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CaseReport

Introduction

Solitary fibrous tumor (SFT) is a mesenchymal tumoroften arising from the pleura and usually showing a be-nign clinical course. We herein report a rare case that wasconsidered to originate in the thymus and showed clini-cally malignant behavior such as pleural disseminationand direct invasion into the lung, although without histo-logically malignant features.

Case

A 74-year-old female presented with an abnormal medi-

Solitary Fibrous Tumor of the Thymus with LocalInvasiveness and Pleural Dissemination: Report of a Case

Departments of 1Thoracic and 2Cardiovascular Surgery, Osaka CityUniversity Hospital, Osaka, Japan

Received October 6, 2006; accepted for publication October 26,2006Address reprint requests to Takashi Iwata, MD: Department ofThoracic Surgery, Osaka City University Hospital, 1–4–3 Asahi-machi, Abeno-ku, Osaka 545–8585, Japan.

astinal shadow in a routine follow-up chest X-ray after apacemaker implantation 9 years previously (Fig. 1). Shewas asymptomatic and had no history of asbestos expo-sure. A chest computed tomography (CT) revealed a largeround mass in the anterior mediastinum, just anterior tothe heart (Fig. 2). All tumor markers were within the nor-mal ranges. Systemic evaluation demonstrated no meta-static lesions. The patient underwent an extended thymec-tomy via median sternotomy on suspicion of thymoma.The tumor had arisen from the left half of the thymuswithout a pedicle, and had invaded directly into the lung(Fig. 3A). The part of the lung with the tumor wasresected. The adjacent pericardium seemed to be invadedand was resected with the tumor. The size of the tumorwas 5.3×4.0 cm. One disseminated lesion, with a diam-eter of approximately 1.5 cm, was observed on the medi-astinal pleura and was also resected. The cut surface re-vealed a yellowish-white solid tumor with partial cysticdegenerations. (Fig. 3B). She complained of muscle weak-ness in the right upper limb 8 hours after the operation.

Takashi Iwata, MD1, Noritoshi Nishiyama, MD,1 Nobuhiro Izumi, MD,1 Takuma Tsukioka, MD,1

and Shigefumi Suehiro, MD2

A 74-year-old asymptomatic female presented with an anterior mediastinal mass incidentallydiscovered on a routine chest X-ray. Systemic evaluation demonstrated no metastatic lesions.The patient underwent an extended thymectomy via median sternotomy on suspicion of a thy-moma. The tumor had arisen from the left half of the thymus without a pedicle and had directlyinvaded into the left lung and pericardium. The tumor was resected with the entire thymictissue, and the invaded lung and pericardium were resected en-bloc. The size of the tumor was5.3×4.0 cm. A disseminated lesion on the mediastinal pleura was also resected. Histopathologi-cally, the lesion mainly consisted of non-atypical spindle-shaped tumor cells in a so-called “pat-ternless pattern” with various densities of collagenous background. Pleomorphism and mitoseswere not significant. Immunohistochemical analysis revealed mesenchymal positive markerssuch as vimentin and CD34. Epithelial markers such as CAM 5.2 and AE1/AE3 were negative.S-100 protein and desmin were not stained. Solitary fibrous tumor of the thymus was diagnosedhistologically. Postoperative adjuvant chemotherapy or radiotherapy was not undertaken be-cause the benefits were uncertain. She is well without recurrence 3 months after the operation.(Ann Thorac Cardiovasc Surg 2007; 13: 198–202)

Key words: adult, neoplasm, solitary fibrous tumor, thymoma, mediastinum

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The patient was diagnosed as having a transient ischemicattack and the symptoms disappeared within a day throughconservative treatment. She was discharged on the 19thpostoperative day without further disturbances.

SFT of the thymus was diagnosed from the histologi-

Fig. 1. Chest X-ray demonstrates a smooth-surfaced, well-demarcated mass overlapping the heart (arrows).

Fig. 3.A: Gross specimen. The tumor seemed to originate from the

thymus.B: The cut surface of the specimen shows a yellowish-white,

well-encapsulated solid tumor tissue with clear demarca-tion; approximately 5 cm in diameter. Cystic degenerationwas observed. Necrotic or hemorrhagic portions were ab-sent.

Fig. 2. Computed tomography shows a large mass in the anteriormediastinum (black arrows). The tumor is poorly enhanced bycontrast medium. A disseminated lesion was demonstrated(white arrow).

cal examination of the specimen and the results of immu-nohistochemical study. The lesion mainly consisted ofnon-atypical spindle-shaped tumor cells in a so-called“patternless pattern” with various densities of collagenousbackground (Fig. 4). Pleomorphism and mitoses were notsignificant. There were no necrotic or hemorrhagic le-sions in the tumor tissue, and lymphocytic infiltration wasnot obvious. Direct invasion into the lung was histologi-cally confirmed. However, invasion into the pericardiumwas not demonstrated.

Immunohistochemical analysis revealed mesenchymalpositive markers such as vimentin and CD34 (Fig. 5A).Epithelial markers such as CAM 5.2, AE1/AE3 were nega-tive. Ki-67 (MIB-1) was positive in approximately 15%of the tumor cells (Fig. 5B). S-100 protein and desminwere not stained. Blood cell markers such as leukocytecommon antigen (LCA; CD45), and Leu7 (CD57) were

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Fig. 4. Histopathological examination (original mag-nification ×200, hematoxylin-eosin stain) reveals thecharacteristic “patternless pattern” with non-atypi-cal spindle or round tumor cells in the collagenousbackground.

