What could cause a rash on the head and neck of a woman with a sharp demarcation at the middle neck?

Head and Neck Rash with Sharp Mid-Neck Demarcation

A rash on the head and neck with sharp demarcation at the middle neck most commonly suggests radiation dermatitis from prior head and neck radiotherapy, where the radiation field boundary creates this characteristic cutoff pattern.

Primary Diagnostic Consideration: Radiation Dermatitis

The sharp demarcation at the mid-neck is the key diagnostic feature that points to radiation-induced skin changes, as radiation fields are precisely defined and create distinct boundaries between treated and untreated skin. This pattern is characteristic of patients who have received radiotherapy for head and neck malignancies, where the radiation portal typically extends from the head to the mid-neck region.

Clinical Features to Assess

The severity and appearance will help guide management:

• Grade 1: Faint erythema or dry desquamation confined to the radiation field 1
• Grade 2: Moderate to brisk erythema with patchy moist desquamation, mostly in skin folds 1
• Grade 3: Moist desquamation beyond skin folds with bleeding from minor trauma 2
• Grade 4: Full-thickness skin necrosis or spontaneous bleeding (rare, <5% of cases) 2

History to Obtain

• Prior cancer treatment: Specifically ask about head and neck cancer and radiation therapy, as this creates the characteristic sharp field boundary 2
• Concurrent medications: EGFR inhibitors (cetuximab, erlotinib) can cause acne-like rash that coexists with radiation dermatitis 2
• Timeline: Radiation dermatitis typically develops during or shortly after treatment 2

Alternative Considerations

While radiation dermatitis is most likely given the sharp demarcation, other possibilities include:

• Contact dermatitis: From necklaces, clothing, or topical products, though this typically has less precise boundaries
• Photodermatitis: Sun-exposed areas versus covered neck, but demarcation is usually less sharp
• Allergic reaction: To cosmetics or hair products, though distribution would be less geometric

The geometric precision of the demarcation strongly favors radiation dermatitis over these alternatives.

Management Approach

For All Grades

Maintain meticulous skin hygiene by gently cleaning the area with pH-neutral synthetic detergent (not soap) and drying with a soft towel before any treatment application 2, 1

Critical timing rule: Never apply topical products shortly before any ongoing radiation treatments, as they create a bolus effect that artificially increases radiation dose to the epidermis 1

Grade-Specific Treatment

Grade 1 (Mild erythema):

• Keep area clean between treatments 1
• Optional non-perfumed moisturizers 1
• Avoid sun exposure using soft clothing or mineral sunblocks 2, 1

Grade 2-3 (Moderate to severe):

• Drying pastes for skin folds where reactions remain moist 2, 1
• Gels for seborrhoeic areas 2, 1
• Creams for areas outside skin folds 2, 1
• Hydrophilic dressings for moist areas to absorb exudate 2, 1
• Silver sulfadiazine or beta glucan cream applied after radiation (evening application after cleaning) 2, 1
• Hyaluronic acid cream or zinc oxide paste if easily removable before treatment 2, 1

Corticosteroid Use

Topical corticosteroids are not contraindicated but should have limited treatment duration to minimize risk of skin atrophy and telangiectasias 1, 3. For acute dermatitis, mid-potency corticosteroids like triamcinolone acetonide 0.1% cream can be used 2-4 times daily 3.

Infection Monitoring

If infection is suspected (increased warmth, purulence, systemic symptoms):

• Swab the affected area for culture 2
• Check blood granulocyte count, especially if receiving chemotherapy 2
• Obtain blood cultures if fever or sepsis signs present 2
• Reserve topical antibiotics for documented superinfection only—do not use prophylactically 2

What to Avoid

• Greasy topical products: These inhibit wound exudate absorption and promote superinfection 2
• Skin irritants: Perfumes, deodorants, alcohol-based lotions 2, 1
• Scratching the affected area 2
• Sun exposure without protection 2, 1

Important Caveats

No prophylactic treatments beyond keeping the area clean and dry have proven effective in preventing radiation dermatitis 2, 1. The sharp demarcation pattern is pathognomonic for a field-based exposure (radiation being most common), making this a clinical diagnosis that doesn't typically require biopsy unless the diagnosis is uncertain or malignancy is suspected.

If the patient has no history of radiation therapy, the sharp demarcation warrants investigation for other geometric exposures (occupational, chemical) or consideration of dermatology referral for alternative diagnoses.