How would you manage the axilla in a patient with Merkel cell carcinoma of the left forearm with clinically negative, axillary sentinel node positive disease?

I would irradiate the axilla with 50 Gy in 25 fractions. I would not recommend completion lymphadenectomy, regardless of number of sentinel lymph nodes involved with carcinoma, or extranodal extension of carcinoma. A randomized controlled trial has demonstrated that 50 Gy in 25 fractions targeting clinically-negative lymph node basins is associated with a very low risk (0%) of regional lymph node recurrence in Merkel cell carcinoma (Jouary et al., Ann Onc 2012). Based on randomized trials in breast cancer (ie, AMAROS, etc), radiotherapy is known to produce fewer adverse events than completion lymphadenectomy after sentinel lymph node biopsy. For these reasons, radiotherapy would be my suggestion.