This transcript provides a microscopic analysis of a nodule identified as Nodular Fasciitis (NF), a benign fibroblastic lesion often found in the subcutis. The characteristic histopathology includes a loose, edematous, and myxoid background separating plump spindle cells identified as myofibroblasts, which exhibit purple amphiphilic cytoplasm. Key diagnostic features discussed include mixoid cystic degeneration, short fascicular arrangements, and the classic "tissue culture" or "feathery" appearance of the cells. While mitosis can be abundant, the crucial differentiator from malignancy (sarcoma) is the absence of atypical mitotic figures or marked cellular pleomorphism. The discussion concludes by noting that NF is now often considered a neoplasm due to the presence of the MYH9-USP6 gene fusion and is typically small, fast-growing, and benign, often resolving spontaneously or with simple excision.
Nodular Fasciitis Histopathology Summary
0:04 Microscopic Appearance: The lesion appears as a nodule in the subcutis with a loose, bluish, edematous, and mixoid background where cells are widely spaced out.
0:15 Cell Type & Morphology: Composed of plump spindle cells with characteristic purple amphiphilic cytoplasm, confirming them as myofibroblasts.
0:34 Diagnosis Confirmation: The presence of scattered erythrocytes, hemorrhage, and small blood vessel proliferation, along with the morphology, confirms the diagnosis of Nodular Fasciitis. Mitosis is present and expected.
0:57 Mixoid Cystic Degeneration: A helpful diagnostic feature is the presence of mixoid cystic degeneration, where cells break apart, leaving small or large cystic spaces.
1:19 Architecture: Cellularity is variable (loose to cellular). Cells may be arranged in short fascicles but never in the broad, sweeping fascicles seen in other lesions. Scattered osteoclast-like giant cells can sometimes be present, particularly near cystic areas.
1:59 Critical Size Constraint: NF lesions are typically 3 centimeters or smaller. A large lesion (e.g., 10 cm) should strongly raise suspicion against NF.
2:10 Tissue Culture Arrangement: A highly typical feature is the loose, feathery arrangement of plump, stretched-out cells (sometimes described as stellate/triangular), analogous to cells growing in a laboratory culture dish.
3:07 Distinguishing from Sarcoma (Key Takeaway): While mitosis may be abundant, the pathologist should not find atypical mitotic figures or marked hyperchromatic pleomorphic cells (ugly cells), which would suggest malignancy (sarcoma).
4:02 Etiology and Behavior: Nodular Fasciitis is benign. Although historically considered reactive, it is now often classified as a neoplasm due to the presence of the MYH9-USP6 gene fusion.
4:32 Prognosis: NF lesions are benign, and patients often have spontaneous regression, or they are treated with simple excision.