Solitary fibrous tumor, or SFT, is a rare mesenchymal neoplasm that can arise in almost any soft-tissue site in the body. It was first characterized by Klemperer and Rabin in 1931 as a pleura-derived lesion, and the pleura remains its most common origin. Epidemiologically, SFTs typically occur in adults between the fifth and seventh decades of life, with an equal distribution between the sexes. Decades of pathological research have identified a specific molecular driver: a fusion between the NAB2 and STAT6 genes, which has been confirmed as the characteristic genetic event behind these tumors. While SFTs arising within the abdomen are uncommon in general, those originating from the greater omentum and mesentery are rare, and the malignant subtype is exceedingly scarce.

The patient in this report was admitted to the general surgery department after the incidental discovery of a left abdominal mass. He reported no accompanying symptoms such as abdominal distension, dull pain, nausea or vomiting, and he had no significant past medical history, no family history of malignancy and no known hereditary diseases. Laboratory work was strikingly unremarkable: a comprehensive panel of serum tumor markers fell entirely within normal reference ranges, including carcinoembryonic antigen at 1.89 ng/mL, carbohydrate antigen 19-9 at 9.32 U/mL, alpha-fetoprotein at 3.73 ng/mL, abnormal prothrombin at 19.00 mAU/mL and heat shock protein 90α at 37.90 ng/mL. The absence of any biochemical red flags underscored how easily such a tumor can remain hidden until it is found by chance.

Contrast-enhanced computed tomography of the abdomen provided the key preoperative clues. The scan revealed a lobulated, hypervascular soft-tissue mass measuring 9.3 by 7.9 centimeters, arising from the greater omentum in the left-middle abdomen. The mass showed uneven, significant enhancement, and the imaging also demonstrated notably tortuous and thickened gastroepiploic feeding veins. Importantly, there was no evidence of local invasion or regional lymphadenopathy. The scan did, however, identify a tiny suspicious low-density nodule in liver segment III, a finding that would need careful follow-up to determine whether it represented metastatic disease or something benign. The marked hypervascularity, lobulated morphology and internal heterogeneity seen on CT align with imaging features previously described for larger SFTs, and these characteristics, together with the tortuous feeding vessels, led the team to consider a mesenchymal tumor in the preoperative differential diagnosis.

After routine preoperative laboratory tests and anesthesia evaluation, the surgical team performed an open laparotomy with en bloc tumor resection under general anesthesia on November 28, 2024. Intraoperative exploration revealed a large mass rooted in the left mid-abdominal greater omentum, with engorged superficial venous vasculature, favorable mobility and a clear boundary with adjacent visceral organs. The tumor was pedicled on the greater omentum and showed no adhesion to the surrounding bowel or liver. Its texture was brittle and prone to bleeding under mechanical traction, and the surgeons also detected small sclerotic nodular lesions in the surrounding omental tissue, which were completely resected and sent for pathological examination. Using an ultrasonic scalpel, the team dissected the whole tumor together with a portion of normal greater omentum. The resected specimen measured 11.2 by 8.3 by 6.5 centimeters and had an intact surface capsule. The patient recovered smoothly and was discharged on postoperative day four without complications.

The pathological workup confirmed the diagnosis and illustrated why immunohistochemistry sits at the center of SFT diagnostics. Hematoxylin-eosin staining demonstrated densely proliferative spindle tumor cells with obvious cellular atypia, mitotic activity of at least four mitoses per ten high-power fields, and tumor necrosis involving at least ten percent of the tumor, meeting the established diagnostic criteria for malignant SFT. Immunohistochemical staining showed diffuse positivity for CD34, strong nuclear positivity for STAT6 and positivity for CD99, while S100, cytokeratin, epithelial membrane antigen, CD117, DOG1, smooth muscle actin and desmin were all negative. That negative panel mattered enormously: the absence of CD117 and DOG1 effectively ruled out a gastrointestinal stromal tumor, while negative SMA and desmin excluded smooth muscle tumors, narrowing the diagnosis to SFT. The surrounding omental sclerotic nodules were confirmed as reactive tissue with no microscopic tumor involvement, and all specimen margins were free of tumor, confirming a true R0, or microscopically margin-negative, resection.

