9593 Background: Merkel Cell Carcinoma (MCC) is a rare, cutaneous neuroendocrine tumor associated with poor prognosis. Adjuvant radiation therapy (RT) is commonly used for the treatment of locoregional disease, but the impact on survival remains controversial. Previous studies from the Survival, Epidemiology, and End Results (SEER) database have shown an overall survival benefit to adjuvant RT without controlling for important prognostic differences in comparison groups such as age. To better control for these variables, we examined the impact of postoperative adjuvant radiation therapy on stage I and II disease-specific survival using the SEER database. Methods: SEERStat was used to obtain and analyze the outcomes of 2423 histologically confirmed MCC patients treated with surgery monotherapy or surgery with postoperative radiation therapy between 2004 and 2020 in the 17-registry (November 2021) SEER database. We compared the 5-year disease-specific survival (DSS) between the surgery monotherapy and adjuvant RT groups for stage I (n=1697) and stage II (n=726) MCC patients. Results: Patients treated with surgery and adjuvant RT were younger than those who received surgery monotherapy (median age 72 versus 77, respectively). Adjuvant RT was not associated with an increase in disease-specific survival. In contrast, adjuvant RT was associated with a statistically significant decrease in 5-year DSS among stage I patients (82.8% for those treated with RT versus 87.5% for those treated with surgery monotherapy, p = 0.023). Additionally, RT was not associated with a significant increase in DSS among stage II patients (77.0% versus 74.4% for patients treated with surgery monotherapy, p=0.490). Conclusions: This study demonstrates that there is no statistically significant disease-specific survival benefit from adjuvant radiation therapy for stage I and II MCC in the SEER database. Interestingly, in stage I patients we find a significant negative association between adjuvant RT and DSS. Younger patients are more likely to receive adjuvant RT, which likely contributes to higher overall survival previously reported from the SEER database, but the explanation for decreased DSS in patients who received RT remains unclear. Further studies regarding the effect of radiation therapy on local disease control and tumor microenvironment are needed to establish the appropriate role of RT in stage I-II MCC patients. [Table: see text]
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