A rare case of a retroperitoneal solitary fibrous tumour

An asymptomatic 65-year-old male patient was submitted to an abdominal ultrasound as part of a study for peripheral vascular disease. An abdominal predominantly hypoechoic mass was found and further characterisation was requested. No headaches, palpitations, diaphoresis or hypertension were present.

The patient underwent an abdominal and pelvic CT with contrast-administration demonstrating a retroperitoneal mass with avid enhancement. It was round, with well-defined margins and measuring 8cm (Figs. 1a-c). No lymph node enlargement was seen, neither were other masses or significant alterations, rather than calcific vascular atherosclerosis.

For further characterisation, a scintigraphy with MIBG was requested as the possibility of a paraganglioma was raised, as well as a Gallium-68 DOTATOC​ PET-CT for evaluation of primary or metastatic neuroendocrine activity.

The retroperitoneal mass was characterised as negative on scintigraphy with MIBG that demonstrated physiological uptake in the liver, salivary glands, urinary bladder, heart and gastrointestinal tract (Fig. 2).

On Ga-68 DOTATOC PET-CT, intense uptake was noted in the referred retroperitoneal mass (Fig. 3). No other foci of abnormal uptake were noted.

The patient underwent resection due to the concern for neuroendocrine tumour and the need for pathology confirmation. Surgery with negative resection margins was performed without major complications.

Solitary fibrous tumours (SFTs) are rare spindle-shaped cell tumours originating from dendritic mesenchymal cells that can be either benign or malignant [1]. The most common location is the thoracic cavity, more precisely originating from the visceral pleura, being rare in the retroperitoneum (1% of all SFTs) [2,3]. They are often asymptomatic with symptoms related to compression of adjacent structures [2].

SFTs may overexpress somatostatin receptors, therefore mimicking neuroendocrine tumours at somatostatin receptor PET/CT as in the case presented [4].

Myxoid degeneration, haemorrhage, and necrosis can be present, contributing to their heterogenous appearance in cross-sectional imaging studies [2]. They are usually well-defined tumours with variable degree of enhancement, being commonly described as hypervascular when located in the abdomen and retroperitoneum [5].

At magnetic resonance imaging, SFTs usually have isointense signal intensity on T1-weighted images and heterogeneous low-signal intensity on T2-weighted images. After contrast administration, SFTs are usually hypervascular tumours with intense enhancement. However, as reported for CT studies, the enhancement pattern on dynamic contrast-enhanced images can vary widely depending on cellularity, vascularity, and the density of the collagenous stroma of the tumour [1].

Therefore, on imaging studies differential diagnosis should include well-defined avid enhancement masses as paragangliomas, neuroendocrine tumours and desmoid tumours and also commonly encountered retroperitoneal masses as nerve sheath tumours.

When imaging is not conclusive as in the case presented, once the Ga-68 PET-CT demonstrated avid enhancement raising the concern for neuroendocrine tumour, the differential diagnosis should be done by pathology. As SFTs are distinctive spindle-cell neoplasms, the differential diagnose will include entities as leiomyoma or inflammatory myofibroblastic tumour. In this setting, immunohistochemistry is very helpful as SFTs are positive for Bcl-2, vimentin, CD99 and CD34 [6].

As with most sarcoma-like tumours, surgery is the mainstay treatment for SFTs. In the case presented, surgery with negative resection margins was performed and the tumour was considered benign on pathology examination.

In conclusion, retroperitoneal SFTs are rare, and radiologists should be aware of its specificities to include them in differential diagnoses. Complete surgical excision is the mainstay of treatment. However, intensive long-term follow-up should be warranted for all extra-thoracic SFTs, particularly for those with positive surgical margins as they have potential for recurrence [3].

Written informed patient consent for publication has been obtained.