Solitary Fibrous Tumor of the Buccal Space: Treatment with Percutaneous Cryoablation

SUMMARY: Solitary fibrous tumors are rare spindle cell neoplasms that typically occur in the thorax but have been described in various locations within the abdomen and head and neck region. The most common extrapleural site is the oral cavity, but these tumors have been also described in the orbit, nasopharynx, paranasal sinuses, salivary glands, and larynx.1–3 We describe a case of a solitary fibrous tumor of the buccal space successfully treated with percutaneous CT-guided cryoablation.

tors were placed simultaneously into the mass in a parallel fashion with the tips approximately 1 cm apart (Fig 2A). Temperatures were measured during the procedure with an attached thermocouple (range, Ϫ94°C to Ϫ136°C) at the tip. Active thawing of the freeze zone was achieved using helium. The patient underwent 2 consecutive 10-minute freeze-thaw cycles. Interval unenhanced CT evaluation showed a small crescent of soft tissue not optimally treated with the freeze zone. The cryoapplicators were then repositioned into this superolateral aspect of the mass, and 2 additional 8-minute freeze-thaw cycles were performed. The freeze zone measured 3.3 ϫ 4.5 ϫ 3.0 cm on immediate posttreatment CT imaging and encompassed the mass along with a peripheral 5-mm margin around the mass (Fig 2B). The patient was observed for a 2-hour period after the procedure and was discharged home in stable condition.
At 2-day follow-up visit, there was some swelling of the malar soft tissues. A small eschar was noted on the skin surface along with sloughing of the buccal mucosal surface related to thermal injury. However, this completely resolved at 6-month follow-up. The patient experienced no residual pain, and there was no evidence of sensory or motor nerve deficit or duct injury. Routine follow-up MR imaging at 6 months revealed an organized fluid collection within the buccal soft tissues measuring approximately 2 cm compatible with posttreatment changes, with only a small area of peripheral enhancement. A sonography-guided fine needle aspiration was performed to evaluate for recurrence. Cytology revealed necrotic cells without evidence of tumor. Follow-up MR imaging at 14 months showed near-complete collapse of the treatment cavity with minimal posterior residual peripheral enhancement (Fig 3). The patient's facial swelling had resolved, and her external appearance returned to normal. Despite the low-grade pathology of the tumor, continued imaging surveillance will be performed, given the recurrent nature of the original neoplasm.

Discussion
SFTs are spindle cell tumors that are typically associated with serosal surfaces, especially the pleura, but have been found to arise in various extrapleural locations, most notably the oral cavity. They typically present as a slow-growing, painless mass. SFTs of the thorax usually have a favorable prognosis. However, they have been reported to be malignant in 13%-23% of cases. 2,3 Factors associated with malignancy include high cellularity, more than 4 mitoses per 10 high-power fields, pleomorphism, hemorrhage, and necrosis. 3 Extrapleural SFTs, however, are almost always benign and are cured with simple surgical excision. In this patient, who developed a recurrence despite low-grade pathology and who refused surgery, percutaneous thermal ablation was proposed as a treatment alternative.
Additional diagnostic considerations for a mass involving the buccal space include tumors of minor salivary gland origin, such as pleomorphic adenoma, adenoid cystic carcinoma, or mucoepidermoid carcinoma, along with hemangioma, lymphoma, squamous cell carcinoma, lipoma, soft tissue sarcoma, abscess, lymph node, and nerve sheath tumor. However, because other buccal space masses can present in the same fashion, and because the imaging appearance of many of these processes is nonspecific, the diagnosis is based on the microscopic appearance and characteristic immunohistochemical staining. CD34, a transmembrane glycoprotein, has been found to be a highly sensitive marker for SFTs. Radiographically, they are characteristically isoattenuated to muscle on noncontrast CT and isointense to muscle and brain on noncontrast T1-and T2-weighted MR imaging. They typically enhance avidly in a homogeneous fashion after contrast administration. Some variability in MR imaging characteristics has been described, however, with T2 heterogeneity being the most commonly described alternative imaging appearance, [4][5][6] as was seen in our case subject.
Percutaneous cryoablative techniques have been described for the treatment of primary neoplastic and metastatic disease in various organ systems, most notably the liver and prostate. Recent literature has also shown benefit in palliating painful bony and extra-abdominal soft-tissue metastases using percutaneous image-guided cryoablation. 7,8 Benefits of this minimally invasive treatment option include the absence of a surgical scar, reduced recovery time, and the ability to visualize the treated tumor both during and immediately after the procedure to determine treatment success. Probe diameters have also decreased in recent years, further making use of this technique to other sites of disease safer and easier with less postprocedural bleeding. Cryoablation offers a few specific advantages over heat-based therapies. These include less procedural pain due to the anesthetic effect of freezing; direct visualization of the ice ball with CT, MR imaging, or ultrasound so that the treatment margins can be monitored in real time, with less disruption of underlying supportive tissue making it safer for lesions next to skin, nerve, and hollow viscera; and the ability to treat lesions with multiple applicators so that large and irregularly shaped masses can be ablated at one sitting.
Within the head and neck region, thermal ablative techniques using radiofrequency energy have been described to treat adenoid cystic carcinoma and recurrent thyroid carcinoma. 9,10 However, to our knowledge, this is the first reported use of percutaneous cryoablation to treat a mass in this region. Further investigation of the possible benefits of cryoablation over excision for benign masses would be beneficial.