Subcutaneous Solitary Fibrous Tumor in the Chin of a Young Adult: A Diagnostic Challenge

Introduction

Solitary fibrous tumors (SFTs) are rare mesenchymal neoplasms that were initially identified in the pleura but are now known to occur at various extrapleural sites, including soft tissues of the head and neck [1–5]. Subcutaneous involvement of the chin is exceptionally rare, particularly in young adults; cases with atypical immunohistochemical (IHC) profiles, such as CD34 negativity, are even less frequently documented [6,7]. SFTs are typically slow growing and indolent, most often diagnosed in middle-aged or older adults, but they may exhibit unpredictable behavior, including local recurrence or distant metastasis [8]. Diagnoses of SFTs in atypical locations are challenging because of morphological and IHC features that overlap with other spindle cell tumors, such as schwannomas or inflammatory myofibroblastic tumors (IMTs). Although CD34, B-cell lymphoma-2 (Bcl-2), and CD99 are frequently expressed and can support initial diagnosis, their presence is nonspecific, particularly in atypical or subcutaneous CD34-negative tumors [9]. Nuclear signal transducer and activator of transcription (STAT)6 expression, driven by NGFI-A-binding protein 2 (NAB2)–STAT6 fusion, is highly sensitive, specific, and reliable across anatomical sites, making it a decisive marker for diagnostically challenging SFTs [10–12].

The present case underscores the diagnostic value of STAT6 in atypical SFTs and contributes to the existing body of knowledge by documenting a rare CD34-negative subcutaneous SFT in a young adult. It highlights important implications for clinical practice, including diagnostic pitfalls and the need for vigilant long-term follow-up in similar presentations.

Case Report

A 22-year-old woman presented to the Department of General Surgery at the Regional Diagnostic and Treatment Center of Orkhon on December 26, 2024, with a chief complaint of throbbing jaw pain and facial asymmetry. She had first noticed a small lump on the chin in October 2024. She attempted to manually squeeze the lesion, which resulted in progressive swelling, pain, and redness.

Clinical examination revealed facial asymmetry, localized swelling, and erythema in the chin region. Palpation identified a large, firm, and tender subcutaneous mass. The patient had no history of smoking, alcohol use, or chronic illnesses. Given the acute onset of symptoms, an infectious etiology was initially suspected, and the patient began receiving a 7-day course of oral cephalexin (500 mg every 8 h).

Laboratory investigations (eg, complete blood count, erythrocyte sedimentation rate, and C-reactive protein) showed results within normal limits, providing no evidence of systemic infection or inflammation. Computed tomography revealed a well-defined, hyperdense focal lesion measuring 3.1×2.0 cm (density: approximately +40 Hounsfield units), located in the subcutaneous soft tissue of the left anterior mandibular region. During the arterial phase of contrast-enhanced computed tomography, the lesion demonstrated prominent, homogeneous enhancement. Mildly decreased attenuation was noted in the surrounding fat. Given the well-circumscribed nature of the lesion and the persistence of symptoms despite antibiotic therapy, surgical excision was pursued to achieve both definitive diagnosis and treatment. The mass was excised under local anesthesia. Although alternative approaches such as observation or adjuvant radiotherapy may be appropriate in certain cases, surgical excision was considered the most suitable strategy in this case due to the patient’s symptomatic presentation and the localized, resectable nature of the tumor.

Macroscopically, the specimen consisted of a well-circumscribed, firm, solid mass measuring 4.0×3.4×2.8 cm. Histologically, the tumor was composed of spindle-shaped cells arranged in fascicular patterns, embedded within a fibrous stroma containing a mild inflammatory infiltrate that included plasma cells and lymphocytes. Thin-walled vessels were present; however, staghorn vascular structures were absent (Figure 1A, ×400). No evidence of mitotic activity or necrosis was observed.

The differential diagnosis included spindle cell tumors commonly observed in the head and neck, particularly schwannoma and IMT. Schwannoma was considered because of the spindle cell morphology and head and neck localization; however, negative S100 staining ruled out this possibility. IMT was considered due to smooth muscle actin (SMA) positivity and the inflammatory background, but activin receptor-like kinase 1 (ALK1) negativity did not support this diagnosis.

IHC analysis (×400 magnification in all images) revealed positive staining for STAT6 (Figure 1B), CD99 (Figure 2A), Bcl-2 (Figure 2B), and SMA (Figure 2C), with focal positivity for H-caldesmon (Figure 2D). In contrast, CD34 (Figure 2E), S100 (Figure 2F), and ALK1 (Figure 1C) displayed negative staining. The MIB-1 (Figure 1D) proliferation index was 5%.

A diagnosis of grade I SFT was made based on the patient’s clinical presentation, histological features, and IHC profile. Six months have passed since the surgery, and she remains in good condition without signs of tumor recurrence.

