Retroperitoneal solitary fibrous tumor with malignant transformation

Case contributed by Ralph Nelson


Lower abdominal pain with intestinal and urologic obstructive symptoms.

Patient Data

Age: 55 years
Gender: Female

Presacral retroperitoneal rounded well-defined mass displacing the rectum and the rectosigmoid junction anteriorly with preserved presacral fat.

The mass measures 10 x 10 x 9.5 cm with a predominant hypointense signal at the periphery and central stellate hyperintense areas on T2WI. No opposed phase signal drop (not shown). A profoundly hypointense focus at the periphery, likely an area of calcification.  Heterogeneous enhancement of the peripheral mass on the post-contrast images, with a hypovascular swirling-shape central area corresponding to the hyperintense stellate area on T2WI. Diffusion restriction in the periphery. 

The uterus and ovaries are unremarkable.

Differential diagnosis includes gastrointestinal stromal tumor (GIST), solitary fibrous tumor of the retroperitoneum, or neurogenic tumor (less likely).

Immunohistochemistry from US biopsy revealed neoplastic cells strongly expressing CD34

S100, c-KIT, and SMA negative.

CT post embolization treatment


Despite multiple attempts of injections and angioembolization prior to surgical resection, the mass grew to 11.6 x 11.8 x 12.2 cm. 

No interval abdominopelvic lymphadenopathy nor distal metastases.

CT post surgical resection


6 weeks post-resection of the pelvic mass and hysterectomy-BSO, the mass measures 11.2 x 13.4 x 19 cm, corresponding to size progression. Development of multiple confluent rim-enhancing nodules centered on the surgical bed surrounding the vaginal cuff and abutting the posterior urinary bladder wall. Interval extension into the bilateral ischioanal fossae, involvement of the right piriformis muscle, and presence of permeative changes in the right lower sacrum. 

Development of small bowel obstruction with a transition point at the mid ileal loops surrounded by the pelvic mass.  

New peritoneal nodules in the right lower quadrant.

New nodules in the prevesical space, laparotomy scar, and subcutaneous tissue of the right abdominal wall.

Worsening pelvic lymphadenopathy.

New bilateral lung nodules, suspicious for metastatic disease (not shown). 

Case Discussion

The patient presented with fullness in her lower pelvis and mild bowel obstruction.

A large retroperitoneal mass was discovered on CT and a subsequent MRI confirmed the mass to be retroperitoneal, not related to either uterus or ovaries. The mass was histologically proven to be a retroperitoneal solitary fibrous tumor. Despite neoadjuvant therapy, including angioembolization, the mass kept on growing. She, therefore, underwent surgical resection of the mass, including hysterectomy and bilateral salpingo-oophorectomy. Surgical resection margins were uninvolved by the tumor; however, the distance to the closest margin was less than 1 mm. A month and a half post-resection she presented to the ED with rectal and perineal pain. Findings of disease recurrence with distant metastases and lymphadenopathy were found on subsequent CT. Ultrasound-guided biopsy yielded malignant solitary fibrous tumor with more significant cytotoxic atypia compared to the previous specimen; interval larger areas of necrosis and increased mitotic figures. 

Unfortunately, the patient passed away less than 2 years later.

Courtesy of the Abdominal division of MUHC radiology department. A special thank you to Dr. Kuk, M., pathology resident, who provided valuable insight.

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