Data The patient, male, 63 years old, was admitted to the hospital with “chest wall pressure pain in left lateral recumbency for more than three months”. The patient had localized chest wall pressure pain when lying on the left side without obvious cause three months ago, and a half spherical mass with a diameter of about 2cm×6cm could be found on the left chest wall, which could be pushed up and down, with slightly poor mobility, accompanied by local pressure pain, no local skin redness and cellulite-like changes, no fluctuating sensation, and chest CT showed: soft tissue mass of the left anterior peroneal muscle. On admission, the CT scan showed that the left anterior cohesive muscle was an oval soft tissue mass with a size of about 42.5px×135px, which was not clearly demarcated from the muscle, and the adjacent rib bone was not abnormal. The ESR was 20 mm/hr, and the rest of the ancillary tests did not show any abnormality. After exclusion of contraindications to preoperative surgery, “chest wall mass excision” was performed under general anesthesia, and a nodular, grayish-yellow mass, 5 cm × 4 cm × 3 cm in size, without obvious envelope, with grayish-yellow and grayish-brown interspersed on the cut surface and soft in texture, was examined. Intraoperative rapid pathology showed malignant spindle cell tumor (sarcoma). The lymph nodes of the chest wall, the muscles around the mass, the ribs and the wall pleura were enlarged and the chest wall defect was repaired with a four-layer compound polyester patch. Postoperative pathology (Figure 2): pleomorphic sarcoma, which was considered to be dedifferentiated liposarcoma (with a dedifferentiated component of malignant fibrous histiocytoma) in combination with immunohistochemical marker results. Immunohistochemical labeling of tumor cells showed: CKpan-, SMA-, Des-, Vim+++, S-100 scatter+, HMB45-, CD34-, Ki-67+ about 30%. Discussion Evans et al [1] first introduced the concept of dedifferentiated chondrosarcoma to the classification of liposarcoma in 1979, introducing the concept of dedifferentiated liposarcoma (DL). It was defined as a malignant adipocytic tumor migrating from an atypical lipoma-like tumor/highly differentiated liposarcoma (ALT/WDL) to a differentially differentiated non-adipose spindle cell sarcoma (dedifferentiated component) [2]. Their specimens on macroscopic examination are often large multinodular yellow or grayish-yellow masses containing scattered solid, often grayish-brown nonfatty areas (dedifferentiated areas). The dedifferentiated areas are accompanied by necrosis, and there may be a gradual migration transition between the fatty and dedifferentiated areas. The amount of dedifferentiated components generally varies microscopically and is often an abrupt transition between the two components, but in some cases it can also gradually migrate and sometimes a mixture of the two components can be seen [3,4], most commonly in unclassified malignant fibrous histiocytoma-like pleomorphic sarcomas and intermediate highly malignant mucinous fibrosarcomas. DL generally has the most common retroperitoneal involvement and less common soft tissue development in the extremities [5]. It is usually a large painless mass, but in this case, there was localized pressure pain when lying on the side, with a history of about three months and a sudden feeling of recent enlargement and significant pressure pain, suggesting dedifferentiation. It is often misdiagnosed as other benign tumors of the chest wall or chest wall tuberculosis, etc. The clinical features of this case are consistent with its gross morphology, microscopic examination and previous experience. This tumor has no specific imaging manifestations, and the diagnosis mainly relies on pathology. Understanding and being familiar with these morphological features of the dedifferentiated region in DL is important for timely and correct differentiation of DL from other pleomorphic sarcomas, reducing misdiagnosis and assessing patient prognosis. In the surgical treatment of chest wall tumors, clarifying the benignity and malignancy of the tumor is the key to treatment [6]. General preoperative puncture biopsy is an important way to obtain pathological diagnosis, but its correct diagnosis rate is low. In this case, the patient had no previous history of primary tumor and no preoperative pathologic diagnosis to support it, so firstly, he should undergo a restrictive radical surgical resection and then undergo a second-stage extended radical surgery after the intraoperative rapid pathologic diagnosis to clarify its malignancy. In this case, both liposarcoma and malignant fibrous histiocytoma components were present in the tissue, which is consistent with the microscopic features of dedifferentiated liposarcoma. In order to reduce postoperative recurrence, the adjacent tissues or even organs can be combined with resection. In this case, the scope of surgical resection included ribs, periosteum, wall pleura, muscles, vascular nerves, soft tissues and regional lymph nodes, and the chest wall defect was repaired with a four-layer compound polyester patch at the same time. The 5-year survival rate of dedifferentiated sarcoma is only 20%, and the long-term results are not very satisfactory because of its high local recurrence rate and distant metastasis rate, so it is necessary to emphasize the multidisciplinary comprehensive treatment based on radical surgical enlargement resection and post-discharge follow-up.