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Solitary fibrous tumor (SFT) is a rare mesenchymal tumor with a highly variable clinical presentation. Extrathoracic SFT carries a malignant potential of 11% to 33%^1- 2 and should be managed with surgical resection if possible. Based on a small number of reported cases, pelvic SFT is generally believed to be clinically comparable with those in other extrathoracic locations. They represent a diagnostic challenge to the surgeon because they are easily mistaken for other more common tumor types based on imaging studies.³ In addition, pelvic SFT may present with more advanced tumor size and carry a higher rate of pathologic malignancy.⁴

In a case report that reviewed all 17 reported pelvic SFT cases to date, Kawamura and colleagues⁴ noted a malignancy rate of 25%, slightly higher than the malignancy rate reported for other extrathoracic SFTs.¹ In addition, the clinical behavior of extrathoracic SFTs is unfortunately not predictable based on pathology. While there are a number of pathologic features reported to roughly correlate with malignancy, including size of the tumor, hypercellularity, mitotic activity, nuclear pleomorphism, necrosis, and Ki-67 and p53 staining,^1,5- 6 none of these are reliable markers for recurrence and metastasis.^7- 8 It follows therefore that in our case, as in all cases of pelvic SFT, even in those without malignant features in the pathology, patients should be followed up closely for any clinical or radiological signs of recurrence.

Extrathoracic SFT is often confused on radiographic imaging with other more common tumors.^9- 10 In this case, based on the size, vascularity, and location noted on computed tomographic imaging, the mass was presumed to be a paraganglioma arising from the organ of Zuckerkandl. This assumption did not adversely affect the overall management, which would have required surgical resection regardless.

In summary, we report a case of pelvic SFT with benign pathological features that was initially thought to be a paraganglioma on radiographic imaging. This is one of only a handful of reports in which this tumor type originates in the pelvis. Because of the inherent malignant potential, more cases with long-term follow-up will be needed to help identify characteristics that predict aggressive behavior. Accordingly, surgeons should have a higher degree of suspicion for this tumor type in patients who present with large pelvic masses.

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