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Competing Interests: Conflicts of interest Dr Nghiem has served as a consultant for the following: EMD Serono, Merck, and Pfizer/Regeneron and his institution has received research funding from Bristol-Myers Squibb and EMD Serono. Dr Bhatia served as a consultant (receiving honorarium) for Bristol-Myers Squibb, EMD Serono, Sanofi-Genzyme, Castle Biosciences, and Exicure; his institution (UW) received research grants from Bristol-Myers Squibb, EMD Serono, Merck, Novartis, Incyte, Nantkwest, Exicure, Nektar, Amphivena, Checkmate/Regeneron, Xencor, 4SC, and Agenus. Dr Singh has received consulting fees from BMS. Dr Zaba has received a speaker honorarium from EMD Serono. Dr Park and Authors Hippe, Zawacki, and Bierma have no conflicts of interest to declare.

Background: Merkel cell carcinoma (MCC) is often treated with surgery and postoperative radiation therapy (PORT). The optimal time to initiate PORT (Time-to-PORT [ttPORT]) is unknown., Purpose: We assessed if delays in ttPORT were associated with inferior outcomes., Methods: Competing risk regression was used to evaluate associations between ttPORT and locoregional recurrence (LRR) for patients with stage I/II MCC in a prospective registry and adjust for covariates. Distant metastasis and death were competing risks., Results: The cohort included 124 patients with median ttPORT of 41 days (range: 8-125 days). Median follow-up was 55 months. 17 (14%) patients experienced a LRR, 14 (82%) of which arose outside the radiation field. LRR at 5 years was increased for ttPORT >8 weeks vs ≤ 8 weeks, 28.0% vs 9.2%, P = .006. There was an increase in the cumulative incidence of MCC-specific death with increasing ttPORT (HR = 1.14 per 1-week increase, P = .016)., Limitations: The relatively low number of LRRs limited the extent of our multivariable analyses., Conclusions: Delay of PORT was associated with increased LRR, usually beyond the radiation field. This is consistent with the tendency of MCC to spread quickly via lymphatics. Initiation of PORT within 8 weeks was associated with improved locoregional control and MCC-specific survival., Competing Interests: Conflict of interest None disclosed., (Copyright © 2023 American Academy of Dermatology, Inc. Published by Elsevier Inc. All rights reserved.)