One dose of adjuvant 8 Gray postoperative radiotherapy for Merkel cell carcinoma excised with narrow margins: Low toxicity and comparable efficacy to conventional radiotherapy

BACKGROUND: Merkel cell carcinoma (MCC) recurs in 17% to 30% of patients with stage I to II disease, with local recurrence rates between 2% and 27%. Per established guidelines, conventional postoperative radiotherapy (cPORT; ∼50 Gray across 25 fractions) should be considered for localized MCC when risk factors are present. cPORT can be morbid and logistically challenging. Single-fraction radiotherapy (SFRT) of 8 Gray has utility and low toxicity for palliation but remains underexplored postoperatively. OBJECTIVE: Evaluate postoperative SFRT for stage I to II MCC, compared to observation and cPORT. METHODS: Local recurrences were assessed for observation (n = 108 patients), cPORT (n = 156), or SFRT (n = 43) groups. RESULTS: Observation patients had fewer per-patient risk factors (mean: 1.2 vs 1.6 for SFRT [P = .020] and 1.6 for cPORT [P = .001]). Despite this, 3-year local control was better for cPORT (97.4%; P < .001) and SFRT (100%; P = .012) groups than observation (91.0%), particularly in patients with narrow surgical margins (≤1 cm). Adverse events occurred in 21% of SFRT patients (all grade 1). LIMITATIONS: Single-center retrospective study. CONCLUSIONS: The SFRT group experienced improved local control relative to observation and had comparable local control to the cPORT group. For early-stage narrowly excised MCC, SFRT is feasible, well tolerated, and warrants exploration in a prospective multicenter study.