What is your approach to adjuvant treatment to the tumor bed and/or regional lymph nodes for an early stage Merkel cell carcinoma status-post WLE and sentinel lymph node biopsy?

Merkel cell carcinoma is almost always beyond what you think are the safe margins of resection. Regardless of the completeness of wide local excision I generally extend the postoperative field two or 3 cm beyond the theoretically clear margin, obviously staying off of critical structures such as the eye. Prophylactic treatment of the regional lymph nodes is generally included in my treatment fields. If the patient has metastatic disease in the lymph nodes the prognosis is grim as Merkel cell has a tendency to metastasize extremely fast. In my experience, surgical resection of Merkel cell is generally futile. This tumor is extremely radio sensitive and radiation therapy is still the ideal treatment for local/regional disease.

I agree with the general principles highlighted by @Dr. First Last. The one caveat to add is that early stage (IA) MCC arising in the limbs and trunk could safely be managed with surgical monotherapy after obtaining clear margins and a negative sentinal node biopsy in a patient with an intact immune system (Frohm et al JAMA Dermatol. June 2016). In the H&N region, it becomes a challenge to obtain wide margins to clear the primary and SLNB becomes less reliable due to the complex and cross draining lymphatic pathways. This is probably why higher rates of local relapses are seen even with early stage MCC arising in the H&N region when one tries to omit RT. (Takagishi et al Advances in Radiation Oncology 2016 - in press)

I agree with both comments above. In my approach to a patient with MCC, I consider a sentinel node biopsy to be mandatory - if thats not possible, then I would try to include the primary draining nodal basin if that can be identified, and where possible would treat the primary site and nodal bed in one continuous field. For even the smallest tumor the risk of occult nodal disease is 15% or higher, so the first approach should always include biopsy or treatment of this site.

If the sentinel node is negative, then we know from the MSKCC surgical series (see D Coit et al, several publications), it appears that small tumors with negative SN and widely negative margins can forgo RT, but there's no consensus on what "small" is. For me, 1 cm or l...