This video provides a detailed pathological overview of Nodular Fasciitis (NF), a benign, rapidly growing spindle cell proliferation, typically arising from the fascia in young adults' upper extremities. Microscopically, NF is defined by its small size (usually < 3 cm) and heterogeneous composition, featuring hypercellular areas intermixed with hypocellular, myxoid change. The single most important diagnostic feature emphasized is the presence of myxoid cystic degeneration. Cytologically, the lesion consists of plump myofibroblasts with bland nuclei and brisk, but non-atypical, mitotic activity. The cells characteristically express Smooth Muscle Actin (SMA) in a "tram track" pattern. While historically considered reactive, the discovery of the USP6-MYH9 fusion gene suggests NF may be a transient, self-limited neoplasm. Familiarity with NF is essential to prevent misdiagnosis as a sarcoma or other aggressive soft tissue lesion.
Nodular Fasciitis: Key Pathological Features
0:00 Definition and Presentation: Nodular Fasciitis (NF) is a benign spindle cell proliferation that presents as a solitary, rapidly growing mass, typically in the upper extremities of young adults.
0:32 Origin and Growth Patterns: NF usually arises from the fascia and may grow upwards (subcutaneous), downwards (intramuscular), or horizontally (fascial tracking).
1:14 Crucial Size Clue: The vast majority of these lesions are small, measuring under 3 centimeters. Pathologists should exercise great caution if diagnosing NF in a lesion much larger than this.
1:40 Low-Power Appearance: The lesion is heterogeneous, showing areas that are hypercellular/fibrous alternating with areas that are hypocellular/myxoid.
1:52 Key Diagnostic Finding (Takeaway): The most useful diagnostic clue is the presence of small pockets of myxoid or mucinous cystic degeneration (filled with hyaluronic acid or ground substance).
2:44 Microscopic Pattern: Spindle cells (myofibroblasts) are arranged in short fascicles or in a haphazard, "feathery," or "tissue culture-like" pattern, especially in myxoid areas.
3:30 Cell Characteristics: Cells are plump myofibroblasts with bland nuclei, open chromatin, and a characteristic bluish/purplish cytoplasm (differentiating them from smooth muscle cells).
4:22 Diagnostic Pitfall: Do not confuse trapped, atrophic skeletal muscle cells (which can appear large and hyperchromatic) with nuclear pleomorphism or atypia.
5:17 Mitotic Activity: Mitotic activity is common and can be brisk, but atypical or abnormal mitotic figures are not usually identified, which is crucial for distinguishing NF from sarcoma.
5:53 Associated Features: Other common microscopic features include dense, hyalinized collagen bundles (keloidal collagen) and extravasated red blood cells (3:30).
10:06 Immunohistochemistry (IHC): Myofibroblasts express Smooth Muscle Actin (SMA), often showing a classic "tram track" or "train track" staining pattern. IHC is typically not required for diagnosis.
11:06 Etiology Update (Genetic Takeaway): Recent data (2011) suggests NF is a transient, self-limited neoplasm (tumor) rather than merely a reactive process, due to the presence of the t(17;22) translocation, resulting in the USP6-MYH9 fusion gene.
11:53 Clinical Significance: NF is an important entity for pathologists because its rapid growth, cellularity, and mitotic rate can easily lead to a misdiagnosis of sarcoma.