Ⅰ. INTRODUCTION

Pseudosarcomatous fibromatosis (PSF) was first reported by Konwaler et al. [1] as subcutaneous pseudosarcomatous fasciitis. Also known as nodular fasciitis and subcutaneous pseudosarcomatous fibromatosis, PSF is a benign proliferative fibroblastic lesion of mesenchymal origin. [2] It occurs frequently in people aged between 20-40 years. There is no sex predilection. Due to its rapid growth without any evidence of infection, PSF is often misdiagnosed as sarcoma, leading to unnecessary and aggressive therapy. [2,3]

Here we report a case of PSF found in the left masseter muscle of a 26-year-old female patient whose initial findings were suggestive of a schwannoma with cystic formation. The mass was surgically excised and finally diagnosed as PSF.

Ⅱ. CASE REPORT

A 26-year-old female patient was referred by local orthopedics to the oral and maxillofacial surgery department of Kyungpook National University Dental Hospital, with left mandibular angle swelling that had presented for a 1 month duration. Clinical examination was performed, and a panoramic radiograph was recorded. There was no specific lesion in the bone. There were no history of trauma and no signs of inflammation.

The initial examination revealed a swelling at the left mandibular angle. The mass was soft, elastic, and painless. Blood tests, electrocardiography, and chest radiograph showed no specific findings. The patient was afebrile, and no lymphadenopathy was noted. Fig. 1, 2

Paranasal sinus computed tomography (PNS CT) with enhancement was performed. The results showed a well circumscribed lesion measuring 18×12 mm in the left masseter muscle. There was no sign of bone destruction in the images. Fine needle aspiration biopsy from the lesion was performed at a local clinic. The available results led to a suspicion of a schwannoma with cystic formation. The mass was excised intra-orally under general anesthesia and diagnosed as PSF.

Histologic examination revealed marked nodular and clustering proliferation of spindle-shaped cells with high mitotic activity. No atypical mitosis was observed. Immunohistochemical stains were negative for S100, SMA, and CK, while focal positive for CD68 and positive for vimentin.

After surgery, a three-month follow up was done. Postoperative healing was good, and there was no recurrence.

Ⅲ. DISCUSSION

PSF, first reported by Konwaler et al. in 1955, is a benign fibrous lesion which involves the proliferation of fibroblasts in subcutaneous tissue and intramuscular space.[1] It is also called proliferative fasciitis, infiltrative fasciitis, pseudosarcomatous fasciitis and necrotizing fasciitis. However, over the years, necrotizing fasciitis has been the more commonly used term to refer to this condition. Until the advent of the ICD (International Statistical Classification of Diseases and Related Health Problems)-9 system, the term nodular fasciitis was used. It was changed to PSF in the ICD-10-CM (2015).

The lesion is uncommon, and occurs in patients aged between 20-40 years with no sex predilection. [2] Usually, PSF does not exceed 4 cm in size. [4] The most commonly affected site is the subcutaneous tissue and muscle fascia. [5] Anatomically, PSF occurs more commonly in the upper extremities (48%), followed by the trunk (20%), the head and neck (15–20%) and the lower extremities (15%). [6] Oral cavity is not a prevalent site for PSF to occur.

The lesions are generally painless, firm in consistency, and well-circumscribed but non-encapsulated. They may present as an ulcer in the oral cavity, and affect the underlying bone. [2,3,9] Rarely, PSF lesions may be tender. [13] Although it is commonly well circumscribed, it can stream into the surrounding fat or muscle, clinically imitating an aggressive sarcoma. [9]

The pathogenesis of PSF is still under study. Some investigators have described it as a reactive lesion originating from trauma. However, others have described it as chromosomal abnormalities that suggest a neoplastic origin. [2,7,8]

Histopathologically, PSF lesions appear to be composed of immature fibroblasts, that is, myofibrolast that form a bundle with short fascicles. Other features include the spindle cells within loose substrates, mitotic activity without atypism, inflammatory cells and red blood cells. [4]

Diagnosis of PSF is difficult because of the rare occurrence in facial region, histologic variations, and abnormal clinical features. Rapid growth, presence of spindle-shaped cells, mitotic activity, high cellularity, and the fact that the lesion may spread along peritoneal spaces may lead to misdiagnosis of sarcoma or any other malignant mesenchymal neoplasm. Thus, differential diagnosis should be considered with fibrosarcoma, leiomyoma, myxoid neurofibroma, myxoid dermatofibrosarcoma which are originated from mesenchymal cells. [10] These differential diagnoses can be achieved by immunohistochemical staining because PSF gives a positive response to vimentin and CD68, but negative response to cytokeratin, S-100 protein, and desmin. [8]

PSF can be treated with complete surgical excision. But, there was a case report that the lesion might be self -limiting. Yanagisawa et al. reported a case of PSF which presented on the buccal cheek and spontaneously regressed after biopsy. [11,14] Intra-lesional corticosteroid injection is recommended when the lesion cannot be completely excised, such as when it extends into the muscle. [12] The prognosis of the PSF is good once healing is initiated after surgical excision, and in case of spontaneous regression after post-biopsy. Recurrence rate is about 1-2%. [15] Kenneth E et al. reported 134 cases of PSF, in which 18 cases were recurred. And out of the 18 cases which were first diagnosed as PSF, 17 cases were misdiagnosed. The recurred cases were re-classified as fibromatosis, fibrosarcoma, benign fibrous histiocytoma, etc. Only one case was reported to have revealed the appropriate histological features of PSF in the primary and recurred lesions. [9]

In the current case, the 26-year-old patient came with a rapid growing mass on the mandibular angle region. The fast growing property of PSF may mislead the clinician to a misdiagnosis of sarcoma. Histopathological investigations with clinical examinations and radiological findings lead to accurate diagnosis. Clinicians should be careful to avoid unnecessary and inappropriate treatment through exact diagnosis.