This pathology commentary focuses on the diagnostic features of nodular fasciitis (NF) and its related spectrum, proliferative myositis/fasciitis, emphasizing microscopic clues and pitfalls. Nodular fasciitis, while classically found on extremities, is well-described in the head and neck region, sometimes presenting with a more cellular appearance, particularly in younger patients. Key diagnostic features include small size (typically < 3 cm), cystic myxo degeneration (mixoid cysts), and a loose, "feathery" or "tissue culture" arrangement of myofibroblasts. While mitosis can be abundant, the presence of hyperchromatic pleomorphic nuclei should rule out NF. The video also discusses the importance of studying reactive post-surgical changes to understand the spectrum of atypical myofibroblast morphology. Finally, the variant known as proliferative myositis/fasciitis is examined, characterized by the inclusion of large, "scary" ganglion-like cells within an otherwise classic NF background, often intermingling with splayed skeletal muscle fibers—a pattern considered highly reassuring for this benign process.
Pathology of Nodular Fasciitis and Proliferative Myositis
0:04 Head and Neck Location: Nodular fasciitis (NF) classically occurs on the extremity but is well-described in the head and neck, sometimes presenting with more cellular and atypical features in children.
0:24 Size Constraint: NF lesions are typically small. If a lesion is larger than three centimeters, pathologists should "think twice" before diagnosing nodular fasciitis.
0:46 Key Clue: Cystic Myxo Degeneration: NF often exhibits cystic mixo degeneration, presenting as subtle, bluish mixoid cysts or pockets where the lesion is breaking down—an "incredibly helpful clue" for diagnosis.
1:02 Key Clue: Tissue Culture Arrangement: The myofibroblasts display a "tissue culture arrangement," described alternatively as "loose and feathery."
2:02 Mitosis vs. Atypia: Mitosis is often present, especially in early, cellular lesions. However, the presence of hyperchromatic pleomorphic nuclei should raise suspicion for malignancy and typically rules out nodular fasciitis.
2:41 Learning Tool: Pathologists are advised to frequently study post-surgical reactive changes to understand the wide and sometimes "wild" range of reactive myofibroblast morphology, which aids in distinguishing benign reactive processes from neoplastic (tumor) processes.
3:36 Proliferative Variant: Proliferative myositis/fasciitis is a related entity characterized by the presence of large, atypical, "scary" ganglion-like cells within a background that otherwise resembles NF (loose/myxoid stroma, extravasated red cells).
4:56 Reassuring Pattern: A highly reassuring low-power pattern for proliferative myositis is the patchy intermingling of splayed-apart (sometimes atrophic) skeletal muscle fibers suspended in the loose fasciitis background alongside the scattered ganglion-like cells.
5:20 Diagnostic Takeaway: Always start diagnosis at low power to assess the overall pattern before moving to high power, which can lead to over-interpreting atypical cells.