Solitary fibrous tumor is a rare, benign spindle cell neoplasm that was first described in the thoracic pleura. This tumor is now known to occur at many extrapleural sites. There are established criteria for the diagnosis of malignant solitary fibrous tumor including ≥4 mitotic figures per 10 high-power fields, increased cellularity, cytologic atypia, infiltrative margins, and/or necrosis. Although all solitary fibrous tumors have the potential to recur or metastasize, those with malignant histologic features tend to behave more aggressively. We report a case of solitary fibrous tumor, with malignant histologic features, in a 21-year-old woman which arose from the serosal surface of the sigmoid colon.
Solitary fibrous tumor is a rare spindle cell neoplasm that most commonly arises from the thoracic pleura; however, solitary fibrous tumor is increasingly being reported to arise in multiple extrapleural anatomic sites including the soft tissues of the head and neck, thoracic wall, mediastinum, pericardium, retroperitoneum, peritoneum, abdomen, meninges, orbit, upper respiratory tract, salivary glands, thyroid, liver, adrenal gland, kidney, spermatic cord, urinary bladder, prostate, uterine cervix, spinal cord, and periosteum [
These rare mesenchymal derived spindle neoplasms are currently classified as “typical” or “malignant.” The criteria to designate the tumor as malignant are dependent on the number of mitoses (≥4/10 high-power fields), cellular atypia, presence of necrosis, hypercellularity, and/or infiltrative margins [
A 21-year-old woman presented with acute onset of abdominal pain with recent hematochezia and constipation. The pain was initially described as being most severe in the suprapubic area, followed by diffuse involvement of the entire abdomen, and ultimately localizing to the right lower quadrant. The clinical differential diagnosis at this point included acute appendicitis, ovarian cyst with possible rupture, urinary tract infection, internal hemorrhoids, inflammatory bowel disease, and pancreatitis. An abdominal computed tomography (CT) scan demonstrated an irregular, right adnexal mass measuring 8.8 × 7.8 cm (Figure
Computed tomography scan: There is a large irregular heterogeneous exophytic mass that appears to be arising from the right adnexa which intraoperatively arose from the serosa of the sigmoid colon.
Grossly, the mass was fragmented and 16 × 11 × 9 cm in size (aggregate measurement). The mass was well-circumscribed but unencapsulated and appeared to arise from the serosal surface of the sigmoid colon (Figure
Gross photograph demonstrating a tan-white mass arising from the serosal surface of the sigmoid colon.
Histologically, the tumor was composed of spindle cells with predominantly hypercellular and focal hypocellular areas and prominent hemangiopericytoma-like vasculature. Cytologically, the spindled cells had a scant amount of cytoplasm, mild to moderate cytologic atypia and pleomorphism, and 8 mitotic figures per ten high-power fields (Figures
Hypercellular spindle cell neoplasm with hemangiopericytoma-like vasculature (H&E, original magnification ×100).
The tumor cells are ovoid to spindled with scant cytoplasm, indistinct borders, and vesicular nuclear chromatin. A mitotic figure is present in the lower-half of the field (H&E, original magnification ×400).
The neoplastic cells demonstrate strong nuclear positivity with antibody to STAT6 by immunohistochemistry (STAT6, original magnification ×200).
Solitary fibrous tumor is a rare benign mesenchymal neoplasm originally described in the pleura, but now known to occur at nearly any extrapleural site [
Macroscopically, the majority of SFTs are well-circumscribed, partially encapsulated masses that are between 1 and 25 cm in size [
The histologic differential diagnosis for extrapleural solitary fibrous tumor is broad [
A recurring and nonrandom fusion of two genes has been recently identified in solitary fibrous tumor and involves the NGFI-A-binding protein 2 (NAB2) and STAT6 genes, both located at chromosomal region 12q13. This fusion event is thought to represent an initial tumorigenic event in SFT [
The prognosis for extrathoracic solitary fibrous tumor is difficult to assess and can be unpredictable but most cases behave in a benign fashion [
Herein, we have reported a case of a malignant solitary fibrous tumor, by current histologic criteria, in a rare extrapleural location: the serosal surface of the sigmoid colon. This is a relatively uncommon site with only two other reported cases, to our knowledge, in the sigmoid mesocolon [
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to acknowledge Girish Venkataraman, M.D., for performing the STAT6 immunohistochemical stain.