Abstract:

This dermatopathology teaching session, led by Dr. Jared Gardner and dermatology residents, covers several fibrohistocytic and spindly skin lesions, focusing on their microscopic features and differential diagnoses. Key cases discussed include Elastofibroma Dorsi (characterized by beaded elastic fibers), Acquired Digital Fibrokeratoma (a polypoid lesion with thick acral skin), hypertrophic scars (noting the trick of new collagen overlaying solar elastosis), Infantile Digital Fibroma/Inclusion Body Fibromatosis (defined by eosinophilic inclusion bodies), Dermatofibroma (including its hemosiderin-rich and aneurysmal variants), Sclerotic Fibroma/Plywood Fibroma (associated with Cowden syndrome), Pleomorphic Fibroma (related to pleomorphic lipoma via RB1 loss), Myofibroma (noted for its pseudo-chondroid appearance and staghorn vessels), Dermatomyofibroma (identified by parallel fascicles), Bite Fibroma (traumatic lesion of the lip), Nodular Fasciitis (characterized by mixoid cystic change), and pigmented DFSP/Bednar Tumor (DFSP containing melanin-producing cells). The session also delves into the interpretation of biopsy site changes, including the appearance of maturing scars and the artifacts caused by hemostatic agents like aluminum chloride.

Summary of Spindly Lesion Cases:

