Nodular fasciitis of the masseter muscle: Radiologic and pathologic correlation of a rare benign lesion

Abstract

Nodular fasciitis rarely arises from the masseter muscle and is frequently misdiagnosed as soft tissue sarcoma. A 43-year-old male presented with a rapidly enlarging, painless swelling below the right mandibular angle over 2 months. Clinical examination revealed a firm, mobile mass without skin fixation or lymphadenopathy. Ultrasound and magnetic resonance imaging were performed, followed by fine-needle aspiration cytology and excisional biopsy. Ultrasound demonstrated a well-defined, hypoechoic lobulated lesion (23 × 13 × 8 mm) within the superficial masseter, showing mild internal vascularity and no bone involvement. Magnetic resonance imaging revealed an iso- to slightly hypointense T1 signal, hyperintense T2/ short tau inversion recovery signal, and homogeneous postcontrast enhancement with smooth margins, features suggestive of a benign myofibroblastic lesion. Excisional biopsy confirmed the diagnosis of nodular fasciitis. This case indicates that masseteric nodular fasciitis, a rare benign lesion, may present with imaging features of small size, smooth margins, homogeneous enhancement, and mild vascularity in the context of trauma history. Recognition of this self-limited myofibroblastic proliferation can prevent unnecessary radical surgery and guide conservative management.

Introduction

Nodular fasciitis, also known as pseudosarcomatous or infiltrative fasciitis, is a benign soft tissue lesion of myofibroblastic origin, ¹ with an usually unknown cause. Clinically, it presents as a rapidly enlarging, occasionally tender, palpable mass in soft tissues ² and most commonly arises in the forearm (27%–29%), the back or chest wall (15%–18%), and the upper arm (12%). ³ Because misdiagnosis at the primary care level may lead to inappropriate management and adversely affect patient outcomes, differentiation between benign and malignant tumors during initial evaluation is of paramount importance.^4,5 Ultrasonography, an accessible and noninvasive imaging modality, can aid in differentiating benign from malignant tumors by assessing the size and anatomical location of the mass, distinguishing solid from cystic lesions, and evaluating its vascularity. This makes it a valuable first-line tool in the assessment of soft tissue masses.^6,7 However, imaging techniques such as X-ray, ultrasound, and computed tomography (CT) often have limitations and can yield inconclusive results. ⁸ Magnetic resonance imaging (MRI), with its multiplanar capability, high spatial resolution, superior soft tissue contrast, and absence of ionizing radiation, represents the modality of choice for evaluating soft tissue lesions. It provides critical diagnostic information and contributes to optimal treatment planning.^9–11 This report has been prepared in accordance with the case report (CARE) guidelines. ¹² Herein, we present a case of nodular fasciitis occurring in the submandibular region, an uncommon site for this benign lesion, and discuss its imaging characteristics on ultrasound and MRI.

Case presentation

On 8 January 2025, a 43-year-old male presented with swelling in the right upper neck, located just below the mandibular angle. The swelling had progressively increased in size over the past 2 months (Figure 1). The lesion was nonpainful, although it exhibited tenderness on palpation. The patient had no significant medical history or family history of cancer. Notably, he reported a history of boxing during his youth.

Figure 1.

Cropped and anonymized clinical photograph showing a well-defined swelling in the right submandibular region.

Ultrasound examination of the mass was performed as the initial investigation by a radiologist. The findings revealed a hypoechoic, soft tissue mass-like lesion with lobulated margins, measuring approximately 23 × 13 × 8 mm, located over the right mandibular angle within the superficial part of the right masseter muscle (Figure 2). Color Doppler study demonstrated minimal internal vascularity within the lesion. No underlying bone involvement was observed. The submandibular and parotid glands appeared normal, without evidence of space-occupying lesions. No collection or inflammatory changes were noted, and no significant jugular lymphadenopathy was detected. MRI evaluation (Figure 3) showed homogeneous enhancement of the mass within the masseter muscle following contrast administration, indicating a solid, vascularized lesion. The margins were smooth and well-defined (Figure 3(a)). On noncontrast T1-weighted images, the lesion appeared iso- to slightly hypointense compared with adjacent muscle (Figure 3(b)). On short tau inversion recovery (STIR) sequences, the lesion was hyperintense (Figure 3(c)), and on T2-weighted images, it showed iso- to hyperintense signal (Figure 3(d)).

