Clinical History
A perimenopausal Chinese woman suffering from urinary frequency, unchanged bowel habits, and pain at evacuation since 6 months, was referred to the radiology department for further investigation of a suspected pelvic mass. There were unremarkable physical findings, apart from firm extrinsic rectal compression at digital exploration. Laboratory assays were within normal limits.
Imaging Findings
In China, ultrasound (not shown) recently described a large pelvic mass with mixed echogenicity, initially interpreted as a non-cystic ovarian tumour.
At our hospital, further investigation with MRI (Fig.1, 2) confirmed a well-demarcated presacral ovoid mass measuring 10x9 cm (axial) and 13 cm (caudocranial) diameters, occupying the mesorectal space without infiltration of the ischiorectal fossa, which displaced and compressed extrinsically the urinary bladder, uterus, and rectum. The lesion appeared mostly T2-hypointense, suggesting desmoplastic tissue, with low-to-intermediate unenhanced T1 signal intensity, and marked heterogeneous contrast enhancement, without fluid or necrotic regions.
Surgery through a posterior (sacral) approach allowed complete excision of the pelvic mass, which corresponded to a solitary fibrous tumour at histopathology.
Discussion
A unique group of uncommon mesenchymal neoplasms originating from submesothelial fibroblasts, solitary fibrous tumours (SFTs) were previously known as “localized fibrous tumours” or “localized fibrous mesotheliomas”. Histologically, SFTs are encapsulated spindle cell proliferations with alternating hyper- and hypocellular regions, dense collagenous stroma and prominent thin-walled blood vessels. Mostly reported in the pleura, SFTs may occur virtually anywhere in the body, with the head and neck as the commonest sites. Abdominal, retroperitoneal and pelvic involvement is sporadic. Extrapleural SFTs generally occur in the fifth and sixth decade of life without gender predominance, and present slow-growing, often asymptomatic masses which exceed 10 cm in over 50% of cases. Alternatively, patients may complain of palpable mass, abdominal fullness, or compression symptoms such as urinary retention, frequency or bowel obstruction. A few (<5%) cases manifest with paraneoplastic hypoglycaemia [1-3].
Imaging plays a key role in (often incidental) detection, characterization and presurgical assessment of SFTs, which usually appear as solitary, well-demarcated ovoid or round masses that displace and compress neighbouring structures (kidneys, ureters, bladder and seminal vesicles, bowel and rectum, bony pelvis and abdominal wall muscles): this expansile behaviour within spaces of the abdomino-pelvic cavity without infiltration is secondary to the submesothelial origin with intact overlying mesothelial lining. CT shows SFTs as solid heterogeneous masses without calcifications. MRI appearances are related to cellularity, vascularity, collagen presence, and necrosis. SFTs generally show intermediate T1 signal intensity, heterogeneous T2 signal with hypointense regions corresponding to myxoid stroma, alternating hyperintense cellular regions, sometimes flow voids representing prominent vascular channels. The fibrotic – collagenous stroma shows mild, progressive contrast enhancement. Conversely, solid hypercellular regions show avid enhancement which persists on venous and delayed phase acquisitions [1-3].
Although prospective imaging diagnosis is challenging, SFT is suggested by a solitary noninvasive mass with predominantly low T2 signal, marked heterogeneous enhancement, and sometimes circuitous vessels along the periphery. The differential diagnosis for abdominal and pelvic retroperitoneal extra-parenchymal masses mostly includes other mesenchymal or hypervascular neoplasms, and extraintestinal gastrointestinal stromal tumours. Furthermore, imaging allows differentiation from more common gynaecological and rectal masses [1-4].
Surgical excision is the treatment of choice and allows 5-year survival rate approaching 100%. Whereas most SFTs are benign, 10-20% of cases exhibit malignant histological features (cellular atypia, high mitotic activity, necrosis and haemorrhage) and/or aggressive clinical behaviour with recurrence or metastatization [1-3].
Differential Diagnosis List
Pelvic solitary fibrous tumour
Desmoid tumour
Malignant fibrous histiocytoma
Vascular tumours (angiosarcoma
angiomyxoma
hemangioendothelioma)
Fibrosarcoma
Extraintestinal gastrointestinal stromal tumours
Exophytic rectal cancer
Lymphoma
Ovarian non-cystic mass (e.g.fibroma
fibrothecoma
Brenner tumour)
Final Diagnosis
Pelvic solitary fibrous tumour
