Small cell malignant tumors are a group of malignant tumors that are not easily distinguishable by light microscopy, not easily diagnosed by tissue origin, and have a predominantly small round cell morphology. Because each of these tumors has different biological characteristics, treatment and prognosis are also different, so the application of advanced pathological methods to further classify them. It has not only academic significance, but also practical value. Malignant small round cell tumor (MSRCT) is a rare tumor, which has been clearly defined in the last decade, as a group of tumors with similar cell morphology and diffuse histological structure. The application of immunohistochemistry provides a new diagnostic method for diagnosing these pathomorphologically difficult tumors with a clear origin and improves the diagnostic accuracy. For example, undifferentiated cancer cells are difficult to distinguish from malignant lymphoma when they appear as round cells, diffusely distributed without nesting, especially when they metastasize to lymph nodes making them structurally disrupted. The combination of epithelial markers (KET, EMA or CEA) and LCA, with a positive result of the former, can confirm the carcinoma originated from epithelial tissue. I. Types and immunohistochemical characteristics of malignant small round cell tumors Neuroblastoma, olfactory neuroblastoma, primitive neuroectodermal tumors, and pulmonary neuroendocrine carcinoma are characterized by positive NSE and negative Des; undifferentiated carcinoma with positive KET, EMA, and CEA; embryonal rhabdomyosarcoma with positive MG, Vim, and Des reactions; Ewing’s sarcoma with positive Vim and in some cases LCA is a specific marker for malignant lymphoma; HCG and AFP can be used as markers for seminomatous cell tumors. The results are as follows: 22 cases of small cell malignant tumors were analyzed clinically, morphologically and by various immunohistochemical and electron microscopic observations, and a scientific pathological classification of small cell malignant tumors was performed. The results were as follows: 22 cases were divided into 4 groups: (1) Ewing tumor and neuroendocrine tumor group 9 cases (3 cases of intraosseous Ewing tumor, 1 case of extraosseous, 1 case of Askin tumor, 1 case of primitive neuroectodermal tumor, 1 case of nodular neuroblastoma and 2 cases of neuroendocrine carcinoma); (2) 8 cases of embryonal rhabdomyosarcoma; (3) 3 cases of extra nodal malignant lymphoma; (4) abdominal One case each of intra-abdominal sclerosing small round cell tumor and small cell malignant mesothelioma. It is concluded that there are many types of small cell tumors, and their diagnosis should be closely integrated with clinical practice and supplemented with various special examinations. Small round cell tumors of bone The incidence of bone tumors with small round cells as the main feature is low, and most of them are highly malignant tumors. Therefore, it is difficult to accumulate experience in diagnosis (especially in making frozen sections to determine the origin, benignity and malignancy of the tumor), and often feel helpless. These tumors generally include: 1) Ewing sarcoma; 2) primitive neuroectodermal tumor of bone; 3) metastatic neuroblastoma; 4) malignant lymphoma: malignant lymphoma originating in bone may be associated with cortical invasion and adjacent soft tissue involvement, but without simultaneous involvement of lymph nodes and viscera, and immunological studies have shown that “primary reticulocytic sarcoma” is actually malignant lymphoma. Immunological studies have shown that “primary reticulocytic sarcoma” is in fact a malignant lymphoma, so this name is no longer used. Malignant lymphoma of bone can also be divided into two categories: Hodgkin’s and non-Hodgkin’s malignant lymphoma, and Hodgkin’s lymphoma originating from bone is extremely rare, and most of them are non-Hodgkin’s malignant lymphoma. 5. Plasmacytoid myeloma: Also known as multiple myeloma, it is a malignant tumor characterized by multiple or diffuse bone involvement composed of plasma cells with different degrees of differentiation. Highly differentiated myeloma needs to be differentiated from reactive plasma cell hyperplasia, while poorly differentiated plasmacytoma or pleomorphic myeloma needs to be differentiated from malignant lymphoma, and the prognosis of myeloma is related to the extent of lesions and the differentiation of tumor cells. 6.Other bone tumors and bone lesions containing small round cells: some primary or secondary bone tumors such as small cell osteosarcoma, mesenchymal chondrosarcoma, embryonal rhabdomyosarcoma, undifferentiated synovial sarcoma and small cell carcinoma bone metastasis can contain small round cells, especially when the tumor tissue is biopsied, only small round cells can be seen, which are not easily distinguished from the above small round cell tumors. In this case, it is not easy to distinguish from the above mentioned small round cell tumor of bone. Some benign bone lesions, such as chronic osteomyelitis and eosinophilic granuloma of bone, are sometimes not easily distinguished from small round cell tumors of bone in terms of clinical and radiographic manifestations, although these lesions may mainly consist of small and medium round cells histologically, their cellular components are diverse and show obvious benign morphological manifestations, which usually do not cause diagnostic difficulties. Desmoplastic small round cell tumor (DSRCT) is a rare malignant tumor that has only been recognized in recent years. This tumor was first reported by Gerald and Rosai in 1989 and is characterized by invasive and disseminated growth along the plasma membrane, a histological structure composed of nested small cells and sclerotic mesenchyme, and immunohistochemistry with a complex expression of specific epithelial, mesenchymal and neurological markers. Initially, different names were reported in the literature, including: sclerosing small cell tumor with anisotropic differentiation, intraperitoneal sclerosing small round cell tumor, malignant small cell epithelial tumor of the peritoneum expressing mesenchymal intermediate filaments, peritoneal sclerosing small round cell tumor with anisotropic differentiation, pediatric abdominal neuroectodermal tumor with anisotropic differentiation, and sclerosing small cell tumor with multidirectional differentiation.