Inflammatory cells in the pseudocapsule of osteosarcoma. A clinicopathologic analysis

The presence of a "pseudocapsule," composed of an inner compression zone and outer reactive zone, is a frequent finding in the periphery of osteosarcoma. Morphologic and immunohistochemical methods were used to determine the type of inflammatory cell most closely related to the formation of this pseudocapsule. Six and five specimens with and without a compression zone, respectively, were obtained from ten patients with osteosarcoma. The type and number of inflammatory cells in the specimens with and without a compression zone were counted and compared. The reactive zone contained mast cells, macrophages, fibroblasts, and lymphocytes with a predominance of T-cells. Macrophages were significantly increased in number in the specimens with a compression zone. These results strongly suggest that macrophages play a significant role in the formation of the compression zone. The survival rate of the patients with thick encapsulation was markedly better than that of the patients without this feature.