Molecular testing then provided definitive confirmation. Fluorescence in situ hybridization detected the NAB2-STAT6 gene fusion that underlies SFT, and nuclear STAT6 immunoreactivity itself serves as a surrogate marker for this driver mutation, with reported sensitivity above 95 percent and high reliability as the primary diagnostic hallmark. The team also performed a supplementary GLI-1 FISH test, which returned negative, rigorously excluding the emerging entity of GLI1-altered malignant mesenchymal tumors that can act as profound morphological mimickers of SFT. This layered approach, from imaging to immunophenotype to molecular confirmation, reflects a broader principle in soft-tissue pathology: cytological testing alone cannot accurately evaluate the malignant potential of an SFT, and fine-needle aspiration is prone to misdiagnosis in the preoperative setting, so a combination of histology, immunohistochemistry and molecular analysis is required.

With malignancy established, the team turned to the 2017 modified Demicco risk stratification model, a four-variable scoring system designed to predict postoperative metastatic risk rather than to serve as a diagnostic tool. The authors emphasize an important methodological point: the maximum tumor diameter used in the calculation came from the definitive gross pathology, approximately 11 centimeters, because macroscopic measurement provides the most accurate anatomical dimension. The patient scored zero points for age, being under 55 at 37 years old; two points for tumor size in the 10 to under 15 centimeter category; two points for a mitotic count of at least four per ten high-power fields; and one point for tumor necrosis of at least ten percent. His total of five points placed him firmly in the intermediate-risk category, corresponding to an approximate ten percent ten-year metastatic risk under the 2017 criteria.

That risk classification shaped the surveillance plan. Because SFTs are known for the potential to recur or metastasize late, long-term postoperative imaging surveillance was arranged, with contrast-enhanced CT scheduled every three months during the first two years as a patient-specific strategy. At twelve months, follow-up CT showed no local tumor recurrence, and the hepatic segment III nodule identified before surgery remained stable without evidence of interval progression. A subsequent 18F-FDG PET/CT assessment revealed no abnormal hypermetabolism in that hepatic nodule, strongly supporting a benign etiology such as a hemangioma and effectively ruling out metastasis. The authors are candid about the limits of this follow-up: twelve disease-free months is insufficient to fully evaluate the long-term recurrence risk of a malignant SFT, and extended surveillance is absolutely necessary for patients in this category.

The clinical message from the case is straightforward. Complete surgical excision remains the primary radical treatment for localized SFT, and for advanced, unresectable, recurrent or metastatic disease, contemporary evidence supports antiangiogenic therapies such as pazopanib and sunitinib to control tumor progression, though these are not established therapies for routine localized cases. Malignant behavior, the authors conclude, may occur across a broad range of patient ages and tumor sizes in primary omental SFT, meaning clinicians cannot dismiss the possibility based on a patient being young or a tumor being of moderate size. Diffuse and strong nuclear STAT6 immunoreactivity remains the most convenient and reliable diagnostic marker, radical R0 resection is the core curative treatment, and the Demicco scoring system should guide postoperative risk grading and follow-up intensity. For intermediate-risk patients, continuous long-term imaging surveillance is essential, and the authors note that more clinical data are needed to explore novel targeted and antiangiogenic therapies for advanced malignant omental SFT. Given the extreme rarity of this tumor type, single documented cases like this one collectively form the evidence base that will shape how the next such patient is diagnosed and treated.

Subject of Research: Malignant solitary fibrous tumor of the greater omentum: a rare case report with pathological diagnosis and Demicco risk stratification

Article Title: Case Report: Malignant solitary fibrous tumor of the greater omentum

Article References: Zhong, D., Ji, J., & Li, J. (2026). Case Report: Malignant solitary fibrous tumor of the greater omentum. Frontiers in Oncology, 16, Article 1968288. https://doi.org/10.3389/fonc.2026.1968288

Image Credits: AI Generated

DOI: 10.3389/fonc.2026.1968288

Keywords: solitary fibrous tumor, greater omentum, case report, STAT6, NAB2-STAT6 fusion, immunohistochemistry, Demicco risk model, surgical oncology, R0 resection, soft-tissue tumor, abdominal mass, surveillance imaging