This transcript provides a detailed histopathological examination of a spindle cell lesion arising from the tendon tissue on the plantar surface of the foot, diagnosing it as plantar fibromatosis. The key microscopic features include an ill-defined nodule of bland spindle cells arranged in broad, sweeping fascicles, separated by pink collagen fibers. The presenter notes the high cellularity (contributing to a "bluer" appearance) and distinguishes this entity from palmar fibromatosis (smaller lesions) and desmoid fibromatosis (deeper, larger, and genetically distinct). While morphology is key, the classic immunohistochemical feature—nuclear expression of beta-catenin—is discussed, noting its diagnostic utility but practical challenges in interpretation.
Summary of Plantar Fibromatosis Histology
0:02 Specimen Location: The sample is taken from the bottom of the foot, showing dense regular connective tissue identified as tendon.
0:23 Lesion Origin and Appearance: An ill-defined spindle cell lesion is observed growing directly out of the tendon, forming a relatively cellular nodule that appears "blue" compared to the pink background collagen.
0:40 Key Morphology – Fascicles: The tumor cells are bland and arranged in characteristic broad, sweeping fascicles, with the spindle cells running parallel to each other.
1:01 Cell Type: The lesion consists of fibroblasts or myofibroblasts, which are separated from neighboring cells by strands of pink collagen.
1:25 Classification: This entity is identified as Plantar Fibromatosis. It is typically much smaller, more cellular, and "bluer" than desmoid fibromatosis.
2:01 Mitotic Figures: Scattered mitotic figures are a common finding in plantar fibromatosis and should not be a source of concern.
2:09 Comparison to Palmar Fibromatosis: Palmar fibromatosis (Dupuytren’s contracture) is usually much tinier, sometimes comprising only a few fascicles, but can cause clinical issues like trigger finger. Plantar fibromatosis is generally bigger and more cellular.
3:05 Comparison to Desmoid: Desmoid fibromatosis is similar in appearance (broad sweeping fascicles) but is bigger and behaves as a separate disease process.
3:16 Classical Immunostain: All forms of fibromatosis (plantar, palmar, desmoid) classically show nuclear expression of beta-catenin.
3:34 Molecular Distinction: Desmoid tumors have actual genetic abnormalities (in beta-catenin or APC genes), whereas palmar and plantar fibromatosis achieve beta-catenin buildup through other mechanisms.
4:14 Takeaway: Although beta-catenin staining is the classic marker, the speaker prefers relying on the H&E morphological features, as nuclear beta-catenin staining can be challenging to interpret cleanly in practice.