Metastatic Solitary Fibrous Tumor of Parapharyngeal Region: A Case Report with Review of Literature

Abstract

Solitary fibrous tumor (SFT) is a rare fibroblastic mesenchymal neoplasm with a low metastatic potential. The parapharyngeal area in the head and neck is one of the rarest subsites. Surgical excision remains the standard of care. The local recurrence rate stands at 4%, while the distant recurrence rate is 7% over a 5-year period. Radiation can be used in adjuvant, metastatic, and palliative settings. In metastatic disease, therapeutic options are limited, with the most common regimen used in the first line being a combination of dacarbazine and doxorubicin. With a better understanding of molecular biology, many centers use bevacizumab with temozolomide in the first line. A 50-year-old man underwent a wide excision of a mass in the parapharyngeal region in 2014. Histopathology showed a SFT with negative margins and a low mitotic index. He was under regular follow-up. He presented in January 2024 with abdominal discomfort and distension, fever, cough, and substantial weight loss. Clinically, he had hepatomegaly, lab investigations revealed transaminitis and elevated serum alkaline phosphatase. 18F fluorodeoxyglucose positron emission tomography–computed tomography (18F-FDG PET CT) revealed multiple lung and liver metastases. Ultrasound-guided tru-cut biopsy of the liver lesion with immunohistochemistry (IHC) confirmed metastasis from SFT (positive for signal transducer and activator of transcription 6 and cluster of differentiation 34). He received bevacizumab and temozolomide for four cycles, and a PET CT scan showed progressive disease as per Choi's criteria. Treatment was changed to pazopanib 800 mg once daily as the patient was not keen on intravenous chemotherapy. The patient progressed clinically and radiologically. His programmed death-ligand 1 IHC showed a tumor proportion score of 0%. Next-generation sequencing 75 gene panel could not be done due to low tumor content in the block. The treatment was changed to doxorubicin. He received three cycles of doxorubicin and progressed clinically with deterioration of general health and hence offered the best supportive care. He succumbed to the illness in September 2024.

Keywords

metastatic solitary fibrous tumor - lung and liver metastases - case report - head and neck - parapharyngeal

Introduction

Solitary fibrous tumor (SFT) is a rare fibroblastic mesenchymal neoplasm that accounts for less than 3.7% of all soft tissue masses.[1] At present, around 50 to 70% of the SFTs occur outside the thorax. The parapharyngeal area is one of the rarest subsites. Surgical excision remains the standard of care. Local recurrence rates at 5 and 10 years are 4 and 7%, respectively. The distant recurrence rates at 5 and 10 years are 13 and 16%, respectively.[2] Radiation can be used in adjuvant, metastatic, and palliative settings. In metastatic disease, therapeutic options are limited, with the most common regimen used in the first line being a combination of dacarbazine and doxorubicin. With a better understanding of molecular biology, many centers use bevacizumab with temozolomide in the first line.[3] [4] Here, we present a rare case of SFT of the parapharyngeal region which recurred after 10 years with lung and liver metastases with a literature review.

