Head and Neck Solitary Fibrous Tumors: Diagnostic and Therapeutic Challenges

Transcription

1 The Laryngoscope VC 2012 The American Laryngological, Rhinological and Otological Society, Inc. Head and Neck Solitary Fibrous Tumors: Diagnostic and Therapeutic Challenges Sarah N. Bowe, MD; Paul E. Wakely, Jr., MD; Enver Ozer, MD Objectives/Hypothesis: Head and neck solitary fibrous tumors (SFTs) are quite rare, with <200 cases reported in the literature. The purpose of this study was to analyze the diagnostic and therapeutic challenges of this rare disease while reporting the largest head and neck case series. Study Design: Retrospective review. Methods: Between 1991 and 2011, 13 cases of head and neck SFTs from the pathology database were identified. Demographic, clinical, radiologic, and pathologic data were then abstracted and compiled for review. Results: SFT presented nearly equally as asymptomatic slow-growing masses or with local symptoms due to compression. All tumors imaged by computed tomography or magnetic resonance imaging with contrast showed a well-defined strongly enhancing mass. Two patients had evidence of bone erosion on imaging, but no histopathologic features of malignancy. Treatment consisted of surgical resection and in two cases postoperative radiation. Two patients had pathologic findings of malignancy, but are alive without recurrence at 26 and 51 months follow-up. Two patients had recurrence, one with paraspinal disease and grossly positive surgical margins, and another with endoscopic resection and poor margin control. Conclusions: SFTs of the head and neck are exceedingly rare and those with aggressive behavior even more so. Diagnosis is often difficult and not definitive until morphologic and immunohistochemical evaluation is performed. In most cases, complete surgical excision is the only necessary treatment. Although in patients with malignant components or positive surgical margins, adjuvant radiation may be beneficial. Regardless, all patients require close clinical follow-up for several years. Key Words: Solitary fibrous tumor(s), head and neck, extrapleural, spindle cell neoplasm, hemangiopericytoma. Level of Evidence: 4 Laryngoscope, 122: , 2012 INTRODUCTION Klemperer and Rabin are attributed with the first description of solitary fibrous tumor (SFT) as a distinct entity of submesothelial origin arising from the pleura. 1 In contrast, Stout and Murray 2 proposed a mesothelial origin to these neoplasms, leading to the wide variety of terms associated with these tumors in the literature, including localized mesothelioma, fibrous mesothelioma, and benign mesothelioma. Around the same time, Stout and Murray were also presenting their landmark paper on hemangiopericytoma (HPC). 3 Further advances in pathology would reveal that SFT is actually a distinctive mesenchymal neoplasm, and its microscopic HPC-like features and immunohistochemical pattern make it nearly indistinguishable from HPC. 4 From the Department of Otolaryngology Head and Neck Surgery (S.N.B.) and Department of Pathology (P.E.W.), The Ohio State University Medical Center, Columbus, and the Department of Otolaryngology Head and Neck Surgery (E.O.), Arthur G. James Cancer Hospital and Richard J. Solove Research Institute, Comprehensive Cancer Center, The Ohio State University Medical Center, Columbus, Ohio, U.S.A. Editor s Note: This Manuscript was accepted for publication March 16, Presented at the Triological Society Combined Sections Meeting, Miami, Florida, U.S.A., January 26 28, The authors have no funding, financial relationships, or conflicts of interest to disclose. Send correspondence to Sarah N. Bowe, MD, 915 Olentangy River Rd., Suite 4000, Columbus, OH sarah.bowe@osumc.edu DOI: /lary With these improvements in pathological classification, extrapleural SFTs have been increasingly reported in nearly all sites, including the retroperitoneum, proximal extremities, abdominal cavity, and trunk. 5 The first case in the head and neck region was presented in In this location, these tumors present as slowgrowing masses, often with local compressive symptoms, that are difficult to distinguish from other soft tissue tumors. Although generally benign, malignant variants have been identified. The goal of the present study was to analyze the diagnostic and therapeutic challenges of this rare disease while reporting the largest head and neck SFT case series. MATERIALS AND METHODS We conducted a comprehensive search of the Ohio State University Medical Center pathology records between 1991 and 2011 under institutional review board approval. Thirteen patients yielding the diagnosis of SFT within the head and neck were identified. To be entirely comprehensive in the setting of evolving concepts regarding SFT and HPC, we also searched the pathology database for HPC. During this time period, HPC was only diagnosed within the sinonasal tract and meninges. As they are still considered to be distinct clinicopathologic entities in these regions, 4,7 they were immediately excluded based on location alone and not analyzed along with the original 13 cases. Demographic, clinical, radiologic, and pathologic data were then abstracted and compiled for review.

