What are the depth requirements for radiation therapy in a patient with mycosis fungoides?

Depth Requirements for Radiation Therapy in Mycosis Fungoides

The treatment depth required is determined by the thickness of the tumor and its depth of invasion, which should be assessed using ultrasound, CT, or MRI, and dictates whether orthovoltage X-rays, electrons, or photons are needed. 1

Depth Assessment and Modality Selection

The depth of invasion must be evaluated before selecting radiation modality. 1 This assessment is critical because:

• Superficial lesions (patches/thin plaques) respond to low-energy electrons or orthovoltage X-rays that penetrate only 3-5 mm into the dermis 2
• Thick plaques or tumors require higher energy electrons (4-6 MeV) or photon beams to reach deeper tissue invasion 1
• Imaging with ultrasound, CT, or MRI should be performed when tumor thickness suggests invasion beyond superficial dermis 1

Electron Energy and Penetration Depth

For Total Skin Electron Beam (TSEB)

Higher energy electrons (4-6 MeV) are associated with superior complete response rates and should be used for optimal depth coverage. 1 The penetration characteristics are:

• 4-6 MeV electrons penetrate to approximately 1-1.5 cm depth, treating epidermis and dermis while sparing deeper structures 3, 4
• Lower energy electrons (< 4 MeV) treat only to 3-5 mm depth 2
• The Stanford protocol uses 6 MeV electrons with dual-field technique to achieve uniform dose distribution 1

For Localized Radiotherapy

Orthovoltage X-rays (100-280 kV) are appropriate for superficial patches and thin plaques. 5 However:

• Electrons (4-12 MeV) should be selected when lesions have measurable thickness 5
• Photon beams are reserved for tumors with deep invasion beyond electron range 1

Stage-Specific Depth Considerations

Stage IA-IIA (Early Disease)

• Solitary patches/plaques: Orthovoltage X-rays or low-energy electrons (4 MeV) are sufficient for superficial disease 5
• Multiple plaques: 4-6 MeV electrons provide adequate depth coverage for typical dermal involvement 1

Stage IIB (Tumorous Disease)

Tumor thickness and depth of invasion must be assessed with imaging before treatment planning. 1 Treatment selection:

• Superficial tumors: Can be treated with electrons after initial photon beam reduction if very thick 2
• Deep tumors: Require photon beams initially, followed by electron boost after tumor flattening 2
• Regional photon irradiation should precede TSEB when tumors exceed electron penetration depth 2

Stage III-IV (Advanced Disease)

• Erythrodermic disease: Standard 4-6 MeV electrons are adequate as disease remains primarily cutaneous 1
• Nodal involvement: Requires photon beam therapy to reach lymph node depth 1, 2
• Visceral metastases: Photon beam therapy using standard non-Hodgkin lymphoma protocols 1

Critical Technical Considerations

Avoiding Common Pitfalls

Never use photon-based total skin irradiation (e.g., tomotherapy) even at low doses, as excessive dose to deeper structures causes bone marrow toxicity that never occurs with electrons. 3 This is a critical safety distinction.

For thick plaques or tumors beyond electron range, use regional photon therapy first to flatten lesions, then follow with TSEB. 2 This sequential approach allows:

• Initial tumor reduction with adequate depth penetration
• Subsequent electron therapy for remaining superficial disease
• Prophylactic treatment of adjacent nodal areas 2

Depth-Dose Optimization

The conventional Stanford TSEB delivers dose to 1-1.5 cm depth, which is adequate for most mycosis fungoides as the disease primarily involves epidermis and dermis. 3, 4 However:

• Hemi-body irradiation with bolus (9-12 Gy) may be needed for multiple regional tumors with large, thick skin infiltration, followed by TSEB boost 2
• Total body irradiation (2.5-3 Gy split-course) combined with TSEB addresses both cutaneous and systemic disease in Sézary syndrome or visceral involvement 2

Practical Algorithm for Depth-Based Treatment Selection

1. Assess lesion characteristics: Measure thickness clinically and with imaging if tumorous 1
2. Superficial disease (< 5 mm): Use orthovoltage X-rays or low-energy electrons 5, 2
3. Standard dermal involvement (5-15 mm): Use 4-6 MeV electrons 1
4. Deep tumors (> 15 mm): Use photon beams initially, then electron boost 2
5. Extensive disease: TSEB with 4-6 MeV electrons provides optimal depth coverage 1