Case Report

A 50-year-old man presented to a multispecialty hospital with dyspnea, hoarseness of voice, and right upper neck swelling in 2014. A contrast-enhanced computed tomography (CECT) showed a mass of size 6 × 5 cm involving the right parapharyngeal region for which he underwent wide excision of the lesion under general anesthesia. Histopathology showed a SFT of size 5.5 × 5 cm, all the margins were negative, low mitotic index, and no necrosis. Immunohistochemistry (IHC) showed positive for cluster of differentiation (CD)34, negative for synaptophysin, S100, EMA, and Ki-67 was 4 to 5%. He was kept on regular clinical follow-up after the tumor board discussion. He has had Type 2 diabetes mellitus for 3 years and is on medication. He was evaluated at an outside hospital with complaints of right hypochondrial pain, dry cough, and significant weight loss of 8 kg over 3 months (11%) in January 2024. On examination, his performance status was Eastern Cooperative Oncology Group (ECOG) 1. Physical examination revealed a 5 cm surgical scar on the upper neck and hepatomegaly, with the liver palpable 3 cm below the right costal margin, firm in consistency with a well-defined edge. Routine blood investigations revealed normal bilirubin levels, mild transaminitis, and elevated serum alkaline phosphatase of 337 U/L (normal range 53–128 U/L). A contrast-enhanced CT scan of the thorax and abdomen revealed multiple well-defined soft tissue density lesions in both lungs with multiple liver lesions with contrast enhancement suggestive of lung and liver metastases. 18F fluorodeoxyglucose positron emission tomography–computed tomography (18F-FDG PET CT) revealed multiple nodules in bilateral lungs (largest 4.2 × 3.3 cm in the right upper lobe) and multiple hypodense lesions in the liver (largest 6.1 × 1.5 cm in segment 7/8) ([Fig. 1B]). Ultrasound-guided tru-cut biopsy of the liver lesion revealed poorly differentiated spindle cell neoplasm, and IHC showed positive for signal transducer and activator of transcription (STAT) 6 and CD34, focally positive for creatine kinase and Tranducin-like enhancer of split 1 (TLE1), negative for S100, synaptophysin, and CD117. The Ki-67 proliferation index was 25%. Morphology and IHC were consistent with metastasis from SFT ([Fig. 2]). He was then referred to us for further management. We reviewed the initial biopsy slides and blocks, liver biopsy slides and blocks, and IHC slides from our pathology department which confirmed the diagnosis of SFT. He was treated with a combination of bevacizumab 7.5 mg/kg in 100 mL normal saline as an intravenous infusion and temozolomide 200 mg/m² days 1 to 5 orally every 28 days. The patient tolerated the treatment well without much side effects. The patient was clinically stable. A PET CT scan with intravenous contrast was taken after four cycles, which showed bilateral lung lesions stable in size with an increase in metabolic activity, liver lesions increased in number, size, and metabolic activity suggestive of progressive disease as per Choi's criteria ([Fig. 1A]). The option of changing intravenous chemotherapy was explained, but the patient and relatives opted for the oral treatment option. He was started on pazopanib 800 mg once daily. The patient progressed clinically with increasing abdominal pain, increasing hepatomegaly, increasing bilirubin, transaminitis, and elevated ALP. CECT scan revealed the progressive disease. His programmed death-ligand 1 (PD-L1) IHC was 0%. Next-generation sequencing (NGS) 75 gene panel was requested to look for tissue agnostic biomarkers, but this could not be done due to tumor content less than 5% in the block. He was then started on single agent doxorubicin 60 mg/m² intravenous push every 3 weeks. He received three cycles of doxorubicin and progressed clinically with deterioration of general health and hence offered the best supportive care. He succumbed to the illness in September 2024.

Fig. 1 (A) PET CT image after four cycles of bevacizumab and temozolomide, which showed progression in the liver and lungs. (B) PET CT image before starting chemotherapy with bevacizumab and temozolomide. PET CT, positron emission tomography–computed tomography.

Fig. 2 (A) Infiltrating neoplasm with spindle cell morphology and high-grade nuclear features. (B) IHC: STAT6 positive. (C) IHC: CD34 positive. (D) IHC: Ki-67 25%. CD, cluster of differentiation; IHC, immunohistochemistry; STAT, signal transducer and activator of transcription.

Discussion

SFTs are myofibroblastic tumors of a rarely metastasizing variety which tend to occur in individuals over a wide age range with a peak incidence in the fifth and sixth decades of life.[5] It accounts for less than 3.7% of all soft tissue masses. The annual incidence is 0.35 per 100,000 individuals.[1] Less than 50 cases of neck SFTs have been reported since 1992. The males and females are affected equally.[6] There are no known associations with environmental risk factors and no known hereditary predispositions.

As per the 2020 World Health Organization (WHO) Classification of Tumors of Soft Tissue and Bone, extrapleural SFT is classified as a fibroblastic/myofibroblastic neoplasm. It has a rare potential to metastasis.[7] The site-wise distribution of SFT is 30% in the thoracic cavity, 30% in the peritoneal cavity, 20% in the head and neck (10% extracranial and 10% intracranial), and the remaining 20% in other areas. SFTs of the extracranial head and neck arise in the sinonasal tract, oral cavity, deep soft tissues, and orbit. Symptoms vary by location: in orbit, they present as an expanding mass with excessive tearing or proptosis; in the oral cavity, they are found beneath the buccal mucosa, tongue, and lower lip; in the sinonasal tract, they appear as a painless obstructive mass; and in deep soft tissues of the cheek or neck, they typically manifest as a painless mass. Local recurrence is as high as 40% whenever complete resection is not possible. Although most of them are benign, malignant behavior has also been reported. In tumors arising in the parapharyngeal space, the most common symptom is a bulge in the overlying pharynx. Paraneoplastic syndrome (PNS) is seen in less than 10% of patients, the most common being hypertrophic osteoarthropathy, especially in pleural SFTs. Another PNS manifestation seen is hypoglycemia in less than 5%.[8]