2 TABLE I. Demographics and Clinical Presentation. Case No. Age, yr/sex Clinical Presentation Location Size, cm Biopsy (Result) 1 59/M Painless mass Left neck 10 Yes (DFSP) 2 45/M Painless mass Left neck 4 Yes (epithelioid neoplasm) 3 44/M Painless mass Right neck 3 Yes (spindle cell neoplasm) 4 52/F Painless mass, hoarseness Right neck/parotid UA No 5 83/M Painless mass Right face/parotid 10 Yes (spindle cell neoplasm) 6 51/M Painless mass Right face/parotid 4 Yes (pleomorphic adenoma) 7 44/M Proptosis, diplopia Left orbit UA No 8 32/F Painless mass, diplopia Right orbit UA No 9 57/M Left nasal obstruction Left nasal cavity UA Yes (HPC) 10 52/F Right nasal obstruction Right nasal cavity UA No 11 41/F Right nasal obstruction, epistaxis Right nasal cavity UA No 12 65/M Dysphagia Right aryepiglottic fold 4 No 13 55/M Neck pain, arm pain, hand weakness Right paraspinal region UA No M ¼ male; DFSP ¼ dermatofibrosarcoma protuberans; F ¼ female; UA ¼ unavailable; HPC ¼ hemangiopericytoma. RESULTS The demographics and clinical presentation of the 13 cases of head and neck SFTs are presented in Table I. Nine patients were male (69.2%) and four were female (30.8%). The mean age was 52.3 years (range, years). Six patients presented with an asymptomatic, slowly enlarging facial or neck mass. Unilateral nasal obstruction and diplopia were common symptoms depending on tumor location. Imaging was available for 10 out of 13 patients. Table II summarizes the pertinent radiologic findings. In nine of the patients, the imaging revealed well-circumscribed masses with discrete margins ranging from cm to cm. The remaining case TABLE II. Radiological Findings. Case No. CT MRI 1 Well-circumscribed cm mass; with contrast, heterogeneous enhancement 2 Well-circumscribed cm mass; with contrast, relatively homogenous enhancement 4 Well-circumscribed cm mass; without contrast, isointense to muscle; with contrast, strong homogenous enhancement; expansile bone remodeling with scalloping of lateral aspect of right occipital condyle and jugular foramen 6 Well-circumscribed cm mass; with contrast, strong homogenous enhancement 7 Well-circumscribed cm mass; without contrast, isointense to brain; with contrast, heterogenous enhancement; expansile bone remodeling of orbit with vertical dimension 4.1 cm compared to contralateral 3.5 cm 8 Well-circumscribed cm mass; without contrast, isointense to brain; expansile remodeling of orbit, as well as orbital roof erosion 9 Well-circumscribed cm mass; without contrast, isointense to muscle; expansile bone remodeling with bowing of medial margin of left maxillary sinus laterally and left inferior ethmoid sinuses 10 Ill-defined cm mass; without contrast, isointense to muscle; expansile bone remodeling with scalloping of right maxilla and pterygoid 11 Well-circumscribed cm mass; without contrast, isointense to muscle 13 Well-circumscribed cm mass; with contrast, homogenous enhancement Well-circumscribed cm heterogeneously enhancing mass; isointense to muscle T1 and T2 Well-circumscribed cm homogenously enhancing mass; isointense to muscle T1 and hyperintense to muscle T2 Well-circumscribed cm homogenously enhancing mass; isointense to brain T1 and T2; expansile remodeling of bone, as well as skull base erosion at jugular and hypoglossal foramen Well-circumscribed cm very strongly homogenously enhancing mass; isointense to brain T1 and T2 Ill-defined cm homogenously enhancing mass; isointense to brain T1 and T2 Imaging was not performed or unavailable for cases 3, 5, and 12. CT ¼ computed tomography; MRI ¼ magnetic resonance imaging; ¼ not performed; T1 ¼ T1-weighted imaging; T2 ¼ T2-weighted imaging. 1749

3 had more ill-defined borders, but this was an individual presenting with recurrent disease whose initial imaging performed prior to her first resection was unavailable. Computed tomography (CT) imaging was performed on all 10 patients, and when performed without contrast, tumors tended to be isointense to surrounding muscle or brain (Fig. 1). Magnetic resonance imaging (MRI) was only performed in five cases. Four were similarly isointense to muscle or brain on T1- and T2-weighted imaging (Fig. 2). Another common radiographic finding was expansile bony remodeling (Fig. 3). In some cases, what appeared to be benign remodeling on CT was actually indicative of bone erosion when evaluated further by MRI (Fig. 4). Overall, there were two cases (4 and 8) with radiographic evidence of bone destruction, neither of which was found to have features of malignancy on pathology. Perhaps the most common and reliable radiographic feature was post-contrast enhancement on both CT and MRI (Fig. 5). Homogenous enhancement occurred in six out of eight cases compared to heterogenous