The use of adjuvant radiation therapy (RT) to the primary site in Merkel cell carcinoma (MCC) is not uncommon. However, the need for adjuvant RT to the primary site in patients at low risk for local recurrence is questionable.

To examine the occurrence of true local, satellite, in-transit, regional, and distant recurrences in patients undergoing surgery alone without adjuvant RT to the primary site. To establish overall survival (OS), MCC-specific survival (MCCSS), and disease-free survival (DFS) relationships in a cohort of patients with MCC.

Our University of Michigan Multidisciplinary MCC Program database was used to obtain characteristics and outcome measures for 104 patients (105 primary MCCs) with tumors less than 2 cm in diameter. The majority of patients were treated between July 2006 and November 2012.

Outcome measures included the occurrence of true local, satellite, in-transit, regional, and distant recurrences. End points included OS, MCCSS, and DFS.

Overall, information for 55 men and 49 women with 105 primary MCCs was obtained; 19 patients developed recurrent disease, and the mean time to first recurrence was 10.7 months. True local recurrence occurred in 1 patient with concurrent in-transit recurrence. Satellite recurrence occurred in 1 patient with concurrent regional recurrence. Four additional patients developed in-transit metastases. Thirteen patients had a regional recurrence component, 4 patients had distant metastases, and 6 patients developed subsequent regional and/or distant recurrences. Stratified by initial pathologic stage, the OS and MCCSS at 48 months were estimated to be 85.0% (95% CI, 71.8%-92.3%) and 94.4% (95% CI, 83.4%-98.2%) for patients with stage 1A/B disease and 63.2% (95% CI, 36.6%-81.1%) and 78.1% (95% CI, 50.0%-91.5%) for patients with stage 3A disease. The OS and MCCSS at 24 months for patients with stage 3B disease were both 50.0% (95% CI, 5.8%-84.5%).

In selected MCC patients with primary tumors less than 2 cm in diameter treated with surgery alone without adjuvant RT to the primary site, we found a low occurrence of true local recurrences and satellite recurrences. This relatively low rate of local recurrence questions the need for adjuvant RT to the primary tumor site in patients with small low-risk lesions.