SFT in ultrasound has a nonspecific appearance. They are typically hypoechoic or occasionally heterogeneous masses with internal vascularity. CT is the first modality imaging study that depicts the lesion and also defines size and local extent, including invasion into adjacent structures and detection of regional metastases. It shows a well-circumscribed, smooth, and lobulated soft tissue mass and may contain scattered calcifications. Tumors are usually hypervascular and enhance avidly, with smaller tumors showing homogeneous enhancement, whereas larger tumors exhibit inhomogeneous enhancement with cystic/necrotic change. Magnetic resonance imaging shows low to intermediate signal intensity on T1-weighted images and heterogeneous low signal intensity on T2-weighted images (due to fibrous tissue). Usually, intense contrast enhancement and nonenhancing areas in large lesions are likely due to necrosis or cystic or myxoid degeneration. The central focus of heterogeneity and variable contrast enhancement points toward high-risk tumors.[8] [9] In 18F-FDG PET CT, benign SFT usually exhibits low-grade activity, and malignant SFTs tend to be strongly hypermetabolic and inhomogeneous. Also, the multiplicity of lesions points to malignant disease. But at times, benign SFT can cause erosions of adjacent bones and thus mimic an aggressive or malignant lesion.[10] The incorporation of maximum standardized uptake value and heterogeneity assessed by 18F-FDG PET CT could be other crucial components in the effort to tailor treatment to an individual patient, providing valuable parameters to guide the selection of the most appropriate management schedule for an individual. Pulmonary SFT shows intense tracer uptake in gallium 68 (Ga) DOTA PET CT in some case reports. Ga-DOTA- fibroblast activation protein inhibitor is a novel PET agent developed for tumors, as fibroblast activation protein is overexpressed in cancer-associated fibroblasts.[11]

As per WHO classification 2020, there are three types of SFT: benign, which is locally aggressive, SFT not otherwise specified, which rarely metastasizes, and a very aggressive malignant variant. Grossly, SFTs are unencapsulated but well-circumscribed masses typically measuring 5 to 15 cm. They show nodular, tan to red-brown, firm-cut surfaces with occasional tumors showing hemorrhage, myxoid change, or cystic degeneration. Microscopically, SFTs are composed of ovoid to fusiform fibroblastic spindle cells with indistinct cytoplasm arranged haphazardly in a patternless pattern with intervening collagenous stroma. Dilated, branching staghorn or hemangiopericytoma-like vascular pattern is often present. The histological spectrum ranges from paucicellular lesions to highly cellular tumors with little intervening stroma. Myxoid change and mature adipose tissue can be seen. The giant cell-rich variant has multinucleated giant cells lining pseudovascular spaces.[9] The abrupt transition to high-grade sarcomatous regions, either with or without heterologous components such as osteosarcoma and rhabdomyosarcoma, can be seen in dedifferentiated (DD) tumors. Malignant tumors show areas of high cellularity, atypia, mitosis (>4 mitotic figures per 10 high power fields), and necrosis.[9] Immunohistochemically, CD34 is almost always expressed diffusely in SFT but lacks specificity. STAT6, which gives strong nuclear staining, is a more sensitive and specific marker. However, STAT6 expression can be lost in DD tumors.[12]

The NAB2 and STAT6 genes are located on chromosome 12q13. The NAB2-STAT6 fusion is the crucial molecular event in the development of SFT. The fusion protein converts NAB2 into a transcriptional activator, activating early growth response 1 (EGR1). This activation triggers the expression of IGF2 and FGFR1, promoting cell proliferation and oncogenesis. EGR1 also enhances cyclin D1 and activates the MAPK/ERK pathway, which accelerates tumor growth. TP53 mutation and loss of APAF1 are linked to malignant transformation and metastasis, highlighting the need for targeted therapies.[13]

The most commonly used risk model is that of Demicco et al, which has four criteria, including age (younger than 55 years or 55 years and older), tumor size (< 5 to ≥ 15 cm), mitotic count (0, 1–3, or ≥ 4/10 high power field) and necrosis (<10% and >10%). Further studies identified male sex and prior exposure to radiation therapy also to be high-risk factors. A recent study showed that the density of Ki-67, CD163 positive cells, and MTOR mutation can accurately identify high-risk patients.[14] For locally resected SFT, clinicians look into these factors and keep the patients under follow-up accordingly.

