This video provides a focused dermatopathology review of Inclusion Body Fibromatosis (IBF), also known as Infantile Digital Fibroma. The entity is characterized clinically by nodules usually found on the 2nd, 3rd, and 4th digits of infants. Histologically, IBF is a bland fibroblastic/myofibroblastic proliferation arranged in weaving fascicles within the dermis and subcutis. The pathognomonic feature is the presence of bright red/pink intracytoplasmic inclusion bodies (composed of condensed actin filaments) located adjacent to the nucleus of the spindle cells. The condition is benign and typically requires only H&E staining for diagnosis, although it is notable for a high rate of local recurrence, primarily due to the difficulty in achieving wide excision on the small digits.
Summarizing Inclusion Body Fibromatosis (Infantile Digital Fibroma)
0:11 Nomenclature: The entity is known as Inclusion Body Fibromatosis (IBF) or Infantile Digital Fibroma/Fibromatosis. It is crucial to note that it is not related to desmoid fibromatosis or palmar/plantar fibromatosis.
0:33 Clinical Presentation: IBF typically presents on the digits (toes or fingers) of babies, showing a predilection for the 2nd, 3rd, and 4th digits, usually on the dorsal aspect. About 30% are present at birth, with the rest developing within the first year of life.
1:12 Histopathology (Architecture): Microscopically, it is a fibroblastic proliferation that fills the dermis and sometimes extends into the subcutis. The fibroblasts run in weaving fascicles.
3:09 Diagnostic Hallmark (Inclusion Bodies): The key diagnostic feature is the presence of eosinophilic (bright red/pink) globules—the "inclusion bodies"—which are condensed actin filaments located in the cytoplasm right next to the nucleus.
3:26 Myofibroblastic Features: Although classically fibroblastic, the cells exhibit myofibroblastic characteristics, showing a wispy, "tram track" pattern when stained for smooth muscle actin.
4:02 Diagnostic Confidence: With the characteristic clinical history (digit nodule in a baby) and the presence of inclusion bodies on H&E stain, the diagnosis is essentially complete, and supplementary immunohistochemistry is typically unnecessary.
4:10 Special Stains: Inclusion bodies can be highlighted by certain special stains, including PTAH (dark purple) and Trichrome (bright red).
5:11 Prognosis: The lesion is benign and not malignant, but it has a significant tendency for local recurrence (historically reported up to 60–70%), often attributed to the anatomical constraints that limit wide, complete surgical excision on the digits.
6:46 Case 2 Analysis: A second case is reviewed, demonstrating that the fascicular pattern may be subtle or less organized, but the presence of the inclusion bodies (7:51) remains the definitive feature for diagnosis.