Fig. 5. Results of immunohistochemical staining (original magnification ×200).A: CD34 is positive.B: Ki-67 is expressed in approximately 15% of the tumor cells.C: MIC2 protein is positive.D: Bcl-2 oncoprotein is expressed. Adding to these results, with a lack of S-100 protein and keratin, solitary fibrous tumor

was diagnosed.

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negative. C-kit protein was not detected. Expression ofboth MIC2 protein (CD99) and bcl-2 oncoprotein wasdemonstrated (Fig. 5, C and D).

From these immunohistochemical results, SFT wasdiagnosed histologically. The resected disseminated lesionwas also histopathologically and immunohistochemicallyconfirmed to be the same SFT.

Postoperative adjuvant chemotherapy or radiotherapywas not undertaken because the benefit of this was un-certain. The patient is well without recurrence 3 monthsafter operation.

Discussion

SFT was formerly called “localized (or solitary) fibrousmesothelioma,” or “benign (fibrous) mesothelioma,” andhad been considered to be a subgroup of mesothelioma.After revision of pleural tumor classification by the WorldHealth Organization in 1999, SFT was classified as anindependent entity and was excluded from the mesothe-lioma subgroups.1)

SFT is an uncommon soft tissue neoplasm that mainlyarises from the submesothelial layer of the pleura. In two-thirds of cases, SFT arises from the submesothelial con-nective tissue of the pleura, however, occurrences arisingfrom various anatomical sites as well as the pleura havebeen recently reported, such as the meninges, orbit, up-per and lower respiratory tract, salivary glands, thyroid,breast, heart, liver, kidney, retroperitoneum, soft tissue,and urogenital organs.

SFT originating in the mediastinum has been rarelyreported.2–5) In our case, because the majority of the tu-mor was in the anterior mediastinum, traced to the thy-mus and encapsulated with macroscopically intact pleura,we diagnosed that the tumor had originated in the thymicmesenchymal tissue, by gross specimen and histologicalexamination. SFT of pleural origin usually has a pedicle,however, in our case it was absent.4) Because the tumorwas large and partially invading into the lung tissue, theexact origin could not be determined and the possibilityof a pleural origin could not be excluded. MediastinalSFT without actual connection to the pleura has beendocumented.6)

SFT is usually a slow-growing tumor with a favorableprognosis. The patient is usually asymptomatic until thetumor grows large enough to compress neighboring or-gans. Although the tumor is usually endocrinologicallyinactive, secretion of insulin-like growth factor II is docu-mented, causing recurrent hypoglycemia.7,8) The clinical

behavior is usually benign, however, aggressive cases withlocal extension and/or recurrence have been reported inthe literature.2,9) Pleural dissemination has also been docu-mented.10,11) Complete surgical removal is the best treat-ment for SFT.

Histopathologically, the “patternless pattern” with non-atypical spindle-shaped tumor cells in the collagenousbackground is the most characteristic, but not unique, find-ing in SFT.12) Other spindle cell tumors, such as mesothe-lioma, hemangiopericytoma, synovial sarcoma, fibrosar-coma, sarcomatoid carcinoma, and malignant peripheralnerve sheath tumor are considerations for differential di-agnosis. In particular, hemangiopericytoma is known tohave a histopathological and immunohistochemical over-lap and it is questionable that it can be separated clearlyfrom SFT.13) In cases with necrosis, hypercellularity, sig-nificant mitoses, atypia, and/or pleomorphism the SFTwould be considered to have malignant features. How-ever, in such cases, these histological findings do not al-ways correlate with the clinical course.14) In our case, al-though there were no obvious malignant features revealedin the histopathological examination, the operative find-ings such as local invasion into the lung and pleural dis-semination suggested that the tumor could have potentialmalignancy might have indicated a poorer prognosis, al-though the tumor has been resected completely.

Immunohistochemically, the tumor expresses CD34 in80–100% of the cases and bcl-2 in more than 80%, thoughit is negative for keratin and S-100 protein.12,14,15) MIC2(CD99) protein is also known to be expressed normallyin the thymic cortex, but positive immunoreactivity forMIC2 in SFT has also been reported.16) Cases with malig-nant transformation were reported to show diminishedCD34 expression and positive p53 immunoreactivity.17)

In our case, Ki-67 was positive in approximately 15% ofthe cellular population. Ki-67 is considered to be an indi-cator of cell proliferation and was not significantly ex-pressed in this case.

In our case, although malignant features were absenthistopathologically and CD34 expression was not dimin-ished, because of the local invasiveness and dissemina-tion, malignant behavior of the tumor is predicted. Care-ful scrutiny will be needed for long-term postoperativefollow-up.

Acknowledgments

We thank Dr. Kenichi Wakasa, Department of Pathology,Osaka City University Hospital, for his kind assistance and

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advice on histopathological diagnosis. We also thank Ms.Yukiko Wakita for help in preparing this manuscript.

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