Radiotherapy

How Ionizing Radiation Works on Skin Cells

Ionization of Water and DNA Double-Strand Breaks

When a low-energy X-ray or electron beam enters the skin, ionizing radiation acts in two ways:

• Indirect effect (the main one): Radiation ionizes water inside the cell (radiolysis), producing highly reactive free radicals such as hydroxyl and superoxide. These attack cell membranes and the phosphodiester bonds of DNA.
• Direct effect: Radiation breaks both strands of the DNA molecule directly (double-strand breaks, DSBs). When the damage exceeds what the cell's DNA repair machinery can fix, the cell is arrested in the G2/M phase of the cell cycle and switches on programmed cell death (apoptosis).

Why Diseased Cells Are More Sensitive Than Healthy Ones

Skin cancer cells and rapidly proliferating fibroblasts divide quickly and repair DNA far less well than healthy keratinocytes and endothelial cells. Fractionation, splitting the total dose into many small daily doses, gives healthy skin cells time to repair DNA damage between sessions, while diseased cells accumulate damage and are destroyed.

How Radiation Helps Prevent Keloids

In keloids, overactive fibroblasts driven by TGF-beta produce far too much type I and type III collagen. Superficial radiation given within 24–48 hours of excision selectively kills dividing fibroblasts, suppresses new blood vessel growth and restores the balance between collagen production and breakdown.

Radiotherapy Techniques in Clinical Dermatology

• Superficial Radiation Therapy (SRT):
  • Characteristics: Uses low-energy X-rays (50–150 kV). Penetration is limited mainly to the top 1–5 mm of epidermis and dermis, with very little energy reaching deeper structures such as bone, cartilage or muscle.
  • Indications: Basal cell carcinoma (BCC), superficial or shallowly infiltrating squamous cell carcinoma (SCC), widespread actinic keratoses, and treatment-resistant keloids after surgery.
• Surface Brachytherapy With Custom Molds:
  • Characteristics: A sealed radioactive source (such as iridium-192 or Re-188) is placed directly against the lesion using a soft plastic mold made for each patient.
  • Advantages: Delivers a very precise dose that conforms to complex curved surfaces (the ear, the side of the nose, the corners of the mouth), preserving appearance and function as much as possible.
• Superficial Electron Beam Therapy (EBT):
  • Characteristics: A linear accelerator produces an electron beam of 4–6 MeV.
  • Indications: Widespread plaque- or tumor-stage mycosis fungoides, Merkel cell carcinoma and large skin tumors.

Clinical Safety, Side Effects and Contraindications

• Acute radiation dermatitis: Develops 2–3 weeks after treatment begins and is graded as follows:
  • Grade 1: Mild redness, dryness and dry peeling.
  • Grade 2: Marked redness, moist peeling limited to skin folds and mild swelling.
  • Grade 3: Widespread moist peeling with oozing and burning pain.
  • Management: Apply a repair cream with hyaluronic acid, zinc oxide and panthenol; avoid harsh soaps and never rub the treated skin.
• Late (chronic) radiation effects: Appear months to years later and include epidermal atrophy, spider-like dilated vessels (telangiectasia), dermal fibrosis, pigment changes (darker or lighter skin) and a risk of new secondary skin cancers decades later.
• Protecting sensitive structures: Dedicated lead shields are always used to protect the eyes and lenses, thyroid, ear cartilage and developing teeth when treating the head and neck.
• Absolute contraindications:
  • Inherited syndromes with defective DNA repair or radiation sensitivity (xeroderma pigmentosum, ataxia-telangiectasia, Gorlin syndrome/basal cell nevus syndrome).
  • Lesions lying directly over exposed cartilage or bone (risk of radiation-induced chondronecrosis or osteonecrosis).
  • Tumor recurrence in skin that has already received the maximum tolerated radiation dose.