Principles of Radiation Therapy

Dose Recommendations for Radiation Therapy:
Primary site:
• Negative resection margins	50–56 Gy
• Microscopic (+) resection margins	56–60 Gy
• Gross (+) resection margins or unresectable	60–66 Gy
Nodal bed:
• No SLNB or LN dissection
• Clinically (−) but at risk for subclinical disease	46–50 Gy
• Clinically evident adenopathy: head and neck	60–66 Gy
• Clinically evident adenopathy: axilla or groin1	--1
• After SLNB without LN dissection
• Negative SLNB: axilla or groin	Radiation not indicated2
• Negative SLNB: head and neck, if at risk for false-negative biopsy	46–50 Gy2
• Microscopic N+ on SLNB: axilla or groin	50 Gy3
• Microscopic N+ on SLNB: head and neck	50–56 Gy
• After LN dissection
• Lymph node dissection: axilla or groin	50–54 Gy4
• Lymph node dissection: head and neck	50–60 Gy
All doses at 2 Gy/d standard fractionation. Bolus is used to achieve adequate skin dose. Wide margins (5 cm) should be used, if possible, around the primary site. If electron beam is used, an energy and isodose line (e.g., 90%) should be used to deliver adequate lateral and deep margins. Extremity and torso MCC: after negative SLNB and WLE, in most instances, radiation therapy is given to the primary site only. SLNB dictates the need for regional irradiation. If SLNB is negative, then regional nodal basins can be observed. If SLNB is not performed, consider irradiating nodal beds for subclinical disease. Irradiation of in-transit lymphatics is usually not feasible unless the primary site is close to the nodal bed. Head and neck MCC: risk for false-negative sentinel node biopsy is higher, because of aberrant lymph node drainage and frequent presence of multiple sentinel node basins. The radiation field to treat the primary site is often overlying the draining lymph node beds. Treatment options for clinically node negative MCC of the head and neck include: Perform SLNB and WLE. If SLNB is negative, options are to irradiate the primary site ± nodal beds and in-transit lymphatics or observe. OR Perform WLE without performing SLNB and irradiate the primary tumor site, in-transit lymphatics, and regional nodal sites.

¹Lymph node dissection is the recommended initial therapy for clinically evident adenopathy in the axilla or groin, followed by postoperative radiation if indicated.

²Consider RT when there is a potential for anatomic (e.g., previous history of surgery including WLE), operator, or histologic failure (e.g., failure to perform appropriate immunohistochemistry on SLNs) that may lead to a false-negative SLNB.

³Microscopic N+ is defined as single-node involvement that is neither palpable clinically nor abnormal by imaging criteria, which microscopically consists of small metastatic foci without extracapsular extension.

⁴RT may be omitted after axillary/groin LN dissection for microscopic disease. Postoperative radiation is indicated for multiple involved nodes and/or presence of more than focal extracapsular extension.