Case 3 -

Solitary Fibrous Tumor Matt Van De Rijn Stanford University Stanford, CA

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Clinical History A 35 y.o. male presented with shortness of breath. An 11 cm pleural based lesion was resected from the right lung. Microscopic description: Sections show a densely collagenized, hypercellular bland spindle cell lesion with focally hemangiopericytomatous appearing blood vessels. The tumor is well circumscribed but no clear capsule is present. Immunohistochemistry shows a strong positive reaction for CD34. The tumor cells are negative for S100, keratin, EMA, and actins. Case 3 - Figure 1 - Gross image of pleural mass. Case 3 - Figure 2 - H&E images from different areas within the tumor. Case 3 - Figure 3 - H&E images from different areas within the tumor. Case 3 - Figure 4 - High magnification of staghorn-like vessels at pleural margin. Case 3 - Figure 5 - CD34 stain. Diagnosis: Solitary fibrous tumor. Discussion: Solitary fibrous tumors are spindle cell tumors that were originally described in the pleura and at that time were also called localized or fibrous mesotheliomas. Immunohistochemical and ultrastructural studies subsequently showed that the cells in these tumors do not display features indicative of mesothelial differentiation [1]. Currently, the histogenesis of these lesions remains controversial, with many favoring a derivation from mesenchymal fibroblast-like cells [1, 2, 3, 4] . Several growth patterns of SFT have been described [5, 6] . Perhaps the most helpful to remember are: the "patternless" pattern in which a proliferation of randomly oriented spindle cells is seen, often with alternating hypocellular and hypercellular areas. the hemangiopericytoma-like pattern, consisting of a cellular neoplasm with "staghorn" vessels. the diffuse sclerosis pattern which shows a paucicellular lesion consisting of dense bundles of collagen permeated by thin strands of spindle cells. Usually, tumors show a combination of different growth patterns in the same lesion [6], a finding that can help in differentiating these tumors from other lesions [7, 8] . The spindled cells in these tumors have a small amount of indistinct cytoplasm. In most cases little nuclear pleomorphism is seen and the behavior is benign. However, cases with increased cellularity, increased mitotic activity (>4/10 hpf), nuclear pleomorphism, hemorrhage, or necrosis have also been described and these lesions have been associated with a malignant behavior in pleural sites [2], where the degree of resectability (polypoid and well circumscribed vs. infiltrative) also is a major prognostic factor [6]. Originally, SFT were negative for most markers tested and the diagnosis of SFT was one of exclusion and was based on its failure to react for a panel of antibodies. Subsequently, several studies showed a strong reaction for CD34 in the vast majority of SFT both in pleural [5, 9, 10] and other sites [3]. As a result, many previously uncategorized spindle lesions could be classified as SFT and there was a subsequent marked increase in the number of extrapleural SFT's described in the literature [4]. The majority of these extrapleural SFT's have behaved in a benign fashion, but it should be mentioned that even bland appearing extrapleural SFT can recur or even metastasize [11]. With longer follow-up, more cases of SFT with a malignant behavior will most likely appear in the literature. Extrapleural SFT have been reported in almost all body sites [11]. While not all SFT of the pleura are CD34+, most pathologists will require CD34 reactivity in extrapleural lesions before calling the tumor a SFT. The reactivity for CD34 can be a useful tool to distinguish the pleural based SFT from mesothelioma, fibrous histiocytoma, fibromatosis, synovial sarcoma, and spindled cell carcinoma, as these lesions are almost uniformly negative for CD34. Depending on the site in which SFT's occur, the differential diagnosis will of course vary, as described by Nascimento [12]. In combination with other antibodies lesions, such as nerve sheath tumors, smooth muscle tumors and others can also be distinguished. Hemangiopericytoma has become a highly controversial diagnosis. The diagnosis had always been one of exclusion as many lesions such as synovial sarcoma can have a similar architectural pattern consisting of a hypercellular tumor with irregular vessels. With the recognition that the vast majority of HPC actually are solitary fibrous tumors, this diagnosis has lost most of its reason to exist, as stated in 2 recent reviews [10, 13] References Said JW, Nash G, Banks-Schlegel S, Sassoon AF, Shintaku IP: Localized fibrous mesothelioma: an immunohistochemical and electron microscopic study. Human Pathology 15:440-3, 1984 England DM, Hochholzer L, McCarthy MJ: Localized benign and malignant fibrous tumors of the pleura. A clinicopathologic review of 223 cases. Am J Surg Pathol 13:640-58, 1989 Suster S, Nascimento AG, Miettinen M, Sickel JZ, Moran CA: Solitary fibrous tumors of soft tissue. A clinicopathologic and immunohistochemical study of 12 cases. American Journal of Surgical Pathology 19:1257-66, 1995 Chan JKC: Solitary fibrous tumor-everywhere, and a diagnosis in vogue. Histopathology 31:568-76, 1997 van de Rijn M, Lombard CM, Rouse RV: Expression of CD34 by solitary fibrous tumors of the pleura, mediastinum, and lung. American Journal of Surgical Pathology 18:814-20, 1994 Moran CA, Suster S, Koss MN: The spectrum of histologic growth patterns in benign and malignant fibrous tumors of the pleura. Semin Diagn Pathol 9:169-80, 1992 Mentzel T, Bainbridge TC, Katenkamp D: Solitary fibrous tumour: clinicopathological, immunohistochemical, and ultrastructural analysis of 12 cases arising in soft tissues, nasal cavity and nasopharynx, urinary bladder and prostate. Virchows Archiv 430:445-53, 1997 Westra WH, Gerald WL, Rosai J: Solitary fibrous tumor. Consistent CD34 immunoreactivity and occurrence in the orbit [see comments]. American Journal of Surgical Pathology 18:992-8, 1994 Hanau CA, Miettinen M: Solitary fibrous tumor: histological and immunohistochemical spectrum of benign and malignant variants presenting at different sites. Human Pathology 26:440-9, 1995 van de Rijn M, Rouse RV: CD34-A review. Applied immunohistochemistry 2:71-80, 1994 Gengler C, Guillou L: Solitary fibrous tumour and haemangiopericytoma: evolution of a concept, Histopathology 2006, 48:63-74 Nascimento AG: Solitary fibrous tumor: a ubiquitous neoplasm of mesenchymal differentiation. Advances in Anatomic Pathology 3:188-95, 1996 Fletcher CD. The evolving classification of soft tissue tumours: an update based on the new WHO classification. Histopathology. 2006;48:3-12.