurrence rate than deeper tumors, and the 5-year survival rate is higher for tumors in the distal limb than in the proximal limb. Some studies reported that tumor depth is an important risk factor for soft tissue sarcoma. Tumors located under the fascia are prone to local recurrence with poor prognosis, and local recurrence can cause increased depth of tumor infiltration. In addition multiple recurrent resections can cause tumor cell dedifferentiation and increase the chances of re-metastasis [6]. The patient in this case requires continued follow-up.