This pathology case presentation details the diagnosis of Nodular Fasciitis (NF) in a 25-year-old female with a rapidly growing 3 cm forearm lesion. The histopathology confirms the classic features of NF, including an encapsulated nodule composed of plump, stellate myofibroblasts within a loose, mixoid matrix, often described as a "tissue culture appearance." Key diagnostic elements include the presence of cystic mixoid microcysts, common but benign mitotic figures, and the lesion’s typical small size (3 cm or less). The video emphasizes that NF often arises from the fascia and, despite its name, is generally not inflammatory. Crucially, the lesion is a benign neoplasm associated with the MYH9-USP6 gene fusion (also seen in aneurysmal bone cysts). Clinically, NF is benign, may regress spontaneously, and does not require complete excision with negative margins, distinguishing it from sarcoma.
Nodular Fasciitis: Case Review and Diagnostic Features
0:16 Clinical Presentation: The case involves a 25-year-old female with a rapidly growing, 3 cm forearm lesion, a classic presentation for nodular fasciitis (NF). NF typically affects the extremities of young adults and is fast-growing but small.
1:07 Histological Features: The core microscopic finding is a "tissue culture appearance"—feathery, loose mixoid areas composed of plump, spindled, and stellate myofibroblasts.
1:46 Key Diagnostic Sign: A highly characteristic feature is the presence of cystic degeneration and mixoid microcysts.
2:19 Cellular Atypia and Mitosis: Cells should lack severe atypia or nuclear pleomorphism. Mitoses are common and can be abundant, but atypical mitotic figures are usually absent, which helps differentiate NF from sarcoma.
2:57 Size Matters: NF is typically 3 cm or smaller. The speaker advises thinking twice before diagnosing NF if the lesion exceeds 3 cm, as size correlation is crucial for ruling out malignancy.
3:30 Misleading Nomenclature: Despite the name "fasciitis," the lesion is not typically highly inflammatory; pathologists should not expect abundant inflammation.
3:51 Fascial Origin: NF frequently arises directly from the fascia (dense, wavy collagen bundles), making it either push into the subcutis, push down into muscle, or grow linearly along the fascia (the fascial pattern).
5:03 Intravascular Involvement: NF can involve the lumen of vessels; this is an acceptable finding in NF and should not be mistaken for malignant vascular invasion.
5:40 Molecular Basis: NF is now considered a neoplasm due to the recurring MYH9-USP6 gene fusion/translocation. This finding links NF to Aneurysmal Bone Cysts (ABC), which share the same fusion.
6:47 Clinical Management and Takeaway: NF is benign and may regress spontaneously. Complete excision with negative margins is optional and not strictly indicated, as recurrence is rare even with positive margins.
7:38 Crucial Differential Diagnosis: NF should never be overcalled as sarcoma, even given the frequent mitotic activity.