Pathologic margin negativity is not a guarantee: The risk of merkel cell carcinoma recurrence can be reduced with a single dose of radiation

Summary: Merkel cell carcinoma (MCC) differs from most skin cancers by having a high overall recurrence rate (~40%) and an aggressive disease course. While surgery with path-negative margins is sufficient for most skin cancers, its effectiveness in early-stage MCC is controversial. We analyzed a prospectively enrolled cohort of 98 patients with early-stage MCC and path-negative margins who did not receive adjuvant radiation therapy (RT). Of these, 10 (10%) developed a local recurrence (LR), typically by one year after surgery (median follow-up: 8.5 years). No significant differences were noted between LR and non-LR groups in demographics, stage, or known LR risk factors (lymphovascular invasion, head/neck site, immunosuppression, >1 cm primary, positive sentinel node). Among patients with LR, 6 of 8 arose on the head/neck and the mean number of risk factors was 1.3±0.7. A 10% local recurrence rate is not extremely high and may not justify the toxicity (nearly all patients experience at least transient side effects) and inconvenience of conventional adjuvant RT typically involving 25 doses. However, path-negative margins are clearly not a guarantee of local control. A well-tolerated alternative approach, one dose of 8 Gray adjuvant RT, has been used in an analogous cohort (mean risk factors: 1.6±0.9) of 43 patients with MCC. None of these 43 experienced LR (median follow-up: 2.7 years) and 82% had no side effects. The risk of recurrence for the single-dose cohort was lower than the no-RT cohort (p=0.032). Adjuvant RT is well known to reduce MCC recurrences, likely by eliminating microscopic discontiguous disease. However, patients with low-risk MCC often forgo a conventional course of RT given inconvenience and significant side effects especially in the head/neck region. This study demonstrates that patients with early-stage MCC and path-negative margins still have an appreciable LR risk and could benefit from a low-toxicity adjuvant RT approach. Alex Fu¹, Peter Y. Ch'en¹, Ariel Finberg¹, Emily T. Huynh¹, Krista Lachance¹, Paul Nghiem¹, Song Y. Park¹ 1. University of Washington, Seattle, WA, United States. Clinical Research: Epidemiology and Observational Research