Surgical excision with negative margins remains the treatment of choice. The surgical approach depends on the site of the disease in the head and neck. Tumors in the head and neck often represent a surgical challenge, and wide surgical margins are rarely possible due to the complex three-dimensional anatomic compartments in the region. Surgeons should take extreme care in removing the tumor with an intact capsule.[15] There is no role for lymph node dissection. Reconstructive surgery support may be required for the repair of surgical defects. The surgical pathology report should mention tumor location, size, margins, differentiation, necrosis, mitotic count, and Ki-67. Size more than 5 cm, poor pathological differentiation, deep tumor location, and nonsurgical treatment are associated with adverse prognostic factors.[16]

Indications for adjuvant radiotherapy include patients with positive surgical margins or whose tumors have a malignant component as they are at increased risk for local recurrence.[17] There is no role for elective nodal irradiation. Volumetric-modulated arc therapy/intensity-modulated radiotherapy is the current standard of radiation therapy in the head and neck site, usually starting 3 to 6 weeks after surgery. In the postoperative setting, the dose to the primary site is 60 Gy in 30 fractions, 2 Gy per fraction. Clinical target volume (CTV) includes encompassing the resection cavity and is modified to include the whole involved anatomical compartment with modification to anatomical boundaries to exclude bone and/or air without evidence of invasion. Planning target volumes are generated by an autoexpansion of 5 mm to the CTV structures. Side effects are determined by the site of radiation. Acute toxicities include oral mucositis, pharyngitis, dermatitis, rhinitis, and conjunctivitis. Chronic toxicities include fistula formation, cataracts, retinopathy, epiphora, nasal obstruction, cellulitis, fibrosis, and osteoradionecrosis. Surgically inoperable/unresectable disease can be offered definitive radiotherapy. At present, there is no role for adjuvant chemotherapy. In recurrent and margin-positive SFT, the option of adjuvant chemotherapy may be discussed in a multidisciplinary tumor board and can be decided on a case-by-case basis. There is no role for adjuvant immunotherapy or tyrosine kinase inhibitors (TKIs) at present.

At the time of a local recurrence, the feasibility of surgical resection should always be explored. Given the anatomic complexities of head and neck sites, the patient may be referred to a high-volume center with expert head and neck surgeons as well as reconstructive and plastic surgeons. Definitive radiation can be offered in case of an unresectable local recurrence without distant metastasis. The dose schedule is 60 Gy in 30 fractions, 2 Gy per fraction. CTV includes a gross tumor with a margin covering the anatomic compartment. The objective response rate was found to be 67%. Five-year local control and overall survival were 81.3 and 87.5%, respectively. Palliative radiation can be offered for palliation of symptoms of distant metastasis and local disease, the typical dose used is 39 Gy, and the objective response rate was around 38%. Five-year local control and overall survival were 62.5 and 54.2%, respectively.[18]

In locally advanced unresectable or metastatic SFT, we should not blindly use traditional chemotherapy options for soft tissue sarcoma. In treatment, naïve patients with performance status ECOG 1, good general health, and limited comorbidities are treated with a combination of dacarbazine and doxorubicin. Choi's criteria are used for response assessment. The response rate was 50% with a median progression-free survival of 6 months. Better response rates were observed in DD SFT.[3] Patients with poor performance status can be treated with single-agent dacarbazine. In case of progression after chemotherapy, TKIs targeting vascular endothelial growth factor (VEGF) pathway can be used.

