What is the role of radiation therapy in the treatment of Merkel cell tumor?

Radiation Therapy in Merkel Cell Carcinoma

Radiation therapy is a critical component of Merkel cell carcinoma treatment and should be used adjuvantly after surgical excision in most cases to minimize locoregional recurrence, with specific dosing based on margin status and nodal involvement. 1, 2

Primary Role of Radiation Therapy

Radiation therapy serves multiple essential functions in MCC management:

• Adjuvant radiation after complete excision significantly reduces local and regional recurrence rates compared to surgery alone, even when surgical margins are negative 1, 3
• Expeditious initiation of adjuvant RT after surgery is critical, as delays have been associated with worse outcomes 1, 2
• RT can compensate for narrow surgical margins - patients receiving adjuvant RT achieve excellent local control (99%) regardless of whether margins are ≤1 cm or >1 cm, whereas surgery alone with narrow margins results in 20% local recurrence 3

Radiation Dosing Algorithm

For the Primary Site:

• Negative resection margins: 50-56 Gy 1, 2
• Microscopic positive margins: 56-60 Gy 1
• Gross positive or unresectable disease: 60-66 Gy 1

For Regional Nodal Basins:

After negative SLNB:

• Extremity/torso: No radiation indicated - observe regional nodal basins 1, 4
• Head and neck: Consider 46-50 Gy if at risk for false-negative biopsy due to complex drainage patterns 1, 4

After positive SLNB (microscopic disease):

• Axilla or groin: 50 Gy 1, 2
• Head and neck: 50-56 Gy 1, 2

After lymph node dissection:

• Axilla or groin: 50-54 Gy 1
• Head and neck: 50-60 Gy 1

Clinically evident lymphadenopathy: 60-66 Gy using shrinking field technique 1

No SLNB performed or unsuccessful: 46-50 Gy to nodal beds at risk for subclinical disease 1, 4

Technical Considerations

• All doses are delivered at 2 Gy per fraction using standard fractionation 1
• Bolus is used to achieve adequate skin dose 1
• Wide margins (5 cm) should be used around the primary site when possible 1
• If electron beam is used, select energy and isodose line (e.g., 90%) that delivers adequate lateral and deep margins 1

Site-Specific Considerations

Head and Neck MCC:

• Higher risk of false-negative SLNB due to aberrant lymph node drainage and multiple sentinel node basins 1, 4
• The radiation field treating the primary site often overlies draining lymph node beds 1
• Even in low-risk Stage IA head and neck MCC (≤2 cm, negative margins, negative SLNB, no immunosuppression), omission of PORT resulted in 26% local recurrence versus 0% with PORT 5

Extremity and Torso MCC:

• After negative SLNB and wide local excision, radiation is typically given to the primary site only 1
• SLNB dictates the need for regional irradiation - negative SLNB allows observation of regional nodal basins 1
• Irradiation of in-transit lymphatics is often not feasible unless the primary site is in close proximity to the nodal bed 1

Radiation as Primary Treatment

Definitive radiation therapy (without surgery) can be an effective alternative for locally advanced MCC, especially in anatomically challenging locations where extensive surgery would cause significant morbidity, disfigurement, or functional compromise 6

• Complete and durable control is achieved in up to 90% of cases when primary MCC is treated with definitive radiotherapy 6
• Generally less morbid than surgery and produces excellent cosmetic and functional outcomes 6

Palliative Radiation

For palliative intent, use a less protracted fractionation schedule such as 30 Gy in 10 fractions 1

Critical Pitfalls to Avoid

• Do not pursue extensive surgery to achieve clear margins if it will significantly delay adjuvant RT - pre-radiation margin status had no impact on locoregional failure in patients receiving adjuvant RT 1, 7
• Do not omit RT in head and neck MCC even for low-risk tumors - the false-negative SLNB rate is higher and local recurrence risk is substantial without RT 5
• Minimize tissue movement during reconstruction that could obscure the radiation target area 1
• Avoid delays in initiating RT - coordinate surgical planning to allow expeditious start of radiation 1, 2