Histiocyte-like cells expressing factor XIIIa do not belong to the neoplastic cell population in malignant fibrous histiocytoma

The term malignant fibrous histiocytoma (MFH) is widely used for pleomorphic soft tissue sarcomas without a specific line of differentiation. MFH is included in the category of fibrohistiocytic soft tissue tumors. MFH has a broad range of histological appearances, and it has several subtypes. All of these subtypes are composed of spindled fibroblast-like cells, undifferentiated cells, and histiocytic or histiocyte-like cells. A large number of fibroblast-like and pleomorphic cells express factor XIIIa in MFH. The cytological pleomorphism of factor XIIIa cells suggests that these cells may belong to the neoplastic population. It is equally possible that the factor XIIIa-positive cells are only activated stromal cells. The relation of factor XIIIa-positive cells to the neoplastic cell population in MFH is addressed in the present study. A morphometric approach compares the measure of nuclear pleomorphism of the factor XIIIa-positive cells with that of the factor XIIIa-negative tumor cells in high-grade MFH. The immunohistochemical approach compares the factor XIIIa-positive and -negative cell populations with regard to mutations of p53 tumor suppressor gene in p53-positive MFH cases. We selected 58 cases of soft tissue pleomorphic or storiform–pleomorphic MFH on the basis of histopathological examinations. A combination of incident light immunofluorescence for factor XIIIa and transmitted light examination for nuclear staining was used for morphometrical analysis. We found cytoplasmic factor XIIIa positivity in at least 2% of cells in 39 cases; the number of factor XIIIa-positive cells was under 0.5% in two cases, and the number of factor-positive cells ranged between 0.5% and 2% in 13 cases. Eighteen cases were analyzed with nuclear morphometry. We found that mean nuclear area and mean nuclear Ferret diameter in factor XIIIa-positive cells differed significantly from those of the tumor cells in all cases. The mean nuclear roundness factor differed significantly only in four cases. The latter finding showed that the microscopic polymorphism of factor XIIIa cells is measurable and is not merely a suspicion. The immunohistochemical positivity for p53 positivity can be accepted as the manifestation of a missense mutation of TP53 gene and as a marker of neoplastic cells. The simultaneous immunohistochemical detection of factor XIIIa and p53 in the same section revealed that factor XIIIa-positive cells were invariably p53 negative in MFH. This finding implies tha