Giant Solitary Fibrous Tumor of Pleura Presenting Both Benign and Malignant Features

Background

Solitary fibrous tumor (SFT) is rare mesenchymal tumor that typically originates from the pleura and pelvis. The incidence of SFT of the pleura (SFTP) is less than 5% of all pleural tumors [1,2]. The prognosis is relatively good, but up to 20% of cases are malignant [3]. It is important to determine whether the tumor is benign or malignant before deciding on treatment options and estimating prognosis, but this can sometimes be difficult. We managed a case of SFTP that showed a discrepancy in prognostic prediction between clinical and pathological findings.

Case Report

A 59-year-old woman with no prior medical history presented with a 4-month history of right back pain and dyspnea. A chest radiograph showed a mass lesion in the middle-to-lower right lung field (Figure 1), contrast-enhanced computed tomography (CT) of the chest revealed a giant oval mass (long diameter, 14 cm) in the right thoracic cavity, with inhomogeneous intensities (Figure 2A–2C), adhering to the Th4-5 level pleura via a pedicle (Figure 2D). A moderate right pleural effusion was also observed, which was bloody and exudative on thoracic puncture. On 18F-fluorodeoxyglucose (FDG) positron emission tomography/CT, the mass showed mild FDG up-take (maximum standardized uptake value=3.1). Also, there were no increases in blood carcinoma-specific tumor markers, including carcino-embryonic antigen, cytokeratin-19 fragment, squamous cell carcinomas, neuron-specific enolase, progastrin-releasing peptide, soluble interleukin-2 receptor, human chorionic gonadotropin, and a-fetoprotein. From these findings, SFTP was clinically suspected and the mass and its pedicle were completely resected thoracoscopically (Figure 3A).

Despite the presence of bloody pleural effusion, no evidence of metastasis and pleural dissemination were found in intra-operative findings, and the pleural effusion had no evidence of malignancy. On the macroscopic examination of the resected tumor, intratumor hemorrhage and necrosis were observed (Figure 3B). Microscopically, irregular fascicles of spindle cells with positive results of CD34 and STAT6 for the diagnostic biomarkers of SFT and staghorn-like blood vessels were observed (Figure 4A–4C). Ki-67 (MIB-1) proliferation index for the detectable biomarker of malignant feature was less than 1% (Figure 4D). The histological diagnosis was SFTP with no recurrence observed for 2 years after symptom onset (Figure 5).

Discussion

SFTP is rare spindle cell tumor typically originated from the visceral pleura. Although its incidence is less than 5% of all pleural tumors, it has been increasingly recognized over the past few years [1,2]. Although the predominant onset age of SFTP is reported to be the sixth and seventh decades of life, it can also occur in any age group [4]. Many patients (about 50%) with SFTP are asymptomatic and tumors are often found incidentally on chest radiograph [5]. However, some patients may have cough, chest pain, and dyspnea related to the size of the lesions. Ten-year overall survival rates widely ranged from 66.9% to 97.5% [1,6]. Complete surgical resection is the mainstay of treatment for both benign and malignant SFTP.

Making the differential diagnosis between benign and malignant SFTP is usually problematic for predicting the disease prognosis. Although criteria for defining malignant SFTP have been discussed in previous reports [7–10], there have been no established biomarkers for distinguish malignancy from benign in patients with SFTP.

Originally, in a clinicopathological review of 223 localized fibrous tumor of pleura, England et al. defined tumor size ≥10 cm, tumor necrosis and hemorrhage, mitoses ≥4/10 high-power field, increased cellularity, and nuclear pleomorphism as the malignant features of SFTP [7]. Recently, in a radiographical review of 60 SFTP cases, You et al. reported that larger tumor size, inhomogeneous intensities, abundant intratumor blood vessels, and pleural effusions in the chest CT were more common in malignant SFTP [8]. The present case had several malignant features, including large size of tumor, intratumor hemorrhage, inhomogeneous intensities, and pleural effusion. However, in the pathological analysis of 78 SFTP cases, Diebold et al. reported that Ki-67 (MIB-1) proliferation index ≥10%, which was expressed in active phases of the cell cycle, including G1, S, G2, and mitosis, appear to have strong association with the degree of histopathological malignancy and are independent prognostic factors [9,10]. In the present case, the Ki-67 (MIB-1) proliferation index was less than 1%. Therefore, we thought the present case showed both malignant and benign features, and careful follow-up was necessary to detect the recurrence, although the patient was without recurrence for 2 years after onset. Furthermore, in patients with SFTP, multi-disciplinary discussion among the clinician, radiologist, and pathologist is important for prognostic prediction and follow-up for detecting recurrence, and occasionally it is helpful for Ki-67 (MIB-1) proliferation index to distinguish between malignant and benign tumors.

Conclusions

We managed a case of SFTP presenting both malignant features, including large size of tumor, intratumor hemorrhage, inhomogeneous intensities, and pleural effusion, and benign feature including low Ki-67 (MIB-1) proliferation index. In patients with SFTP, multi-disciplinary discussion among clinicians, radiologists, and pathologists was considered to be needed for deciding on treatment options and estimating disease prognosis.