Nodular fasciitis (NF) is a benign self-limited neoplasm of fibroblastic and myofibroblastic derivation. It primarily arises within the subcutaneous tissue. It arises less frequently in the skeletal muscle, and very rarely in the skin. Due to its rapid growth, high cellularity, and mitotic activity, NF can be confused with malignant lesions. Histologically, NF involving the skin needs to be distinguished from several entities including atypical fibroxanthoma (AFX), sarcomas, melanoma, and squamous cell carcinoma, all of which have vastly different clinical managements. Nodular fasciitis has propensity to occur in children and young adults on head and neck, trunk and upper extremities, but it is extremely rare to occur in the auricular region. Its etiology was thought to be reactive, but it is now found to have USP6-MYH9 fusion gene in most cases, thus favoring a neoplastic process.

In this study, we will discuss a case of intradermal NF in the pinna of a middle-aged man, which is a very rare presentation. The purpose of this paper is to broaden the clinical differential of post-auricular lesions of the skin and provide a succinct histological and immunohistochemical guide to distinguish NF from other more common clinical and pathologic mimickers.

A 53-year-old man presented with a tender oval-shaped lesion of the right posterior auricular region that developed over several weeks. The computed tomography (CT) scan revealed hyperdense well-defined oval-shaped structure in the external ear inseparable from the skin measuring 1.9 × 1.8 × 1.4 cm with no bony erosion or destruction of the underlying mastoid bone, which was separated from the lesion with a fat plane. Moreover, there were no lymphadenopathy or any other suspicious soft tissue or osseous lesion. The histology revealed atypical spindle cell neoplasm with fascicular arrangement with occasional epithelioid cells and multinucleated giant cells involving the superficial and deep dermis (Figure 1). The lesional cells showed variable degrees of nuclear pleomorphism with occasional mitoses, approximately 7 per 10 high-power fields (HPF) (Figure 1C). The lesion showed slight-to-moderate fibrotic stroma, which was seen expanding the superficial and deep dermis. Immunohistochemistry was positive for vimentin, CD10, CD99, alpha smooth muscle antigen, B-catenin, and focal positivity for D2-40 (Figure 2). Ki-67 proliferation showed 10% of nuclear expression. Immunohistochemistry was negative for S100, PAN-ME, HMB45, muscle-specific actin (MSA), desmin, myosin, CD34, MCK, CAM5.2, epithelial membrane antigen (EMA), p63, CD31, ALK1, HHV8. The histological and immunohistochemistry findings were consistent with the diagnosis of NF.

Nodular fasciitis is a benign solitary, circumscribed, subcutaneous nodule that affects adult males and females equally [1]. Although NF grows rapidly over several weeks, it usually presents smaller than 2 cm in size [2],[3]. The dermal variant of NF is very rare and can present with additional symptoms of pain [4]. In our case, the patient presented with tenderness. The most common sites for NF occurrence are the upper extremities, especially the forearm, followed by the trunk, and head and neck [5]. Nodular fasciitis of the ear auricle or pinna is very rare. In a case series of 50 patients on NF of the ear region, only 2 patients or 4% were of the pinna [3]. Based on our literature search, only 8 cases of auricular NF have been reported (Table 1). All the cases reported thus far and including this case, the average age of patients with auricular NF is 35.1 years and this affects males 85.7% of the time. Interestingly, auricular NF involves the dermis in most cases with subcutaneous involvement reported only in two cases (Table 1). Intradermal NF is itself a rare entity with only 36 cases reported in literature, of which 5 cases are from the ear auricle. Like subcutaneous NF, the dermal variant also seldom presents greater than 2 cm in size (Table 1 and Table 2).

Nodular fasciitis was first described in 1955 by Konwaler et al., as subcutaneous pseudosarcomatous fibromatosis due to its ability to mimic sarcomas histologically [15]. Clinically, dermatofibroma, lipoma, neuroma, and fibrosarcoma are often parts of the differential diagnoses [16]. Microscopically, the lesion was a spindle cell neoplasm with atypical spindle cell density and extravasated erythrocytes (Figure 1B). Therefore, our differential diagnoses for NF included AFX, spindle cell melanoma, poorly differentiated squamous cell carcinoma, and sarcomas such as dermatofibrosarcoma protuberans (DFSP) and nodular stage of Kaposi sarcoma (KS). Furthermore, the immunohistochemistry showed positive expression with vimentin, CD10, CD99, alpha smooth muscle antigen (aSMA), and negative expression for S100, HMB45, MSA, desmin, CD34, MCK, CAM5.2, EMA, CD31, ALK1, and HHV8. Therefore, mimics including melanoma, and poorly differentiated squamous cell carcinoma were sufficiently excluded. Furthermore, CD34 and HHV8 staining sufficiently excluded DFSP and KS, respectively.

Overall, the features were suggestive of atypical fibrohistiocytic proliferation with the diagnosis consistent with NF and the differential diagnosis of AFX and pleomorphic sarcoma. Although this lesion showed atypical spindle cells with variable pleomorphism and mitoses, there was no necrosis, ulceration, lymphovascular invasion, or perineural involvement that would make it suspicious for a malignant process. Furthermore, the patient’s young age and lack of bizarre-appearing and highly atypical epithelioid cells made the diagnosis of AFX unlikely, which largely occurs in patients above the age of 70 years [17].

Although auricular intradermal NF is a very rare entity, but pathologists and clinicians should be aware to include it as part of the differential diagnosis for rapidly growing dermal nodules of the ear. It may initially appear ominous for malignancy both clinically and histologically, so careful analysis of clinical history and risk factors, supplemented by histological and immunohistochemical workup is essential to reach the correct diagnosis.

Intradermal auricular NF is extremely rare entity. It should be distinguished from AFX, sarcomas, melanoma, and squamous cell carcinoma due to significantly different clinical outcomes. The diagnosis is dependent on morphology and distinct immunohistochemistry staining pattern. Complete excision is standard of practice for symptom management and to exclude other differential diagnoses.