Discussion

SFTs are rare mesenchymal neoplasms that most commonly arise in the pleura; however, extrapleural involvement, including the head and neck, has been increasingly recognized [4,6,13]. Subcutaneous SFTs of the chin are exceptionally uncommon, particularly in young adults [5,7,14], highlighting the novelty of the present case. In a review by Smith et al, only 7 of 88 head-and-neck SFTs involved subcutaneous tissue: 3 in the cheek, 2 in the eyelids, 1 in the external auditory canal, and 1 in the chin [15]. Silva et al described a CD34-positive subcutaneous SFT in a 28-year-old woman, which differed from our CD34-negative case [16]. Dermawan et al reported that CD34-negative SFTs are disproportionately represented in the head and neck; they may demonstrate more aggressive behavior [17]. These observations emphasize that CD34-negative SFTs in young adults are particularly rare and clinically significant, underscoring the importance of our report.

Histologically, SFTs typically show a patternless architecture, dense collagenous stroma, and branching staghorn vessels, although these features may be absent [6,18,19]. In the present case, the tumor lacked the typical vascular architecture, reinforcing the need to integrate morphology and IHC findings for accurate diagnosis.

The main differential diagnoses considered were schwannoma and IMT. Schwannoma was excluded because S100 staining results were negative; such findings typically are strongly positive in schwannomas. IMT was supported by SMA positivity and the inflammatory background; however, ALK1 negativity did not support this diagnosis. Although ALK1 is expressed in 50% to 60% of IMTs, its absence – together with strong nuclear STAT6 positivity – supported SFT rather than IMT. Importantly, STAT6 is not expressed in IMTs or myofibroblastic tumors [10,13,14,20–22], making it a decisive diagnostic tool in this context.

IHC analysis is critical. Although CD99 and Bcl-2 positivity supported the diagnosis, these markers lack specificity because they can be expressed in various spindle cell tumors [9]. Given these limitations, STAT6 should be incorporated into routine IHC panels for spindle cell tumors. Unlike supportive but nonspecific markers, STAT6 analysis provides a direct link to the NAB2–STAT6 gene fusion, offering a molecularly anchored diagnostic advantage. Its inclusion early in diagnostic workups can reduce unnecessary staining procedures, prevent delays, and improve the evaluation of diagnostically challenging cases (eg, CD34-negative SFTs). Furthermore, routine STAT6 testing ensures consistency across institutions and strengthens diagnostic confidence in both typical and atypical presentations [6,10–12]. In addition to its diagnostic accuracy, STAT6 narrows the differential diagnosis when SFT is considered along with histologic mimics such as synovial sarcoma, fibrosarcoma, or IMT, which often lack nuclear STAT6 expression [6,12]. By clearly distinguishing SFT from these entities, STAT6 improves efficiency while directly influencing clinical management, given that treatment approaches and prognoses substantially differ. Accordingly, its systematic inclusion in routine IHC panels is justified, even in resource-limited settings, where misdiagnosis must be avoided. CD34 negativity occurs in 5% to 10% of cases and may be associated with more cellular or atypical variants [17,23–26], but it is not diagnostic of malignancy. In this context, strong nuclear STAT6 positivity remains the most reliable basis for confirming a diagnosis of SFT [10–12].

Treatment for head-and-neck SFTs relies on complete surgical excision. Radiotherapy may be considered in unresectable cases or when margins are positive, although it does not consistently improve survival. Even low-risk tumors may recur, particularly when excision is incomplete. Vigilant long-term follow-up is essential, especially for CD34-negative tumors, which may exhibit more aggressive clinical behavior [27,28].

The present case demonstrates 3 key points: CD34-negative SFTs in young adults may pose considerable diagnostic challenges; STAT6 should be included in all relevant IHC panels; and CD34-negative status warrants careful surveillance. Reports of such cases enhance understanding of SFTs and may inform future diagnostic strategies and management protocols.

Conclusions

An SFT can be challenging to diagnose when common histological features are absent and the immunoprofile is atypical, as in this case. Although the NAB2–STAT6 gene fusion and corresponding STAT6 IHC staining findings are highly specific, the rarity and bland morphology of SFTs require a high index of suspicion. Strong nuclear STAT6 expression remains the most decisive marker, particularly in CD34-negative or morphologically atypical tumors. CD34-negative SFTs in young adults may carry a greater risk of recurrence or aggressive behavior, underscoring the importance of vigilant long-term follow-up. The present case highlights the need for integrated diagnostic approaches that combine morphology, IHC assays, and molecular markers to avoid misclassification. Future efforts should focus on clarifying mechanisms that underlie CD34 loss and establishing standardized diagnostic and management protocols, including tailored follow-up for atypical cases. By documenting a rare, atypical SFT presentation, this report contributes to the evidence base; it may inform future diagnostic algorithms, prognostic assessment, and treatment strategies.