This pathology video examines a large, painful, and slowly growing mass excised from the plantar aspect of the foot, diagnosed as Plantar Fibromatosis (Lederhose's disease). The surrounding tissue exhibits dense regular connective tissue, characteristic of deep structures, exemplified by the "Ramen Noodle Sign" (wavy, dense collagen). The tumor itself displays classic features of fibromatosis, including broad, sweeping fascicles of bland, uniform spindle cells separated by delicate collagen bundles.
The presenter highlights key diagnostic challenges specific to this location: Plantar Fibromatosis is often larger, more cellular, and more mitotically active than its palmar counterpart, which can mistakenly suggest malignancy (like Synovial Sarcoma). A major pitfall is identifying areas where fascicles are cut in cross-section, causing spindle cells to appear as round cells. While IHC for Beta-Catenin is discussed as an option for desmoid and plantar/palmar lesions, the diagnosis is generally established based on the characteristic H&E morphology, with additional stains (e.g., Keratin/TLE1) sometimes used to confidently exclude Synovial Sarcoma in highly cellular cases.
0:11 Initial Histology & Location: The lesion originated from the plantar foot. Examination of the surrounding tissue shows dense regular connective tissue (tendon sheath/fascia level), indicating a deep location.
0:41 The "Ramen Noodle Sign": The dense connective tissue is noted to be extremely wavy, a sign that helps distinguish dense regular connective tissue (tendon, fascia, or fibroblastic tumors) from nerve bundles, which are less wavy.
1:44 Core Tumor Morphology: The tumor is cellular and arranged in multiple nodules composed of monotonous, bland spindle cells.
2:18 Key Feature: Broad Fascicles: The cells run in extremely broad, parallel, sweeping fascicles, likened to a "100 lanes wide road." The cells are separated by fine, delicate collagen bundles, a key differentiator from denser malignancies.
3:28 Cell Lineage: The cells are often fibroblasts and myofibroblasts, noting that many fibroblastic lesions show actin expression.
4:10 High Cellularity/Mitosis: This specific case shows high cellularity and occasional mitotic activity (e.g., 4:17), which is common in plantar fibromatosis and can raise concern for aggressive tumors like Synovial Sarcoma.
Takeaway: The separation of cells by collagen is crucial for distinguishing fibromatosis from denser Synovial Sarcoma.
5:07 Histological Pitfall (Cross-Section): Fascicles cut 90 degrees in cross-section will cause spindle cells to appear round, potentially leading to confusion with a round cell neoplasm. Finding the classic long sweeping fascicles is essential for correct diagnosis.
6:14 Palmar vs. Plantar Fibromatosis: Palmar fibromatosis (Dupuytren's) is typically small and hypocellular. Plantar fibromatosis (Lederhose's) is usually large, more cellular, hyperchromatic, and mitotically active, often because surgical discomfort leads patients to allow them to grow longer before removal.
7:48 Mitosis is Acceptable: The presenter corrects the myth that fibromatosis should lack mitosis; occasional mitotic figures are expected, as the tumor must grow.
8:11 Role of Beta-Catenin IHC: Beta-Catenin IHC often shows nuclear positivity in desmoid, palmar, and plantar fibromatosis. However, it is a challenging stain due to non-specific cytoplasmic staining and is not always required for diagnosis.
10:17 Exclusion Stains: IHC like Keratin or TLE1 is considered more useful in highly cellular cases to confidently exclude a subtle Synovial Sarcoma, which can occur in the hands and feet.
11:01 Key Takeaway: The diagnosis of plantar fibromatosis is usually achievable based on the characteristic H&E morphology (wavy background, bland spindle cells, broad fascicles, collagen separation).