Reticulum cell fleshy tumor (primary reticulum-cell sarcoma of bone) is a primary bone tumor originating from the bone marrow or cancellous bone. Although its morphology is consistent with that of diffuse lymphoma-type reticulum cell fleshy tumor, they are two distinct diseases. Reticulum cell fleshy tumor presents as a localized, solitary bone lesion, accounting for 5% of primary malignant tumors. The peak incidence occurs between the ages of 20 and 30, with rare cases in children. Fifty percent of cases occur in individuals under 40, though cases above 40 are also observed. The male-to-female ratio is 2:1. While any bone can be affected, the most common sites are the femur, tibia, and humerus. One-third of cases occur around the knee joint. The humerus accounts for approximately 9%, and flat bones such as the pelvis, scapula, and spine are also frequently involved.

bubble_chart Pathological Changes

Histologically, it is difficult to distinguish lymphoma-type reticular cell fleshy tumor, or even impossible to differentiate. The main component consists of sheets of cells with relatively large, spherical nuclei, occasionally showing nucleoli. Mitotic figures are easily found. Some cells have distinct, sharp boundaries, abundant pale cytoplasm, and nuclei that may exhibit serrated outlines. The intercellular reticular fibers appear to encircle the cells, and the characteristic feature is their negative silver staining for reticular fibers. However, some tumors may lack reticular fibers, so diagnosis cannot rely solely on this feature.

In some cases, there is diffuse cellular proliferation, and these cells may be poorly differentiated lymphocytes or lympho-histiocytes, with nuclei typically larger and rounder than those of Ewing's fleshy tumor, and cytoplasm more clearly outlined than the latter. However, Ewing's fleshy tumor lacks reticular fibers and contains glycogen in the cytoplasm for differentiation.

The main symptom is localized pain, which is not relieved by rest, but some patients only experience mild intermittent pain. The clinical characteristic is that despite severe bone destruction, the general condition remains good. Although pathological fractures are the most common in malignant bone tumors in this disease. Vertebral collapse can cause severe neurological dysfunction. However, the natural history indicates that its prognosis is far better than that of diffuse reticular cell fleshy tumors. This tumor tends to metastasize to other bones, regional lymph nodes, distant lymph nodes, liver, spleen, and kidneys. Lung metastases are not common but do exist.

**X-ray findings**: There are no specific features, although they are diverse. Typically, the main manifestation is focal bone destruction, whether in long bones or flat bones, occurring in the medullary cavity or cancellous bone, presenting as penetrating, speckled destructive areas that merge to form large osteolytic defects, eventually invading and perforating the cortex. Metaphyseal lesions occur earlier than diaphyseal lesions, and when the cortex is eroded or perforated, the medulla shows more extensive infiltration. Bone membrane reactions may be visible, but they are not as pronounced as the onion-skin-like changes seen in Ewing's fleshy tumors. In some cases, the only X-ray manifestation is a layered or sunburst-like bone membrane reaction. Reactive new bone formation can be very prominent, or the tumor may appear as a completely destructive focus.

bubble_chart Treatment Measures

The prognosis of reticular bone cell tumors is better than that of diffuse sexually transmitted disease with extensive bone involvement. Radiotherapy is generally recommended as the primary treatment, covering the affected bones, joints, and adjacent soft tissues, supplemented by chemotherapy. Unless the tumor is massive and unresponsive to radiotherapy, destructive surgery is usually unnecessary. Due to the involvement of the meninges, some also advocate for prophylactic intrathecal chemotherapy.