Solitary Fibrous Tumor of Hard Palate: A Case Report and Literature
Review
Solitary fibrous tumor (SFT) is a mesenchymal tumor with fibroblastic or
myofibroblastic origin, accounting for less than 2% of soft tissue tumors. Although
more than 50% of SFT occur intra-thoracically, it has been diagnosed in other
parts of the body such as liver, adrenal gland, skin, head, and neck, and in rare
occasions, in the oral cavity, , affecting mostly buccal mucosa
and tongue. To the best of our knowledge, only 4 cases of SFT in hard palate have been
documented in the literature. Here, we present an additional case of SFT of hard
palate. 

A 32-year-old man presented to the Oral and Maxillofacial Clinic of our institute in
early 2021 with a painless right posterior hard palatal lesion adjacent to the last
maxillary molar tooth which grew in size since 2017. The lesion was biopsied in 2020
and initially diagnosed as a benign fibrous tumor. Patient underwent maxillofacial
CT scan with contrast prior to surgery in 2021 which showed approximately 2.0 cm
enhancing mass projecting inferiorly from the right side of the hard palate. No
adenopathy or osseous destruction was identified. The excisional biopsy was performed. Gross pathologic examination revealed a 2.7 ×
2.6 × 1.1 cm lesion with a homogeneous cut surface and an overlying tan-white smooth
mucosa. Histologic examination demonstrated a well-circumscribed cellular lesion in
the submucosa composed of bland spindle cells with admixed staghorn vessels in a
patternless pattern. There was no evidence of necrosis or mitotic activity . Immunohistochemical staining was performed with spindle cells strongly positive for
STAT6 and CD34 while being negative for AE1/AE3, S100, desmin, and smooth muscle
actin ( and ). Based on the histological
and immunohistochemical findings, a SFT was diagnosed. The patient recovered well
from the surgery with no recurrence within 1 year post-surgery follow-up. 

