Abstract:

This dermatology and pathology residency teaching session, led by Dr. Jared Gardner, focuses on the histopathological diagnosis of various fibrohistocytic and spindly skin lesions. The video guides viewers through several cases, highlighting key microscopic features to differentiate between benign and potentially malignant tumors. Diagnoses covered include Elastofibroma Dorsi (identified by beaded elastic fibers), Acquired Digital Fibrokeratoma (characterized by acral skin lining and dense collagen), a hypertrophic scar (noted for its dense collagen wiping out solar elastosis), Infantile Digital Fibroma (marked by inclusion bodies), Dermatofibroma (emphasized by collagen entrapment and epidermal changes, including hemorrhagic variants), Sclerotic Fibroma (characterized by dense, sclerotic collagen with a "plywood" pattern), Pleomorphic Fibroma (linked to lipomas via RB1 loss), Myofibroma (defined by pseudo-chondroid nodules and staghorn vessels), Dermatomyofibroma (distinguished by parallel fascicles of bland spindle cells), Bite Fibroma (a reactive process from trauma to the lip), Nodular Fasciitis (key features being mixoid cystic change and high mitotic activity), and Dermatofibroma variants (including those mimicking scars or showing aneurysmal change). The session concludes with a detailed review of a biopsy site scar, emphasizing the reactive changes like vertical vessels and myofibroblast proliferation, and how to distinguish it from other dermal lesions.

