Multimodality imaging of incidental pleural fibroma: usefulness for surgical planning

A routine chest X-ray performed in an asymptomatic 46-year-old woman showed an extrapulmonary lesion. Careful assessment of the lesion by the chest radiology team provided useful preoperative information for surgical planning.

The chest x-ray showed a small well-defined opacity in the upper left hemithorax. The mass abutted the chest wall and formed an obtuse angle (Figure 1, A-B), suggesting an extrapulmonary origin of the lesion.

Chest CT revealed a well-circumscribed and relatively homogeneous intense background enhancing mass (rich vascularization) arising from the left chest wall. Obtuse angles with the chest wall with tapered margins and mild displacement of pulmonary vasculature adjacent to the lesion suggested a pleural origin (Figure 2, A-D). The fat plane of the intercostal muscles was preserved. No bony destruction of the thoracic cage was detected.

PET-CT showed low metabolic activity (SUV max=2.5) of the pleural-based lesion (Figure 3). The mass was surgically removed by robot-assisted procedure (da Vinci® surgical system) (Figure 4). Histological images showed spindle cells with prominent stromal collagen and hemangiopericytoma-like vessels (Figure 5, A-B). Immunohistochemical analysis revealed a negative staining for CD34 and nuclear expression of STAT6 with positive staining for BCL2 (Figure 6, A-C). These findings confirmed the diagnosis of pleural fibroma.

Solitary fibrous tumors (SFT), also known as pleural fibromas, are rare mesenchymal tumors which arise from the pleura [1]. Most of them (80%) originate from the visceral pleural (SFTPs) [2,3]. SFTs generally occur during the fifth and sixth decades of life without significant sex predilection. They are usually benign and slow growing tumors. In 20% of the cases, SFTs can be malignant.

Most patients with SFTs are asymptomatic. The tumor is usually discovered as an incidental finding on a routine chest radiograph. Other patients can present non-specific pulmonary symptoms such as: chest pain (more common in those tumors originated from the parietal pleura), cough or shortness of breath [4]. Digital clubbing and hypertrophic pulmonary osteoarthropathy can be associated in 20% of pleuropulmonary SFT.

Radiologists must be aware of the imaging characteristics of these tumors to determine the lesion origin and thus contribute to surgical planning. CT imaging features of SFTs are not specific. They usually appear as well-defined and peripheral soft-tissue masses showing an obtuse angle with the chest wall [6]. Larger lesions are heterogeneous and may present calcification (seen in up to 26% of cases) [7,8].

On imaging, the tumor is attached to the lung by a narrow pedicle when originates from the visceral pleura (2/3 of the cases), while it presents a broad-based attachment when arises from the parietal pleura. The origin of the tumor provides important information for the ulterior surgical management: the small and pedunculated SFTs can be removed by minimally invasively surgery (robot-assisted procedure), while the larger ones, usually arising from the parietal pleura, require thoracotomy [5]. Uncommonly, a tumor arising from the parietal pleura can be "inverted" and appear to grow within the lung parenchyma [9].

On MRI, SFTs typically show low T1 signal intensity, variable T2 signal, and avid contrast enhancement [10].

Based on FDG uptake, PET-CT can sometimes help to differentiate benign from malignant SFT (mean SUVmax of 3.02±1.02 for benign SFTs vs 4.89±2.12 for malignant SFTs). Nevertheless, if unclear, the size, presence of necrosis and hilar lymphadenopathy on CT images are key findings indicating malignancy [11, 12]. Both CT and PET-CT can be helpful to exclude other malignant tumors of the pleura.

Surgical excision is the treatment of choice. Recurrence is rare [1, 2].

The immunohistochemical (IHC) profile of the lesion markers of SFT include a negative expression of CD34 (endothelial marker) and positive staining for BCL2 (B-cell lymphoma 2). Although these markers have been historically useful to distinguish SFT from histologic mimics, they are not specific for SFT. STAT-6 positive nuclear immunoreactivity is essential for distinguish solitary fibrous pleural tumor from sarcomatoid mesothelioma and synovial sarcoma, relatively frequent lesions in this location, which entail different treatment and prognosis[13].