Answer: Fibrous pseudotumor

Histology:

Discussion: Sections show a fibro-vascular nodule with focal mild chronic inflammation and scattered stromal neutrophils. The nodule has a smooth round outline and is composed of predominantly small caliber vessels some of which are branching and are separated by dense collagen containing scattered fibroblasts. Multiple small vessels with perivascular concentric fibrosis are also noted. The stroma is paucicellular and typical morphologic (vascular) features of solitary fibrous tumor are not seen. No calcification is seen. A stalk like area with fibrotic granulation tissue/scar is present. Immunohistochemical stains demonstrate that the stromal (spindle) cells are negative for CD34, BCL-2, CD99, S100 and pankeratin, while they are positive for Vimentin. An immunostain for ALK is negative. The cytokeratin stain highlights rare positive cells near the surface consistent with reactive mesothelium. This is an unusual fibrous lesion of the pleura that is morphologically similar to fibrous paratesticular masses where the name fibrous pseudotumor is used.