Solitary fibrous tumor is rich in factor XIIIa+ dendrocytes.

Abstract

To the Editor: In regard to the interesting paper from Vadmal and Pellegrini (1) on vaginal solitary fibrous tumor (SFT), we present a case of oral mucosal SFT demonstrating the previously undescribed participation of factor XIIIa (FXIIIa)+ dendrocytes. A 52-year-old woman had a 3-year history of a slow-growing painless nodule on the lingual gingiva of the left anterior mandible. This smooth-surfaced submucosal mass, 2 cm × 1.5 cm in size, showed no involvement of periosteum or bone. The patient was free of recurrence 6 years after excision. The well-circumscribed tumor was composed of a haphazard proliferation of bland-looking spindle cells exhibiting alternating hypercellular and hypocellular areas admixed with prominent vascular and stromal hyalinization and a hemangiopericytomatous vascular pattern (Fig. 1). There was no inflammation. As expected, the tumor spindle cells reacted diffusely and strongly with vimentin, CD34, and CD99, although bcl-2 showed focal and weak positivity (Fig. 2A). Notably, FXIIIa was intensely positive in interstitial dendritic cells distributed throughout the lesion (see Fig. 2B). Double immunostaining revealed no significant coexpression of CD34 and FXIIIa by individual cells. Smooth muscle actin, S-100 protein, epithelial membrane antigen, and cytokeratin were negative in both types of lesional cells. Another interesting finding was the presence of numerous mast cells. They stained for toluidine blue and mast cell tryptase.FIG. 1.: (A) Histologic scanning view of a well-circumscribed lobulated tumor. (B) Hypercellular area composed of bland spindle cells and numerous vessels.FIG. 2.: Immunohistochemical staining characteristics of tumor with CD34 (A) and FXIIIa (B).In spite of the expanding spectrum of anatomical involvement, oral mucosal SFT is exceptional (2,3). Our case is unique in its abundant content of FXIIIa+ dendritic cells, in keeping with the immunospectrum of SFT. As shown in Figure 2B, their number is sufficient to be easily picked up at low-power magnification, suggesting a principal component of the tumor. In this regard, its scanning appearance closely recapitulates dermatofibroma (4). To our knowledge, the active involvement of FXIIIa+ dendrocytes in SFT, irrespective of tumor location, has received little attention (2,5). Cowper et al. (5) briefly mentioned the presence of scattered FXIIIa+ cells in cutaneous SFT. Recently, Alawi et al. (2) reported that FXIIIa, although positive in most cases of oral SFT, is usually identified in a smaller percentage of the lesional cells; however, they did not discuss the role of FXIIIa+ dendrocytes in the etiology or propagation of SFT. Indigenous FXIIIa+ tissue dendrocytes have been described as increasing in number in a variety of inflammatory, reactive, and neoplastic conditions of the oral mucosa (6,7). It is worth mentioning that the previously reported oral SFT (2,3) and our tumor showed a high number of mast cells. This is in accordance with the observations that FXIIIa+ dendrocytes increased when mast cells degranulated and released tumor necrosis factor-α on cell activation (8,9). Mast cells and their proteolytic enzyme may contribute significantly to tissue matrix breakdown during SFT growth (3). The present case suggests that at least some superficial SFTs of the skin and oral mucosa appear to be composed of a bimodal proliferation of immunophenotypically distinct cells: CD34+ fibroblast-like cells and FXIIIa+ dendrocytes. Whether the latter cell type is neoplastic or reactive in nature remains controversial at present. There is growing evidence that a pathogenetic link among CD34/FXIIIa fibrohistiocytic dendritic cell subsets is replicated by many cutaneous mesenchymal tumors (10,11). Unfortunately, other than in the skin (12,13), their biologic role is largely unknown. Although recent studies have demonstrated that CD34+ hematopoietic progenitor cells can differentiate into not only mast cells (14) but FXIIIa+ dendrocytes (15), their putative relation has not yet been fully characterized. In summary, because FXIIIa has been shown to regulate the proliferation of fibroblasts and some tumor cells in vitro (10–13,16), we speculate that FXIIIa+ dendrocytes are potential candidates for playing an interactive role during the development of an SFT. Conceivably, local excessive secretion of FXIIIa by ubiquitous tissue dendrocytes can initiate or promote proliferation of CD34+ uncommitted mesenchymal cells in cooperation with mast cells. We are interested in reports of further studies. Fumio Ide, D.D.S. Kaoru Kusama, D.D.S.