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

Abstract

Merkel cell carcinoma (MCC) recurs in 17-30% of patients with stage I-II disease, with local recurrence rates between 2-27%. Per established guidelines, conventional postoperative radiotherapy (cPORT; ∼50 Gray across 25 fractions) should be considered for localized MCC when risk factor(s) are present. cPORT can be morbid and logistically challenging. Single fraction 8 Gray radiotherapy (SFRT) has utility and low toxicity for palliation but remains underexplored postoperatively. Evaluate postoperative SFRT for stage I-II MCC, compared to observation and cPORT. Local recurrences were assessed for observation (n=108 patients), cPORT (n=156), or SFRT (n=43) groups. Observation patients had fewer per-patient risk factors (mean: 1.2 versus 1.6 for SFRT [p=0.001] and 1.6 for cPORT [p=0.020]). Despite this, 3-year local control was better for cPORT (97.4%; p<0.001) and SFRT (100%; p=0.012) 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). Single-center retrospective study. 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 multi-center study.