Figure 2.

Ultrasound scan of the right mandibular region demonstrating a well-defined hypoechoic lesion within the superficial part of the masseter muscle.

Figure 3.

MRI of the right masseter region. (a) Postcontrast T1-weighted image; (b) noncontrast T1-weighted image; (c) STIR sequence; and (d) T2-weighted image. MRI: magnetic resonance imaging; STIR: short tau inversion recovery.

Given the rapid growth of the lesion, fine-needle aspiration (FNA) was performed, yielding a nondiagnostic result. Immunohistochemical analysis showed positivity for smooth muscle actin (SMA) and calponin, with a low Ki-67 proliferation index of approximately 4%. Tumor cells were negative for H-caldesmon, desmin, myogenin, cytokeratin AE1/AE3 (CK AE1/AE3), beta-catenin, SRY-related HMG-box gene 10 (SOX10), S100, cluster of differentiation (CD) 34 (CD34), and CD99. Due to clinical suspicion of soft tissue sarcoma, an excisional biopsy was performed. Histopathological examination confirmed nodular fasciitis, an unusual finding in the mandibular angle. No recurrence was observed at 6-month follow-up.

Discussion

Nodular fasciitis is a benign myofibroblastic proliferation that can clinically and radiologically mimic sarcoma because of its rapid growth. ¹³ Although various mechanisms, such as reactive or inflammatory processes, have been proposed, the exact pathogenesis of nodular fasciitis remains unclear. Trauma has been suggested in some reports as a possible triggering factor; however, a definitive causal relationship has not been established. ¹⁴ In the present case, a history of boxing was noted, although its etiological relevance remains uncertain.

Nodular fasciitis in the head and neck accounts for approximately 13%–20% of cases.^15,16 Al-Hayder et al. ¹⁵ described a medial canthus lesion in a 64-year-old woman, whereas Rodrigues-Fernandes et al. ¹⁷ reported a painless zygomatic mass in a 32-year-old woman, both confirmed as nodular fasciitis after excision. Pediatric facial involvement has also been documented, including a cheek lesion in a 12-year-old girl reported by Li et al. ¹⁸ and a maxillofacial lesion in a 1.5-year-old child described by Merchavy et al. ¹⁹ These reports highlight the rarity and heterogeneous presentation of nodular fasciitis in the head and neck, ²⁰ which may contribute to diagnostic uncertainty.

In the present case, given the unusual site of the lesion within the masseter muscle, ultrasonography was performed to further evaluate its characteristics during the initial assessment. The lesion’s ultrasound features demonstrated a well-defined hypoechoic lesion with minimal internal vascularity. There are limited imaging data on nodular fasciitis. Typically, on ultrasonography, nodular fasciitis presents as a well-defined hypoechoic to isoechoic mass compared with the adjacent muscle. ²¹ The lesion is usually superficial and may exhibit a heterogeneous echotexture with internal septations. In some cases, it appears slightly lobulated, reflecting its benign and reactive nature. Unlike malignant tumors, nodular fasciitis generally lacks significant necrosis or hemorrhage. Doppler ultrasound may show mild to moderate internal vascularity, as observed in our case, which can aid in differentiating it from highly vascular malignant tumors. ²² As the ultrasound findings in this case were nondiagnostic, MRI assessment was performed.

On MRI, the lesion within the masseter muscle demonstrated iso- to slightly hypointense signal on noncontrast T1-weighted images, hyperintense signals on STIR sequences, and iso- to high signal on T2-weighted images, with homogeneous enhancement following contrast administration.

The MRI appearance of nodular fasciitis is generally nonspecific and can be misinterpreted as a soft tissue sarcoma. ²³ On MRI, nodular fasciitis lesions most commonly exhibit isointensity on T1-weighted images and hyperintensity on T2-weighted images. They may demonstrate rim enhancement, homogeneous enhancement, or minimal to no enhancement, ²⁴ findings that were consistent with the present case.