Dermatopathology Review: Fibrohistocytic and Spindly Skin Lesions

• 0:00:07 Introduction & Case 1: Elastofibroma Dorsi: Dr. Gardner introduces the session with dermatology/pathology residents, focusing on fibrohistocytic lesions.
  • Key Feature: Deep lesion with background of beaded, bunched, or "pipe cleaner"-like elastic fibers (confirmed with Elastic Stain/Verhoeff-Van Gieson).
  • Location: Almost always occurs on the back, often between the scapula and rib cage.
  • Clinical Note: Usually diagnosed radiographically and only removed if symptomatic.
• 0:03:37 Case 2: Acquired Digital Fibrokeratoma: Discussed as a polypoid lesion resembling an acrochordon (skin tag).
  • Key Features: Lined by thick, acanthotic/orthokeratotic skin; dermis shows dense, vertical bundles of collagen and bland fibroblasts/myofibroblasts with dilated vessels.
  • Differential: Shares overlap with Angiofibroma or Periungual Fibroma (Koenen tumor, associated with Tuberous Sclerosis), but this lesion is sporadic and benign.
• 0:07:47 Case 3: Hypertrophic Scar: Characterized by dense collagen.
  • Key Feature: Dense collagen nodule showing a "world pattern" (swirled) in some areas, extending to the surface.
  • Age Clue: The new pink collagen nodule wiping out underlying blue solar elastosis indicates a relatively recent lesion (post-adult onset).
  • Incidental Finding (10:36): Presence of refractile material surrounded by foreign body giant cells, identified as injected corticosteroid.
• 0:14:12 Case 4: Infantile Digital Fibroma (Inclusion Body Fibromatosis):
  • Key Features: Dome-shaped papule with acanthotic epidermis; bland spindle cell proliferation in fascicles; characteristic finding of pink balls ("inclusion bodies," tangled actin fibers) within the cells.
  • Clinical Note: Benign but can recur; occurs in infants on digits.
• 0:18:04 Case 5: Dermatofibroma (DF):
  • Low Power Clues: Hazy, purplish, ill-defined dermal nodule with overlying epidermal hyperplasia (blunting/tabling of rete ridges).
  • Peripheral Feature: Entrapment and splaying into surrounding collagen bundles.
  • Cellular Features: Plump, sometimes round/pleomorphic cells (unlike thin DFSP cells); often contains Touton giant cells (despite being a buzzword for JXG), hemosiderin (golden-brown, chunky), foam cells, and scattered mitoses (which are acceptable).
  • Subcutaneous Involvement (26:00): When trickling into fat, typically shows foamy histiocytes and fat necrosis (unlike the clean invasion of DFSP).
• 0:28:20 Dermatofibroma Variants & Granzone Discussion:
  • Atrophic DF (6:47): Light on spindle cells, more collagenous; key diagnostic clue is the overlying epidermal changes (elongated, blunted rete ridges, follicular induction).
  • Hemorrhagic/Aneurysmal DF (59:26): Large, deep lesions with large pools of blood, hemosiderin, and often foam cells/giant cells. Rare risk of metastasis.
• 0:53:42 Case 6: Pleomorphic Fibroma:
  • Key Features: Polypoid/sessile lesion, hypocellular, with scattered atypical, often stellate/multi-nucleated cells.
  • Pathogenesis: Likely related to spindle cell/pleomorphic lipoma due to loss of nuclear RB1 staining (deletion on 13q).
  • Management: Benign; excision may be warranted if mitosis is present on a partial sample, to rule out deeper entities like myxofibrosarcoma.
• 0:58:46 Case 7: Keloid:
  • Key Feature: Thick, "bubble gum pink" collagen bundles set against a background of scar tissue.
  • Note: Can occur in all skin types, not just darker skin.
• 0:40:03 Case 8: Sclerotic Fibroma (Plywood Fibroma):
  • Key Features: Well-circumscribed, dense, pink, sclerotic collagen nodule with very low cellularity; cracks create a "plywood" or wood-grain pattern.
  • Association: Can be sporadic or associated with Cowden Syndrome (PTEN mutation).
• 0:43:48 Case 9: Myofibroma:
  • Key Features: Nodular, biphasic appearance with nodules of bland spindle/oval cells (sometimes with a bluish/chondroid look—pseudo-chondroid nodules) separated by more cellular areas containing prominent staghorn/branching vessels.
  • Behavior: Benign, may regress; often seen in children.
• 0:49:29 Case 10: Dermatofibroma (Revisited - Superficial): Highlights epidermal changes like basaloid follicular induction mimicking Basal Cell Carcinoma.
• 0:53:42 Case 11: Pleomorphic Fibroma (Revisited): Confirms link to lipomas via RB1 loss.
• 0:58:46 Case 12: Aneurysmal Dermatofibroma (Revisited): Emphasizes the blood-filled spaces and hemosiderin.
• 1:04:14 Case 13: Atrophic Dermatofibroma (Revisited): Focuses on subtle spindle cells wrapping collagen, easily missed if one only focuses on the epidermal changes.
• 1:08:02 Case 14: Angiofibroma (Fibrous Papule): Dome-shaped, collagen-rich dermis with spindle/stellate cells; pushes out elastosis in sun-damaged skin.
• 1:09:16 Case 15: Fibrous Hamartoma of Infancy:
  • Key Features (Tripartite): Mature fat, parallel fascicles of bland fibroblasts, and immature mesenchyme (bluish/grayish, primitive-looking spindle cells/myxoid background).
• 1:11:49 Case 16: Dermatofibrosarcoma Protuberans (DFSP):
  • Key Features: Thin, bland, monotonous spindle cells in a swirling/storiform pattern; clean entrapment/squeezing of fat islands without significant fat necrosis.
  • Eponym: Pigmented DFSP is called Bednar Tumor (due to entrapped melanin-producing dendritic cells).
• 1:16:52 Case 17: Giant Cell Tumor of Tendon Sheath (Localized Type):
  • Key Features: Plump, massive mononuclear cells with abundant cytoplasm and large nuclei, scattered among numerous osteoclast-type giant cells, usually attached to dense regular connective tissue (tendon/fascia).
• 1:19:52 Case 18: Tuberous Xanthoma:
  • Key Features: Large dermal/subcutaneous nodule composed of cells with abundant, foamy cytoplasm, indicating xanthoma cells (histiocytes); associated with serum lipid abnormalities.
• 1:21:41 Case 19: Dermatomyofibroma:
  • Key Features: Discrete, parallel fascicles of bland, hypocellular spindle cells running parallel to the epidermis (not vertical like scar); may have fat entrapment, leading to differential with CD34+ Fibroblastic Connective Tissue Nevus.
• 1:26:35 Case 20: Bite Fibroma (Traumatic Fibroma):
  • Location: Mucosal lip; characterized by thickened, reactive mucosa with underlying fibrosis and dilated vessels due to trauma.
• 1:28:04 Case 21: Nodular Fasciitis:
  • Key Features: Spindle cell proliferation adjacent to fascia; crucial finding is mixoid cystic degeneration (pockets of empty space); high mitotic activity is common, but the lesion is usually small (<3cm).
• 1:34:15 Biopsy Site Scar Analysis:
  • Key Features: Parallel myofibroblasts running toward the epidermis, vertically oriented blood vessels, and loss of rete ridges due to tissue healing; mitotic activity is normal here.
  • Aluminum Chloride Effect: Finding granular histiocytes/collagen calcification can indicate prior chemical hemostasis at the biopsy site.