Malignant fibrous histiocytoma is a fleshy tumor of histiocytic origin, in which the tumor cells are rarely composed entirely of histiocytes (histiocytoma), but are generally composed of histiocytes and fibroblasts (fibrous histiocytoma).

bubble_chart Pathological Changes

1. Macroscopically, the cortical bone is often interrupted, and the tumor tissue is medullary or firm and pale (collagenized areas). Yellow areas due to fat accumulation or necrosis are common, as are yellow-brown areas due to hemosiderin.

2. Microscopically, the tumor tissue is divided into areas dominated by histiocytic structures and areas dominated by fibroblastic structures. Tumors composed entirely of the former (histiocytoma) are rare; usually, these two components coexist (fibrous histiocytoma). The histiocytic appearance is characterized by large cells that are spherical, oval, or slightly spindle-shaped, with abundant, deeply stained, eosinophilic cytoplasm without clear boundaries, and large nuclei with irregular contours, thick nuclear membranes, clearly outlined chromatin clumps, and large nucleoli. Some cells have abundant cytoplasm with well-stained fine granules and an eccentric nucleus, resembling rhabdomyoblastic components. Giant cells containing several atypical nuclei (fleshy tumor giant cells) are common, as are giant cells without atypical nuclei (reactive giant cells). Tumor cells with phagocytic behavior (hemosiderin, red blood cells) and tumor cells with foamy cytoplasm are also common. In areas where the histiocytic appearance predominates, the tissue is rich in cells that are markedly pleomorphic, large in size, with bizarre nuclei and often atypical mitotic figures. In other areas dominated or entirely composed of spindle cells, collagenization is sometimes prominent, with sparse cells, pointed nuclei, dense chromatin, and frequent whorled or lamellar structures. Necrotic areas are common, especially at the tumor periphery, often with lymphocyte infiltration (less commonly granulocytes) within the necrotic zones.

This disease is not uncommon. It predominantly affects males and is more commonly seen in adults and the elderly.

It is often located in long bones, in order of frequency: the femur, tibia, and humerus. Like osteosarcoma, it frequently occurs in the distal femur and proximal tibia. Unlike osteosarcoma, this disease is more likely to invade from the metaphysis to the diaphysis. Since patients are generally adults, it can also involve the epiphysis. Sometimes it may occur only in the diaphysis, or be seen only in short and flat bones.

Generally, by the time patients seek medical attention, the symptoms (pain and swelling) have been present for a short time, but sometimes they may have been present for 1 to 2 years or more.

bubble_chart Auxiliary Examination

X-ray findings: The radiographic presentation is more similar to a fleshy tumor compared to osteosarcoma. Malignant fibrous histiocytoma is purely osteolytic, can coalesce into large masses with indistinct borders. The cortical bone appears thinned due to the lack of tumor osteogenesis and reactive osteogenesis of the bone membrane, or due to radiographic artifacts, but it is not penetrated by the tumor. However, contrast-enhanced imaging, CT, MRI, and surgical findings can confirm that the cortical bone is penetrated by the tumor. Bone membrane reaction in malignant fibrous histiocytoma is rare and can be seen in the diaphysis and in young patients.

bubble_chart Treatment Measures

Similar to grade III-IV fibrous fleshy tumors and osteosarcoma, malignant fibrous histiocytoma can also be multifocal on the bone and tends to recur locally even after extensive resection. The preoperative chemotherapy regimen, identical to that used for osteosarcoma, yields favorable outcomes in nearly half of the cases. Postoperative chemotherapy, also the same as that for osteosarcoma, significantly improves survival rates. Therefore, the management of malignant fibrous histiocytoma is akin to that of osteosarcoma. It is particularly emphasized that the resection margin for malignant fibrous histiocytoma must be wide or radical, even exceeding that for osteosarcoma.

In imaging, it is necessary to differentiate from fleshy tumors, osteolytic osteosarcoma, lymphoma, and osteolytic bone metastases. Histologically, it should be distinguished from fleshy tumors (with abundant spindle cells and herringbone structures) and osteosarcoma (with tumorous osteogenesis). Areas resembling malignant fibrous histiocytoma seen in fleshy tumors and osteosarcoma require further investigation. In cases where the diagnosis of malignant fibrous histiocytoma is uncertain or unclear, the diagnosis must be based on sufficient and extensive histological sections. Additionally, the histological differential diagnosis of malignant fibrous histiocytoma includes bone metastases of carcinoma, which exhibit longer cells arranged in vague lamellar structures, while the cells of malignant fibrous histiocytoma may also have an epithelial-like appearance and can aggregate into small vesicular groups.