A phase II trial of pazopanib in DD/malignant SFT showed a partial response of 51% and stable disease in 26%; 70% were alive at 2 years. In another phase II trial, 40 patients with metastatic or unresectable typical SFT were treated with pazopanib. Fifty-eight percent had a partial response at a median follow-up of 18 months. Median progression-free survival (PFS) and overall survival were 10 and 50 months, respectively.[19] Another VEGF and platelet-derived growth factor receptor alpha inhibitor, sunitinib, demonstrated its activity in SFTs. In a retrospective study including 31 patients with advanced SFT who received sunitinib at a dose of 37.5 mg daily, 48% achieved partial response (Choi's criteria). Median PFS was 6 months.[20] Temozolomide and bevacizumab combination was tested in progressive SFT after chemotherapy and demonstrated a partial response of 79% and a median PFS of 10 months.[4] Hence, this regimen can be used in patients who are not exposed to dacarbazine in prior lines. Some studies use anthracycline-based regimens, sorafenib or axitinib.

One case report of multiple recurrent cranial SFT shows stable disease after 2 years of immunotherapy and low-dose radiation.[21] A combination of immunotherapy and antiangiogenic therapy shows stable disease in two patients.[22]

A study by Kamamoto et al found that diffuse PD-L1 staining with no CD8 expression was associated with a poor prognosis.[23] There are limited data for immune checkpoint inhibitors in SFT. In one case report, pembrolizumab has given a durable near-complete response after 18 months.[24] A randomized trial testing a combination of nivolumab and ipilimumab to pazopanib is being done in advanced SFT and other rare soft tissue sarcomas (NCT04741438). Data on microsatellite instability (MSI) expression, PD-L1, and tumor mutational burden (TMB) are limited in SFT. In soft tissue sarcoma, MSI-H status varies with the test used, 0.4, 3.1, and 7.33% for NGS, IHC, and polymerase chain reaction, respectively.[25] PD-L1 expression in sarcomas is around 23%. Generally, TMB is low except for angiosarcoma and malignant peripheral nerve sheath tumor.[26] DD SFT in theory has high active immunomodulation and higher genomic instability. This creates a high TMB. So, PD1 inhibitors can be tried.

IGF1 is overexpressed in SFT. Figitumumab, an IGF1 R immunoglobulin G2 monoclonal antibody, was tried in various combinations and demonstrated tumor responses.[27] Further development of this molecule was stopped in 2011 by Pfizer. Many therapeutic agents targeting IGF1 R inhibition using antisense oligonucleotide, TKIs, and monoclonal antibodies conjugated with cytotoxic drugs are under preclinical and clinical studies.[28]

Telomerase reverse transcriptase (TERT) promoter mutations are strongly associated with malignant SFTs and are also associated with aggressive behavior and poor prognosis.[14] This mutation can be targeted by various molecules such as small molecule inhibitors, antisense oligonucleotide, GABPβ1L inhibitors, and FOS 1 inhibition. TERT still lacks effective specific inhibitors even though multiple studies have been conducted. The FOS/GA-binding protein subunit beta GABPB/(mutant) TERT cascade plays a critical role in the regulation of mutant TERT, in which FOS acts as a transcriptional factor for GABPB to upregulate its expression, which in turn activates mutant TERT promoter, driving oncogenesis. FOS 1 inhibition inhibits mutant TERT cancer cells through the induction of apoptosis. FOS inhibitors, such as T-5224, are under development for use in clinical trials in various cancers and arthritis.[29]

As EGR1 forms an important part of the NAB2 STAT6 pathway, targeting EGR1 forms one of the promising future approaches. TCR-T cell therapies are under development for PRAME (preferentially expressed antigen in melanoma) positive SFTs. Trabectedin with low-dose radiation, which is being tried in leiomyosarcoma, can also be tested in SFT. CD99, which is expressed in early SFT, can also be developed as a new target.[8] The study's strength is that it is an in-depth discussion of a rare case of recurrent metastatic SFT with a review of published articles focusing on the latest advancement. The limitation is that we could not do NGS due to low tumor content in the tissue sample, which could have provided insights into the molecular abnormality and treatment based on that data.

Conclusion

In metastatic SFTs, the first-line treatment of choice typically involves a combination of dacarbazine with or without doxorubicin. Second-line options are limited; however, temozolomide combined with bevacizumab may be considered for patients who have not previously received dacarbazine. The role of immunotherapy and tissue-agnostic therapies utilizing NGS is still under investigation.

Conflict of Interest

None declared.

Patient's Consent

Written consent has been obtained from the patient.

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Publication History

Article published online:
30 April 2025