Accurate radiologic diagnosis of nodular fasciitis requires awareness of the imaging features of other lesions that may mimic its appearance, as differentiation from other benign or malignant soft tissue tumors can be challenging. For example, desmoid tumors share certain imaging similarities with nodular fasciitis but typically exhibit heterogeneous MRI characteristics, reflecting variability in cellularity, collagen composition, and the presence of myxoid stroma. Collagen-dense regions usually appear as nonenhancing curvilinear or swirling hypointense bands on both T1- and T2-weighted images, whereas more cellular or myxoid areas exhibit intermediate to low signal intensity on T1 and intermediate to high signal intensity on T2 sequences, often showing variable degrees of contrast enhancement. ²⁵ Regarding soft tissue sarcoma, the most significant MRI predictors of high-grade lesions include pronounced intratumoral heterogeneity on T2-weighted imaging and the presence of a fascial tail sign. MRI features suggestive of malignancy include poorly defined margins, invasion or infiltration of nearby structures, lesion size exceeding 5 cm, deep anatomical location, and heterogeneous signal intensity on both T1- and T2-weighted images. Additional concerning features comprise pronounced perilesional edema disproportionate to the lesion size (except in cases with prior surgery, radiation, or inflammation), areas of necrosis or intralesional hemorrhage, involvement of bone or neurovascular structures, and early contrast enhancement followed by a plateau or washout pattern. Enhancement that is peripheral, nodular, or internally heterogeneous further supports the likelihood of malignancy.^26,27

Histologically, nodular fasciitis is typically an unencapsulated but well-circumscribed lesion. ²³ Microscopically, it consists of irregular nests and cords of plump spindle-shaped fibroblasts and myofibroblasts exhibiting a high mitotic rate but without atypical or abnormal mitoses. Cellular atypia, a feature of true neoplasia, is absent. The lesion generally demonstrates strong positivity for α-SMA and muscle-specific actin (HHF-35), indicating a myofibroblastic nature, whereas staining for S-100 and desmin is negative, excluding neural and myogenic origins. The background stroma is typically myxoid and tissue culture–like, often with microcyst formation, extravasated erythrocytes, and a mild lymphocytic infiltrate. Compared with sarcomas, nodular fasciitis lesions are usually smaller (less than 40 mm), sharply demarcated, and lack pleomorphism or cellular atypia.^28,29

Clinically and histologically, important differential diagnoses include sarcoma, ²⁹ fibrosarcoma, fibrous histiocytoma, schwannoma, and myofibromatosis. ³⁰ Although nodular fasciitis lesions are generally small and may follow minor trauma, sarcomas tend to be larger and more aggressive. FNA can assist in diagnosis but is often inconclusive, as reported in many cases. Misdiagnosis has been documented, with some lesions erroneously labeled as desmoid-type fibromatosis, resulting in unnecessarily extensive surgery. Definitive diagnosis of nodular fasciitis is ultimately achieved through histopathological and immunohistochemical evaluation, with excisional biopsy serving both diagnostic and therapeutic purposes; recurrence is rare. ²⁰

Recognition of nodular fasciitis is important not only for establishing the correct diagnosis but also for guiding appropriate management. In our patient, awareness of characteristic imaging features could have suggested the diagnosis. However, tissue sampling was ultimately necessary due to concern for malignancy. Radiologic evaluation proved invaluable in defining the lesion’s extent and benign imaging behavior. In summary, this case highlights that nodular fasciitis in an uncommon location can closely mimic a malignant tumor; however, features such as small lesion size, homogeneous enhancement, and fasciitis-like growth pattern may raise suspicion for this benign entity. Familiarity with these imaging characteristics, supported by current literature, is essential to avoid overly aggressive treatment and to manage nodular fasciitis appropriately.

Limitations

As a single-patient observation, this report primarily aims to raise awareness rather than establish generalizable diagnostic criteria. The rarity of masseteric nodular fasciitis limits opportunities for comparative imaging and histopathologic evaluation across multiple patients. Future accumulation of similar cases may help refine diagnostic criteria.

Conclusion

Nodular fasciitis of the masseter muscle is a benign, self-limiting myofibroblastic proliferation that often mimics malignancy because of its rapid growth and cellularity. Well-circumscribed borders, homogeneous enhancement, and lack of invasion on imaging, together with excisional biopsy, allow definitive diagnosis. Excisional biopsy is both diagnostic and curative in most cases, and recurrence is rare. Awareness of this entity is essential to avoid unnecessary aggressive surgery.