General radiological characteristics
Radiography studies are generally not very helpful in assessing extrapleural SFTs, besides showing internal calcifications in some cases [18].
The ultrasound appearance is non-specific. The limited available ultrasound data states that SFTs are heterogeneous masses with internal vascularity [19].
CT evaluation is generally the first modality imaging study to depict the lesion and is good for defining size, local extent, including invasion into adjacent structures, and detection of regional metastases.
Generally SFTs appear as well-defined, hypervascular mass that often compress adjacent tissues and organs.
Scattered intratumoral foci of hypoenhancement or non-enhancement usually represent regions of necrosis, haemorrhage or cystic change. Nevertheless, smaller lesions typically demonstrate homogeneous enhancement.
Calcifications are uncommon, occurring both in large benign or malignant tumors.
MR evaluation is the preferred modality study for tissue characterization. STFs generally present intermediate signal intensity on T1-weighted images and heterogeneous low signal intensity with flow voids on T2- weighted images.
Contrast enhancement is usually avid, with nonenhancing areas occurring in larger lesions, which can represent foci of tissue necrosis or cystic or myxoid degeneration [20].
Extra-pleural SFT by site
The extrapleural SFTs diagnosis is challenging due to the wide differential diagnosis and the lack of specificity of extremity SFTs imaging features [5]. One unifying important extrapleural SFTs differential diagnosis is fibromatosis. Imaging features of SFTs generally also overlap with fibrosarcoma, synovial sarcoma and fibrous histiocytoma [19].
The series of extra-pleural SFT will be presented along the different anatomical locations.
Extra-pleural thoracic SFT
Small SFT of the pleura typically demonstrates at CT imaging a homogeneous well-defined, lobular, soft-tissue mass, adjacent to the chest wall or within a fissure, showing an obtuse angle with the pleural surface. Larger lesions are typically heterogeneous and may not exhibit CT features suggestive of pleural tumors.
An intermediate to high spontaneous attenuation is generally the case on unenhanced CT studies, which is due to the high density of collagen structure and the abundant vascularity of these lesions.
The contrast enhancement is typically intense and heterogenous conferring a geographic pattern of necrosis, hemorrhage and cystic and myxoid degeneration.
MR usually shows heterogeneous signal intensity on T1 and T2 weighted images with avid contrast enhancement following gadolinium administration. Areas of low T2 signal and lack of invasion of adjacent structures are some helpful features in suggesting this tumor.
Thoracic extra-pleural SFT generally present similar CT and MR characteristics as their pleural counterpart with no obvious connection with the pleural surfaces (Figs. 1 and 2).
To be noted that intrapulmonary location is exceeding rare for extrapleural SFTs [5].
SFT of abdominal and pelvis peritoneum
Abdominal and pelvic SFT commonly arise from the peritoneum presenting clinical and imaging features similar to those of the other extrathoracic location. They are more common in patients above 50 years old, often asymptomatic and large at presentation. Patients may experience symptoms associated to compression on contiguous organs, presence of a palpable mass, or hypoglycaemia [15]. Large pelvic SFTs have been reported to result in large bowel obstruction and various urinary symptoms including urinary retention and bilateral hydronephrosis [21].
Local recurrence and malignant degeneration has been reported in SFTs arising from the abdominal and pelvic peritoneum. SFTs of the peritoneum (Figs. 3 and 4) appear as hypervascular masses with intratumoral cystic changes, necrosis or hemorrhage like other extra-thoracic locations. Low signal intensity with flow voids on T2-weighted images, representing fibrosis or collagen, are the most useful MR imaging features.
When arising in gastrointestinal system, SFT has to be essentially differentiated from gastrointestinal stromal tumors (GISTs).
Some pelvic organs such as prostate and the urinary bladder are very unusual locations for SFTs. Gross hematuria, urinary retention and pain are common symptoms. Regarding the bladder, a heterogeneous vascular tumor intravesicular or exophytic can be encountered (Fig. 5). The differential diagnosis in these locations comprises inflammatory pseudotumor, leiomyosarcoma, sarcomatoid transitional cell carcinoma, and pheochromocytoma.
SFT of the head and neck
The head and neck regions are the most frequent extra-thoracic locations of SFTs. Small series report that they tend to be more likely benign than SFTs of other locations [22].
The nasal cavity and paranasal sinuses are frequently involved (Figs. 6 and 7). There are also reports of SFTs in the larynx, nasopharynx and buccal space (Figs. 8, 9 and 10).
Intracranial SFTs are usually of dural origin. Dural tails and hyperostosis of the overlying calvaria is generally observed. The main differential diagnosis are meningioma, neurogenic tumor and soft-tissue sarcoma.
CT and MR imaging generally show, as in other locations, a well-circumscribed tumor, strongly enchanting after contrast administration.
At MR imaging, the usual signal features are observed, with the highlight that a low-signal-intensity rim representing a pseudocapsule around the lesion may be seen on T2-weighted images.
SFT of soft tissue and bones
Somatic soft tissues involving various locations, mainly the extremities and the head, are the origin of 10% of SFTs [23] (Figs. 11, 12, and 13). Only a few reports have described SFT involving the extremities. Most occur in the proximal lower extremities.
Imaging features of extremity SFTs lack specificity and correlate with the variable histopathological appearances.
Extra-thoracic SFTs generally show lobulated well defined margins, tending to displace adjacent structures. They also tend to be highly vascular with avid contrast enhancement.
Although rare, we should raise suspicion of a soft tissue SFT when a deep heterogeneous-mixed soft tissue mass with marked enhancement is observed.
Either benign and malignant tumors like neurogenic tumor, desmoid tumor, fibrosarcoma, synovial sarcoma and malignant fibrous histiocytoma may present overlap imaging and clinical features with SFTs.