enhancement in the remaining two (Fig. 6). Most tumors were similar on gross examination as relatively well-circumscribed, lobulated, rubbery masses that appeared tan-pink on the surface and whitish-gray on cut section. Characteristic microscopic features seen with hematoxylin-eosin staining included spindled cells in a patternless pattern, with alternating hypocellular and hypercellular areas separated by hyalinized collagen and branching HPC-like vessels (Fig. 7). Table III shows the immunophenotypic staining pattern for all patients. Nearly all tumors were positive for the presence of CD34 (92%) and/or Bcl-2 (82%) (Fig. 8). In case 8, which was negative for CD34, immunophenotyping was positive for both Bcl-2 and CD99. A malignant component was identified in cases 1 and 2, focal necrosis (Fig. 9) and Fig. 2. Isointensity to muscle is seen on this T1-weighted magnetic resonance imaging of a left posterior neck mass. Fig. 1. Non contrast-enhanced computed tomography of the tumor in case 10 showing isointensity to surrounding muscle. Solitary fibrous tumors are usually well-circumscribed, but this lesion also shows variation in this typical characteristic because it is more ill-defined and lobulated in nature. Fig. 3. Case 9 illustrates the common finding of expansile bony remodeling. In this case, there is bowing of the medial margin of left maxillary sinus and left inferior ethmoid sinuses. 1750

4 Fig. 4. In some cases, computed tomography may indicate expansile bone remodeling, as in this case with scalloping of the lateral aspect of the right occipital condyle and jugular foramen (A). When magnetic resonance imaging is used, however, erosive changes may be identified, as can be seen in case 4 (B). Despite the radiographic evidence of bone destruction, this case was not found to have features of malignancy on pathology. increased mitotic rate (Fig. 10), respectively. Neither of these patients has been found to have a malignant solitary fibrous tumor from a clinical standpoint. Table IV shows the treatment and clinical course of each patient. Every patient underwent surgical resection as planned definitive management. Surgical margins were negative in all cases except one. Case 13 had an extensive tumor of the right paraspinal region with bilateral compression of the thecal sac and foraminal compromise. A wide local excision of the paraspinal region was performed, but gross disease was left due to its location and risk to surrounding structures. He subsequently underwent postoperative external beam radiation, but recurred both locally at C7 and regionally at the skull base. He did undergo another procedure aimed at debulking the tumor for improvement of symptoms, namely arm weakness, and is currently alive with disease at 25 months follow-up. Fig. 5. Post-contrast enhancement was universally seen on both computed tomography (A) and magnetic resonance imaging (B). 1751

5 TABLE III. Immunohistochemistry Findings. Antibody Positive (%) Negative (%) CD34 12/13 (92) 1/13 (8) Bcl-2 9/11 (82) 2/11 (18) CD99 7/8 (88) 1/8 (12) EMA 1/9 (11) 8/9 (89) SMA 0/8 (0) 8/8 (100) S-100 1/13 (8) 12/13 (92) Denominator <13 indicates number of tests not performed. EMA ¼ epithelial membrane antigen; SMA ¼ smooth muscle actin. Two patients underwent endoscopic resection, and thus true assessment of surgical margins is impossible. Case 11 had a relatively well-circumscribed mass of the right nasal cavity for which complete gross resection was apparent during the initial surgery. The patient is currently free from disease at 28 months follow-up. In contrast, case 10 presented with signs and symptoms of chronic sinusitis and underwent right maxillary antrostomy with tissue removal and excision of a right nasal mass that was likely, in retrospect, incomplete. When the final pathology showed SFT, there was no gross evidence of disease and she was monitored clinically. About one year later, she began having recurrent symptoms with evidence of a right nasal mass again on examination. She underwent another, more extended endoscopic Fig. 6. The majority of cases had homogenous enhancement, but heterogenous enhancement can also be seen, as shown here in case 1. Fig. 7. The microscopic appearance of a typical solitary fibrous tumor stained with hematoxylin-eosin is shown at low power. Fig. 8. Solitary fibrous tumor will commonly show positive immunohistochemistry staining with antibodies against CD34 (A) and Bcl-2 (B). 1752