• 0:00:07 Case 1: Elastofibroma Dorsi:
  • 0:01:02 Characterized by beaded and bunched elastic fibers, resembling pipe cleaners in longitudinal section and small flowers in cross-section (confirmed with Verhoeff-Van Gieson stain).
  • 0:02:36 Typically occurs on the back, often between the scapula and rib cage, likely due to motion friction. Diagnosis is often made radiographically.
• 0:03:37 Case 2: Acquired Digital Fibrokeratoma (ADFK):
  • 0:04:07 A polypoid lesion resembling an acrochordon (skin tag) but lined by thick, acanthotic, acral-type skin.
  • 0:04:31 Dermis shows fibrotic tissue with vertical collagen bundles and bland fibroblasts/myofibroblasts.
  • 0:05:04 Differential includes Periungual Fibroma (Koenen tumor) seen in Tuberous Sclerosis, though ADFK is not associated with the syndrome.
• 0:07:47 Case 3: Hypertrophic Scar:
  • 0:08:25 Identified by dense, whorled collagen pattern in the dermis.
  • 0:09:18 Key feature for dating: new pink collagen replacing blue solar elastosis indicates the lesion formed recently (after sun damage developed).
  • 0:10:36 Incidental finding: Foreign body giant cell reaction around injected corticosteroid material within the scar tissue.
• 0:14:12 Case 4: Infantile Digital Fibroma / Inclusion Body Fibromatosis:
  • 0:15:29 Classic dome-shaped papule with acanthotic epidermis.
  • 0:15:38 Dermis replaced by bland spindle cell proliferation in fascicles.
  • 0:15:53 Key diagnostic feature: Pink, tangled intracytoplasmic inclusion bodies (actin fibers).
  • 0:16:40 Benign, but can recur; mainly affects babies' digits.
• 0:18:04 Case 5: Dermatofibroma (DF):
  • 0:18:54 Low power shows a hazy, slightly purplish, ill-defined dermal nodule with overlying epidermal hyperplasia (blunting/tabling of rete ridges).
  • 0:20:20 Characteristic feature: Entrapment of collagen bundles at the periphery.
  • 0:21:10 Often contains Touton giant cells and hemosiderin (golden-brown, refractile, chunky pigment).
  • 0:23:17 Cells are variably plump spindle/oval cells; mitosis is acceptable.
  • 0:26:26 When involving fat, DF shows reactive changes (fat necrosis, lymphocytes), unlike DFSP which shows clean entrapment.
• 0:40:03 Case 6: Sclerotic Fibroma / Plywood Fibroma:
  • 0:41:13 Lesion is completely replaced by dense, pink, sclerotic collagen with very low cellularity.
  • 0:41:46 Artifactual cracks in the collagen create a whorled/wood-grain appearance ("plywood pattern").
  • 0:42:33 Can be sporadic or associated with Cowden syndrome (germline PTEN mutation).
• 0:43:48 Case 7: Pleomorphic Fibroma (PF):
  • 0:54:37 Polypoid or sessile lesion, hypocellular, resembling a skin tag but with atypical cells.
  • 0:54:54 Dermis has variable numbers of hyperchromatic, atypical, stellate/multi-nucleated cells.
  • 0:55:39 Related to spindle cell/pleomorphic lipomas; often shows loss of nuclear RB1 staining due to 13q deletion.
• 0:58:46 Case 8: Myofibroma:
  • 0:46:45 Classic finding: large nodules (pseudo-chondroid nodules) with a bluish, myxoid/chondro-myxoid background, often compressing adjacent vessels.
  • 0:47:29 Intervening areas show cellular spindle/round cells with prominent "staghorn" (branching, antler-shaped) vessels.
  • 0:47:37 Cells are bland and mitotically active; usually benign and indolent, often seen in infants/children.
• 0:58:46 Case 9: Dermatomyofibroma:
  • 0:59:15 Key feature: Parallel fascicles of bland fibroblastic/myofibroblastic cells running parallel (east to west) to the epidermis, leaving adnexal structures intact.
  • 0:59:58 Often has a large hemorrhagic space with hemosiderin, sometimes called Aneurysmal Dermatofibroma.
• 1:04:14 Case 10: Atrophic Dermatofibroma:
  • 1:05:21 Light on spindle cells with increased collagen, easily mistaken for normal dermis or scars.
  • 1:06:26 Clues are epidermal changes: hyperplasia, elongated/blunted rete ridges, and basaloid follicular induction.
• 1:07:46 Case 11: Angiofibroma / Fibrous Papule:
  • 1:08:12 Dome-shaped papule with collagen-rich dermis and bland spindle/stellate cells.
  • 1:08:56 Can show epidermal induction and variable dilated vessels.
  • 1:09:00 In sun-damaged patients, the lesion is new because it has replaced the solar elastosis with new collagen.
• 1:09:16 Case 12: Fibrous Hamartoma of Infancy (FHI):
  • 1:09:54 Haphazard mixture of mature fat, parallel spindle cell fascicles, and small balls of bluish/myxoid "immature mesenchyme."
  • 1:11:14 Must be distinguished from DFSP due to fat involvement.
• 1:11:49 Case 13: Dermatofibrosarcoma Protuberans (DFSP):
  • 1:12:18 Characterized by very bland, thin, monotonous spindle cells in a swirling/storiform (whirling) pattern.
  • 1:13:09 Cleanly entraps islands of fat; little fat necrosis/inflammation.
  • 1:14:07 Infiltrative with high recurrence risk; requires wide local excision or Mohs.
  • 1:14:57 Bednar Tumor: A pigmented variant of DFSP containing spindled melanocytes (melanin pigment).
• 1:16:52 Case 18: Giant Cell Tumor of Tendon Sheath (TGCT):
  • 1:17:18 Neoplastic cells are plump, massive mononuclear cells with abundant cytoplasm, often accompanied by numerous osteoclastic giant cells.
  • 1:18:24 Often shows hemosiderin that forms a ring around the mononuclear cells ("ladybird cells").
  • 1:19:01 Usually deeper; uncommon to be seen in the dermis by dermatology.
• 1:19:52 Case 19: Tuberous Xanthoma:
  • 1:21:07 Large nodule filling the dermis/subcutis, composed of cells with abundant foamy cytoplasm (histiocytes), giving a pale grayish/blue color.
  • 1:21:21 Associated with serum lipid abnormalities; requires lipid panel workup.
• 1:21:41 Case 20: Dermatomyofibroma:
  • 1:22:43 Key feature: Parallel fascicles of bland, hypocellular spindle cells running parallel to the epidermis (unlike DF's whorled pattern).
  • 1:24:13 Differential: CD34-positive Fibroblastic Connective Tissue Nevus, which also shows fat entrapment but lacks the characteristic discrete parallel fascicles.
• 1:26:35 Case 21: Bite Fibroma / Traumatic Fibroma:
  • 1:27:35 Mucosal lesion (lip) characterized by thickened mucosa, fibrosis, dilated vessels, and reactive epithelial hyperplasia due to biting/trauma.
• 1:28:04 Case 22: Nodular Fasciitis:
  • 1:28:52 Spindle cell proliferation with a loose, blue myxoid background, often showing "mixoid cystic degeneration" (pockets of empty space).
  • 1:30:17 Cells are plump/stellate myofibroblasts; mitotic activity is common.
  • 1:32:57 Typically small (usually < 3 cm) and rapidly growing.
• 1:34:15 Biopsy Site Change:
  • 1:34:44 Maturing scar tissue shows parallel myofibroblasts, vertical vessels, and loss of epidermal rete ridges.
  • 1:37:54 Aluminum chloride application (for hemostasis) can cause artifactual granular histiocytes and collagen calcification in the scar.