Nodular fasciitis (NF) is a benign, soft tissue tumor composed of myofibroblasts that typically occurs in subcutaneous tissue, fascia, and/or muscles. The literature sometimes titles rare NF variants according to their tissue locations. The most frequently used and important of these are cranial fasciitis and intravascular fasciitis. In 2020, the World Health Organization classified nodular fasciitis as in the category of benign fibroblastic/myofibroblastic tumors. NF is the most common of the benign fibroblastic proliferative tumors of soft tissue. Nodular fasciitis is a rapidly growing, usually self-limiting neoplasm that occurs primarily but not exclusively in adults. Due to its rapid growth, NF is often misdiagnosed as a malignant tumor, usually a sarcoma. Indeed, NF was originally termed subcutaneous pseudosarcomatous fibromatosis when first described in 1955 by Konwaler et al. The correct diagnosis of a tumor as NF is pivotal to prevent its overtreatment as a more aggressive or malignant growth. While nodular fasciitis may be precipitated by localized injuries, recent studies indicate that NFs are true neoplasms (i.e. abnormal proliferations of cells without any precipitating event). Up to 92% of NF cases have a specific type of fusion gene in their tumor cells which may be responsible for disrupting the regulation of cell growth and death.
Presentation Nodular fasciitis occurs in all age groups but most often affects those between 20–40 years old. Males and females are equally affected. NF tumors, which may be tender or painful, typically present as rapidly growing solitary lesions that reach their final size (usually 2–3 cm) within a few weeks. They are located in the upper limbs (39–54% of cases), trunk (15–20% of cases), lower limbs (16–18% of cases), and head or neck area (20% of cases). Involvement of the head or neck area is more commonly observed in children. Cases of NF in joints or nerves are rare, but do occur. Individual cases of NF have been reported to occur in the bladder, prostate, tongue, lower female genital tract, and parotid gland. In some cases, NF tumors have regressed after incisional biopsy. The cranial fasciitis variant of NF occurs in the soft and hard cranial tissues of the outer layers of the skull. Patients with this variant are more commonly males than females, and almost exclusively between 3 weeks and 6 years of age. They typically present with a tumor in areas of the head that lay directly over the temporal or parietal bones. Individual cases have been reported to occur in the lower jaw, frontonasal region, anterior nasal spine, the orbit, and maxilla. Characteristically, the tumor is rapidly enlarging, non-painful, rarely regressing without treatment, and potentially expanding into the skull's interior. In a review of 50 cases, the intravascular fasciitis variant of NF occurred in individuals aged 6 months to 66 years (median age 27 years), with males and females being equally affected (52%:48%). Individuals with this variant commonly present with a blood vessel-localized tumor in the head and neck area (34% of cases), lower extremities (32% of cases), upper extremities (20% of cases), or trunk (14% of cases). The tumors originate in the small blood vessels of the oral mucosa, eyes, lips, cheeks, tongues, and subcutaneous tissue of the extremities (78% of all cases). About 18% of cases involve major veins. The presenting symptoms of these tumors are strictly dependent on their locations and impacts on the involved vasculature. Most cases involving superficial sites present with a small (mean diameter of 1.5 cm), painless, slowly growing mass. However, tumors growing in deep tissues can go unnoticed until they became large enough (e.g. 15 cm) to obstruct blood flow. Cases involving the ascending aorta can present with the signs and symptoms of acute aortic dissection (e.g. severe pain, heart failure, cardiac arrest, fainting, stroke, ischemic peripheral neuropathy, and/or paraplegia), while cases involving large veins may present with acute swelling, pain, and tissue/organ dysfunctions in the areas drained by the involved veins.
Pathology The microscopic histopathology of hematoxylin and eosin stained nodular fasciitis tumors consists of spindle-shaped myofibroblastic cells. These cells are in a myxoid or a collagenous (high content of collagen fibers) tissue background. The neoplastic myofibroblasts are arranged in whorls and/or short bundles. These cells may show high rates of replicating as judged by their mitotic index, but these mitoses are normal in appearance. The tumor tissues often contain red blood cells, lymphocytes, and giant osteoclast-like cells, and may contain sites of bone-like tissue. NF is sometimes classified into three subtypes based on its predominant histopathological pattern: myxoid or reactive (type I), cellular (type II), and fibrous (type III). These patterns appear related to the duration of the lesion, with the myxoid variant tending to have the shortest duration and the cellular and fibrous variants tending to have progressively longer durations. Immunohistochemical analyses indicate that the cells in NF usually express smooth muscle actin, muscle specific actin, and vimentin proteins but generally do not express CD34, S-100 protein, desmin, trypsin, factor VIII, F4/80, or HLA-DR1 proteins. Uncommonly, the cells in NF tumors express the CD68 (a histiocyte-specific marker) protein.
The histopathology and expressions of marker proteins in cranial fasciitis tumors tend to be more organized and have higher levels of inflammatory cell infiltrates, vascularity, and involvements of underlying bone than NF. The histopathology and expressions of marker proteins in intravascular fasciitis tumors tend to be arranged in a storiformed or haphazard pattern and have vesicle-containing nuclei with prominent nucleoli.
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