6 Fig. 9. Hematoxylin-eosin stain in case 1 exhibiting a necrotic focus within the left and center portions of the field and a thin component of viable tumor on the right. Fig. 10. An increased mitotic rate was found in case 2. A mitotic figure can be seen here in the center of this hematoxylin-eosin stain. sinonasal resection with suspected more definitive control of disease. She did well for 36 months, but then began having complaints of right gum pain and facial swelling and was found to have a large recurrence within the pterygopalatine and infratemporal fossa. She underwent an extended endoscopic exploration combined with an open preauricular/bicoronal transzygomatic approach for disease control. She is currently without evidence of disease with recent MRI at 13 months. Overall, follow-up ranged from 0 to 92 months. All patients were alive at the time of article preparation, except one, who unfortunately suffered from an immediately postoperative thromboembolic event with right cerebellar and brainstem infarct. The family elected to withdraw care on the second postoperative day. Only one patient has persistent evidence of disease following both initial and revision surgery due to incomplete resection within the paraspinal region. Cases 1 and 2 had evidence of a malignant component by pathology, focal TABLE IV. Treatment and Clinical Course. Case No. Surgical Procedure Surgical Margins Adjuvant Treatment Follow-up, mo Status Recurrence 1 WLE, suboccipital approach None 51 A, NDP None 2 WLE, selective neck dissection None 26 A, NDP None 3 WLE, transcervical approach None 0 A, NDP None 4 Resection parapharyngeal/infratemporal space, None 0 D, NDP None transcervical/transparotid approach 5 WLE, extended superficial parotidectomy None 1 A, NDP None 6 Partial parotidectomy with accessory lobe resection None 2 A, NDP None 7 Lateral orbitotomy None 0 A, NDP None 8 Anterior orbitotomy External 60 A, NDP None beam radiation 9 WLE, partial maxillectomy removal/reinsertion, None 92 A, NDP None ethmoid/sphenoidectomy, nasopharynx resection 10 Endoscopic resection; extended endoscopic resection; combined preauricular/bicoronal transzygomatic approach to infratemporal fossa and endoscopic approach to pterygopalatine fossa NA, NA, NA None 10, 36, 13 A, NDP Local 11 Endoscopic resection NA None 28 A, NDP None 12 Partial supraglottic laryngectomy None 0 A, NDP None 13 WLE, paraspinal region; surgical debulking, paraspinal region þ, þ External beam radiation 55, 25 A, DP Local/regional WLE ¼ wide local excision; ¼negative; A ¼ alive; NDP ¼ no disease present; D ¼ deceased; NA ¼ not applicable (piecemeal resection); þ¼positive; DP ¼ disease present. 1753

7 necrosis and increased mitotic rate, respectively, but are both alive without evidence of disease at 51 and 26 months follow-up. Last, two cases (4 and 8) exhibited radiographic evidence of bone destruction, but neither was found to have features of malignancy on final pathology. Despite the lack of aggressive features and presence of negative margins for case 8, referral and treatment with postoperative external beam radiation was completed based solely on the findings of orbital roof erosion from imaging. At 60 months follow-up, the patient is alive without evidence of disease. DISCUSSION Extrapleural SFTs are uncommon mesenchymal neoplasms that may be found in any location. The first reported cases in the head and neck were presented in SFTs generally present as slow-growing painless masses. In the head and neck, compressive symptoms may develop while the tumor is still small, especially in the nasal cavity and orbit. In our study, half of the patients presented with an asymptomatic mass, whereas the remaining patients had local compressive symptoms. Both patients with an orbital tumor presented with diplopia, and all three patients with a sinonasal mass had nasal obstruction. Gold et al. 5 compared extrathoracic and thoracic SFTs and identified a significant difference regarding symptoms on presentation, 83% and 23%, respectively. In fact, the majority of thoracic SFTs are diagnosed incidentally on chest x-ray or CT. The earlier presentation within the head and neck region, due to the visibility of a mass or local symptoms, tends to limit the extent of disease, such that all tumors in our series were <10 cm in diameter. Diagnosing SFT by clinical presentation alone is impossible. There are, however, certain radiographic findings suggestive of the diagnosis that should prompt inclusion of SFT into the differential. The most characteristic feature on imaging within our series was that of a well-defined enhancing mass on both CT and MRI. A homogenous enhancement pattern was more common within our patients (75%), which is in contrast to the series evaluated by Wignall et al. 8 They found a heterogenous enhancement pattern within 79% of their patients with SFTs, but this did not include any patients with head and neck lesions. In contrast, Ganly et al. 9 evaluated 12 patients specifically with SFTs