USMLE Guide: Skin Cancer

Introduction

Skin cancer is a common malignancy that arises from the cells of the skin. It is primarily caused by excessive exposure to ultraviolet (UV) radiation from the sun or artificial sources, such as tanning beds. This USMLE guide provides an overview of skin cancer, including its types, risk factors, clinical presentation, diagnostic procedures, and treatment options.

Types of Skin Cancer

1. Basal Cell Carcinoma (BCC):

   • Most common type of skin cancer.
   • Arises from basal cells in the epidermis.
   • Typically presents as a pearly or translucent papule with telangiectasias.
   • Slow-growing with low metastatic potential.
   • Treatment options include excision, Mohs surgery, and topical therapies.

2. Squamous Cell Carcinoma (SCC):

   • Second most common type of skin cancer.
   • Arises from keratinocytes in the epidermis.
   • Presents as a scaly, erythematous plaque or nodule.
   • May metastasize to regional lymph nodes.
   • Treatment involves excision, Mohs surgery, or radiation therapy for high-risk cases.

3. Melanoma:

   • Less common but more aggressive form of skin cancer.
   • Arises from melanocytes, pigment-producing cells.
   • Can develop from a pre-existing mole or as a new lesion.
   • ABCDE mnemonic helps identify suspicious features: Asymmetry, Border irregularity, Color variation, Diameter >6mm, Evolution.
   • Early detection is crucial for favorable prognosis.
   • Treatment options include wide excision, sentinel lymph node biopsy, and systemic therapies for advanced cases.

Risk Factors

• UV radiation exposure: Prolonged sun exposure, tanning bed use.
• Fair skin: People with light skin, hair, and eye color are at higher risk.
• Personal or family history: Previous skin cancer or family members with skin cancer.
• Immunosuppression: Organ transplant recipients, HIV/AIDS patients.
• Precancerous skin lesions: Actinic keratosis, dysplastic nevi.
• Arsenic exposure: Occupational or environmental exposure to arsenic.

Clinical Presentation

• Basal Cell Carcinoma:

  • Pearly or translucent papule with telangiectasias.
  • May have a central depression or ulceration.
  • Commonly found on sun-exposed areas (face, neck, scalp).
  • Rarely metastasizes.

• Squamous Cell Carcinoma:

  • Scaly, erythematous plaque or nodule.
  • May have central ulceration or crusting.
  • Commonly found on sun-exposed areas (face, ears, lips).
  • May metastasize to regional lymph nodes.

• Melanoma:

  • Asymmetric lesion with irregular borders and variable colors.
  • Diameter usually larger than 6mm.
  • Evolution or change in size, shape, or color.
  • May have associated symptoms like itching, bleeding, or pain.

Diagnostic Procedures

1. Skin Biopsy: Gold standard for diagnosis.

   • Incisional or excisional biopsy depending on lesion size.
   • Histopathological examination to determine cancer type and depth.

2. Sentinel Lymph Node Biopsy (SLNB): For melanoma staging.

   • Injection of a radioactive tracer and/or blue dye near the primary lesion.
   • Identification and biopsy of the first draining lymph node.
   • Helps determine the extent of disease spread.

Treatment Options

1. Surgical Excision:

   • basal cell carcinoma: Complete excision with negative margins.
   • Squamous cell carcinoma: Wide excision with consideration of margins based on risk factors.
   • Melanoma: Wide excision based on Breslow thickness and sentinel lymph node status.

2. Mohs Surgery: Used for high-risk or cosmetically sensitive areas.

   • Microscopically controlled surgery for complete tumor removal while sparing healthy tissue.

3. Radiation Therapy: For cases where surgery is not feasible or as adjunct therapy.

   • Used in advanced or high-risk squamous cell carcinoma.

4. Topical Therapies:

   • Superficial basal cell carcinoma: Topical imiquimod or 5-fluorouracil cream.
   • Actinic keratosis: Topical 5-fluorouracil, imiquimod, or photodynamic therapy.

5. Systemic Therapies: For advanced or metastatic melanoma.

   • Immunotherapy (e.g., anti-PD-1 antibodies, BRAF/MEK inhibitors).
   • Targeted therapy