Solitary fibrous tumors are an uncommon group of neoplasm with an unknown origin. It
has variable clinical behavior and was first described in pleura by Klemperer and
Rabin in 1931. , This entity has been since identified in many anatomical locations of the body,
including upper respiratory tract, breast, spinal cord, liver, adrenal gland, skin, and
head and neck. , Due to the overlapping histopathological features with many other soft tissue
tumors, SFTs have been assigned many different names over the years such as benign
mesothelioma, solitary fibrous mesothelioma, and localized fibrous tumor. Solitary fibrous tumors usually have no significant sexual predominance, - although some studies showed
slight predication to female adults. , They may arise at any age but
most commonly occur during fifth to seventh decades of life. To date, no
known environmental elements have been found as predisposing risk factors to
SFTs. Generally, SFTs are well-circumscribed slow-growing masses with majority of cases
discovered as incidental radiologic findings or due to the symptoms associated with
a mass effect. Unusual symptoms including bleeding and tooth mobility were
reported in the SFT of oral cavity. Solitary fibrous tumors in the head and neck are rare and have been reported in the
nose and paranasal sinuses, nasopharynx, major salivary glands, larynx, thyroid,
skin, oral cavity, and orbit. According to the literature,
although rare, oral cavity is the most common site of occurrence in the head and
neck. Approximately 150 cases have been reported until May 2019, and the most common sites of
involvement are buccal mucosa, tongue, and lower lip. Incidence of SFT in the palate
and specifically in the hard palate is extremely rare, and to the best of our
knowledge, only 4 cases of hard palate SFTs have been reported in the
literature. In our case and 4 previously reported cases, patients were young with age ranging 26
to 35 years , , , and showed female
predilection. The tumors in all 5 cases were located at right side of hard palate
with sizes ranging 0.7 to 5.0 cm . Histopathologic and
immunohistochemical evaluations of the reported cases showed similar features to our
case. The histologic spectrum of SFTs is broad. Classically, SFTs are
variably cellular and consist of oval to fusiform spindle cells with minimal nuclear
atypia and mitotic figures that are arranged haphazardly or in short, ill-defined
fascicles. There is dilated, branching, staghorn-like (hemangiopericytoma-like)
vasculature. The tumor may show alternating hypercellular foci and hypocellular
sclerotic foci. Immunohistochemical staining plays an important role in diagnosing SFTs. Solitary
fibrous tumors are immunoreactive to CD34 and STAT6, and variably positive to CD99
while being negative for muscular, epithelial, and neural markers. It is important
to note that CD34 lacks specificity for diagnosis of this entity. Strong nuclear
STAT6 staining has been used to reliably differentiate SFT from other soft tissue
tumors. Presence of positive staining for CD34 and STAT6 with negative staining for
S100, AE1/AE3, smooth muscle actin, and desmin, along with histological features,
supports the diagnosis of SFT within the hard palate for our case. As in our case, complete surgical excision is the treatment of choice. Radiotherapy
is suggested as a standard adjuvant therapy for tumors that are difficult to
complete resection due to the complexity of anatomical site (ie, head and
neck). , Completely excised tumors with no evidence of any malignant
components show an excellent outcome. Local recurrence and distant metastasis are expected in malignant SFTs, which are
extremely rare. In a retrospective study of 110 SFTs, clinicopathological factors
associated with metastasis and survival were assessed, and a metastasis risk
stratification model was developed. In this model, scores were assigned for age,
tumor size, and mitotic figures; total scores were tabulated to determine the risk
of aggressive disease (low, moderate, or high). Based on the proposed model,
our patient is classified as having a low risk of aggressive disease and
metastasis. In reported cases of SFTs of hard palate as well as our case, no recurrence was
reported in up to 15 years follow-up. , The follow-up data are not
available for 2 cases. , Studies believe that intraoral SFTs tend to follow a more
benign non-recurring course. However, certain cases of
SFTs with benign histological features do recur. Therefore, long-term follow-up of
all patients regardless of anatomical locations is highly recommended. Solitary fibrous tumors harbor a characteristic gene fusion between NAB2 and STAT6
which is a highly sensitive and specific molecular marker and is a driver mutation for
this entity. This fusion is identified in all cases of SFTs and results
from inversion of long arm of chromosome 12 (12q13) and induces activation of early
growth response (EGR) transcription factors, cellular proliferation, and
tumorigenesis. No definitive association with prognosis and malignant
potentials has been identified with NAB2-STAT6 fusion variants, , , although, in 1 study, fusion
type NAB2 exon 2 /STAT6 exon 16/17 was found to be
associated with younger age of onset and malignant behavior. In addition,
studies showed that loss of NAB2-STAT6 chimeric protein expression was associated
with SFT dedifferentiation, and hence negative STAT6 immunohistochemistry does not
rule out SFT diagnosis. Due to the small size of the 12q13 inversion, multiplex
polymerase chain reaction (PCR) or next-generation sequencing is required for
adequate sensitivity for detection of the NAB2-STAT6 fusion. Thus, proof of
NAB2-STAT6 fusion is not required or recommended for diagnosis. No genetic
analysis was performed in our case. Other molecular alteration that is thought to be
involved in the tumorigenesis of this entity includes platelet-derived growth factor
receptor beta (PDGFRB) gene which was identified in cases of pleural SFT. Mutations in
telomerase reverse transcriptase (TERT) promoter have been also reported in 20% to
30% of SFTs in all sites and were found to be associated with more aggressive tumors
and adverse clinical outcomes. , P53 mutation has been also
observed in cases of dedifferentiated and aggressive types of SFT. - Due to the broad histologic patterns, SFTs could be difficult to distinguish from
other soft tissue tumors and should be considered in the differential diagnosis of
well-demarcated submucosal solid mass located in oral cavity and hard palate such as
hemangioma, angiofibroma, irritation fibroma, and giant cell fibroma. 

We presented a rare case of SFT in the hard palate. Review of our case and previously
reported cases with SFT of hard palate showed occurrence in young patients,
right-side predominance, and no recurrence on follow-up (between 2 months and 15
years period). Histopathological evaluation together with immunohistochemial
analysis is necessary for the diagnosis of this entity. Due to its rarity and
variable clinical behavior including recurrence and malignant transformation,
periodic follow-up of patients diagnosed with SFTs is highly recommended. 