of the head and neck, eight of whom had preoperative imaging. They found homogenous enhancement within 80% of their patients. Thus, within the head and neck, homogenous enhancement appears to be a more reliable pattern for the basis of differential diagnosis. The signal characteristics as compared to surrounding structures may also aid in diagnosis. Ganly et al. 9 found that almost all of their tumors showed isointensity to muscle or brain on precontrast T1-weighted and T2- weighted MRI sequences. One T2-weighted image of a right paraspinal mass was uniformly hyperintense to nearby paraspinal muscles. Our study also showed both 1754 T1 and T2 isointensity to surrounding muscle or brain, with the exception of one left neck mass that was hyperintense compared to adjacent muscle. Noncontrast CT imaging is also characterized by isointensity to surrounding muscle or brain, both within our series and that of Ganly et al. 9 The most common radiographic osseous finding in cases of SFTs is regressive remodeling of adjacent bone due to the long-standing pressure effect of the slowgrowing mass. 9 Five patients in our series exhibited expansile bony remodeling of nearby structures on CT imaging. There are cases, however, where CT imaging may indicate more benign-appearing changes and fail to identify smaller foci of bone erosion. For example, the CT scan from case 4 showed scalloping of the lateral aspect of the right occipital condyle and jugular foramen. MRI, on the other hand, revealed erosion of the skull base in the region of the jugular and hypoglossal foramen (Fig. 4). One additional patient, case 8, had evidence of bony erosion of the orbital roof that was identified on both CT and MRI. Despite these concerning destructive bone changes, neither of these patients had evidence of malignant tumor on pathologic analysis, nor have they shown evidence of recurrence on clinical follow-up. In contrast, cases 1 and 2, which showed pathologic evidence of malignant features, and focal necrosis and increased mitotic rate, respectively, did not have any evidence of bony remodeling or destruction. These findings are similar to other series, 9 such that the presence of erosive changes should prompt concern for a malignant tumor, whereas the absence of these findings does not negate the possibility of malignancy. On gross examination, our series was similar to most SFTs that present as well-circumscribed, lobulated, rubbery masses that appear tan-pink on the surface, but whitish-gray on cut section. 10 Histopathology generally shows a patternless pattern with alternating hypocellular and hypercellular areas separated by thick bands of hyalinized collagen and branching HPC-like vessels. Despite what many pathologists would now consider a fairly recognizable constellation of histological features, allowing a diagnosis based on morphology alone, immunohistochemistry is a significant contributor in distinguishing SFT from other soft tissue tumors. Table III presents a summary of the immunohistochemistry results of our patient population. Solitary fibrous tumors commonly express CD34 (80% 95%) and CD99 (70%). 4,10 Less commonly, they will express Bcl-2, epithelial membrane antigen, and smooth muscle actin (20% 35%). There have been a few isolated cases of focal and limited reactivity for S-100. Our findings are generally consistent with previous results, although our specimens tended to have a much higher positivity for Bcl-2 compared to the literature. It is also interesting to note that in the one case that was negative for CD34, immunophenotyping was positive for both Bcl-2 and CD99. Malignant SFTs are generally hypercellular with moderate to marked cytological atypia, infiltrative margins, necrosis, and increased mitotic rate (4 or greater per 10 high-power fields). 10 Cases 1 and 2 had evidence of focal necrosis (Fig. 9) and an increased mitotic rate

8 (Fig. 10), respectively. Gold et al. studied SFTs for adverse prognostic factors affecting local recurrence and metastasis and found that the presence of a malignant component was associated with both worse local recurrence-free survival and metastasis-free survival. 5 In their study, tumor size and the presence of a malignant component were not only highly correlated, but also appeared to be compounding factors in regard to poor prognosis. Only two of 15 tumors with a malignant component were <10 cm in size, and neither of these recurred at 8 and 32 months follow-up. Most SFTs of the head and neck, including those of our series, are <10 cm in size as they tend to present sooner due to visibility or compressive symptoms. 