Dermal tumors and proliferations

Benign Fibrous & Histiocytic - Common Dermal Bumps

• Dermatofibroma (Benign Fibrous Histiocytoma)

  • Firm, pigmented papule/nodule on extremities.
  • Fitzpatrick's sign: Lateral pressure creates a central dimple.
  • Histo: Dermal spindle cell proliferation entrapping collagen bundles.

• Acrochordon (Skin Tag)

  • Fleshy, pedunculated papule in intertriginous areas (axilla, neck).
  • Associated with obesity, insulin resistance.

• Keloid & Hypertrophic Scar

  • Overgrowth of dense fibrous tissue after skin injury.
  • Keloid: Extends beyond original wound borders.
  • Hypertrophic Scar: Confined to wound borders.
  • Histo: Thick, haphazardly arranged eosinophilic collagen bundles.

⭐ Fitzpatrick's sign is a classic clinical finding for dermatofibroma, demonstrating dermal tethering.

Vascular Proliferations - Red & Purple Lesions

• Cherry Angioma:

  • Most common benign vascular tumor; increases with age.
  • Bright red, dome-shaped papules.
  • Histology: Well-circumscribed proliferation of capillaries in the papillary dermis.

• Pyogenic Granuloma (Lobular Capillary Hemangioma):

  • Benign, rapidly growing red papule; often ulcerates and bleeds.
  • Common on skin, gingiva (pregnancy tumor).
  • Frequently follows minor trauma.

• Bacillary Angiomatosis:

  • Bartonella infection in immunocompromised (CD4 <100).
  • Red-purple papules/nodules.
  • Histology: Capillary proliferation with neutrophils & nuclear dust.

• Kaposi Sarcoma (KS):

  • Malignant; associated with HHV-8.
  • Affects AIDS patients, transplant recipients.
  • Purple patches, plaques, or nodules.

⭐ Differentiating look-alikes: Bacillary Angiomatosis shows neutrophilic inflammation, while Kaposi Sarcoma is characterized by lymphocytic and plasma cell infiltrates with atypical spindle cells.

Neural, Fat & Muscle - The Softer Side

• Neurofibroma
  • Benign nerve sheath tumor; proliferation of Schwann cells, perineurial cells, and fibroblasts.
  • Associated with Neurofibromatosis Type 1 (NF1).
  • Plexiform neurofibroma ("bag of worms" feel) is pathognomonic for NF1.
• Schwannoma (Neurilemmoma)
  • Encapsulated tumor of Schwann cells. Biphasic histology: cellular Antoni A areas with Verocay bodies and myxoid Antoni B areas.
• Lipoma
  • Most common soft tissue tumor; composed of mature adipocytes (fat cells).
  • Presents as a soft, mobile, painless subcutaneous nodule.
• Leiomyoma
  • Benign tumor of smooth muscle, often from arrector pili muscle (pilar leiomyoma). Can be painful.

⭐ Both Neurofibromas and Schwannomas are S-100 positive, reflecting their neural crest origin.

Malignant Dermal Tumors - Deep Trouble

• Dermatofibrosarcoma Protuberans (DFSP)

  • Low-grade sarcoma, locally aggressive with high recurrence.
  • Presents as a firm, slow-growing plaque/nodule, often on the trunk.
  • Histology: Storiform (pinwheel) arrangement of spindle cells.
  • Genetics: Characteristic translocation $t(17;22)$ fusing COL1A1-PDGFB genes.
  • Immunohistochemistry: Strongly positive for CD34.

• Angiosarcoma

  • Aggressive malignancy of endothelial cells.
  • Presents as a bruise-like patch or ulcerated nodule, typically on the scalp/face of the elderly.
  • Associated with chronic lymphedema (Stewart-Treves syndrome) and post-radiation.

⭐ Stewart-Treves Syndrome: Angiosarcoma arising in the context of chronic, non-filarial lymphedema, classically in a post-mastectomy arm.

High‑Yield Points - ⚡ Biggest Takeaways

• Dermatofibroma demonstrates the "dimple sign" on lateral compression.
• Dermatofibrosarcoma Protuberans (DFSP) is locally aggressive, driven by a t(17;22) translocation, and has a storiform pattern.
• Glomus tumors are benign but exquisitely painful, often subungual and sensitive to cold.
• Neurofibromas are hallmark lesions of Neurofibromatosis type 1 (NF1).
• Schwannomas are S-100 positive, showing Antoni A/B patterns and Verocay bodies.
• Kaposi Sarcoma is an HHV-8 associated vascular tumor in immunosuppressed individuals.