Solitary fibrous tumour of the thyroid J.H.KIE, J.Y.KIM, Y.N.PARK, M.K.LEE, W.I.YANG, & J.S.PARK* Departments of Pathology and *General Surgery, Yonsei University College of Medicine, Seoul, Korea Date of submission 21 August 1996 Accepted for publication 4 November 1996 Keywords: thyroid, solitary fibrous tumour, CD34 Introduction Solitary fibrous tumours of the thyroid are quite rare; only four cases have been reported recently 1,2 . Although originally regarded as a serosa-associated neoplasm since the first description by Klemperer and Rabin in 1931 3 , recent reports of a solitary fibrous tumour arising from the various extraserosal and even solid organs support its origin from mesenchymal rather than mesothelial cells 4 . We report a case of solitary fibrous tumour of the thyroid with histologic, ultrastructural and immunohisto- chemical findings. Case report A 48-year-old woman presented a palpable mass in the right thyroid of 4-years duration. The mass was mobile and firm. Thyroid function tests including serum levels of T3 and T4 were within normal ranges. The patient underwent a right hemithyroidectomy and ismuth- ectomy for a suspected thyroid tumour. PATHOLOGICAL FINDINGS The right lobe of the thyroid was mostly replaced by a well-circumscribed pinkish grey, rubbery and solid mass. The mass measured 80 mm in the largest dimension and the cut surface showed multifocal cystic changes (Figure 1). On histologic examination, the tumour mass was composed of a patternless proliferation of spindle cells in a collagenous stroma (Figure 2). Alternation of cellular and hypocellular areas was observed. The tumour was richly vascularized (Figure 2) and a haemangiopericytoma-like pattern was also present (Figure 3). The tumour cells displayed vesicular nuclei with fine chromatin, and neither mitotic figures nor pleomorphism were found. A few normal appearing thyroid follicles were seen at the periphery of the tumour (Figure 4). Immunohistochemical staining showed that the tumour cells strongly expressed CD34 and vimentin (Figure 4). Thyroglobulin was expressed only in the entrapped follicular cells. Calcitonin, carcinoembryonic antigen, cytokeratin, alpha smooth-muscle actin, and S-100 protein were not expressed in the tumour cells. On ultrastructural examination, the neoplasm was composed of mostly parallel spindle cells separated by bundles of collagen. The nuclei were slightly indented and elongated with moderate amounts of hetero- chromatin. Although intracytoplasmic organelles were sparse, there was a moderate amount of rough endoplasmic reticulum. A few mitochondria and occasional lipid droplets were present. The cellular borders were indistinct. We could not find any specialized junctional structures such as desmosomes (Figure 5). Histopathology 1997, 30, 365–368  1997 Blackwell Science Limited. Address for correspondence: Dr. W.-I.Yang, Department of Pathology, Yonsei UniversityCollege of Medicine, C.P.O. Box 8044, Seoul,120-750, Korea. Figure 1. The right lobe of the thyroid was mostly replaced by a well-circumscribed, pinkish grey, rubbery, solid mass with areas of cystic change.