9,11 Cases 1 and 2, now clinically free of disease at 51 and 26 months, respectively, have similarly shown that malignant histologic features do not necessarily correlate with a poor outcome if tumor size is small. Another important factor relating to size that has a role in disease prognosis is resectability. Surgery is generally regarded as the treatment of choice for SFT. Cases 1 and 2 both presented with left neck masses that were easily resectable with negative surgical margins. Gold et al. noted that the presence of macroscopically or microscopically positive surgical margins was associated with a statistically worse outcome regarding local recurrence and metastasis. 5 Similarly, Cox et al. found in their review that recurrence occurred only in cases that had been incompletely excised. 11 Two patients in our series have exhibited aggressive behavior with local and/or regional recurrence. Case 10 presented with signs and symptoms of chronic sinusitis and underwent right maxillary antrostomy with tissue removal and excision of a right nasal mass that was likely, in retrospect, incomplete. As she underwent an endoscopic procedure, true margin analysis was impossible. The patient appeared to be grossly free of disease, but likely had microscopically positive disease. She recurred 10 months later and underwent an extended endoscopic resection. Following this, she again did well until a more advanced locoregional recurrence 36 months later within the pterygopalatine and infratemporal fossa. After undergoing a combined endoscopic and open approach, she is clinically free of disease at 13 months follow-up. Case 13 actually presented having already undergone a wide local excision at an outside hospital for extensive tumor of the right paraspinal region with bilateral compression of the thecal sac and foraminal compromise. Gross disease was left at his initial surgery due to its location and risk to surrounding structures. He had undergone postoperative external beam radiation, but presented to our institution with recurrence both locally at C7 and regionally at the skull base. He underwent another debulking procedure aimed at symptom relief and was referred for additional radiation to manage the skull base disease. He is still alive with active disease at 25 months follow-up. Although the overall size of tumors within the head and neck may be smaller, portending a better prognosis, clearly resectability is affected due to the nature of delicate surrounding structures. Postoperative radiation has been used to assist in disease control in patients with known residual disease after resection or microscopically positive margins that return on final pathology, but the small number of cases and short follow-up make any conclusions regarding use of this postoperative adjuvant treatment impossible. CONCLUSION Extrapleural SFTs have been increasingly reported in nearly all sites, but still remain rare within the head and neck. They generally present as slow-growing masses, which due to direct visibility of the mass or local symptoms due to compression, tend to present earlier than other locations, which may limit the extent of disease. Although there are certain radiographic findings suggestive of the diagnosis, namely a well-defined, homogenously enhancing mass on both CT and MRI, definitive diagnosis is generally not made until review of the final pathology for both morphologic and immunohistochemical characteristics. Complete surgical resection is the primary treatment for SFT, but the importance of preserving vital structures within the head and neck may make this difficult. Local recurrence and metastasis have been correlated with tumors possessing malignant features on histology as well as those with macroscopic or microscopic residual disease after resection. Tumors with malignant features and/or positive surgical resection margins clearly require close follow-up and likely benefit from consideration for postoperative radiation treatment. Further studies are needed on this rare disease entity regarding endoscopic margin control, efficiency of adjuvant treatment regimens, and longer-term disease control. BIBLIOGRAPHY 1. Klemperer P, Rabin CB. Primary neoplasms of the pleura: a report of five cases. Arch Pathol 1931;11: Stout AP, Murray MR. Localized pleural mesothelioma: investigation of its characteristics and histogenesis by the method of tissue culture. Arch Pathol 1942;34: Stout AP, Murray MR. Hemangiopericytoma: a vascular tumour featuring Zimmermann s pericytes. Ann Surg 1942;116: Gengler C, Guillou L. Solitary fibrous tumour and haemangiopericytoma: evolution of a concept. Histopathology 2006;48: Gold JS, Antonescu CR, Hajdu C, et al. Clinicopathologic correlates of solitary fibrous tumors. Cancer 2002;94: Witkin GB, Rosai J. 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