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Merkel cell carcinoma (MCC) is a rare and aggressive skin cancer where Merkel cell Polyomavirus (MCPyV) contributes to the pathogenesis. In an adjuvant setting, radiotherapy (RT) is believed to give a survival benefit. The prognostic impact of sex related to MCPyV-status and adjuvant RT were analyzed in patients referred to Karolinska University Hospital. Data were collected from 113 patients&rsquo, hospital records and MCPyV analyses were made in 54 patients (48%). We found a significantly better overall survival (OS) for women compared to men and a significant difference in OS in patients receiving adjuvant RT. Furthermore, we found that men with virus negative MCC have an increased risk for earlier death (HR 3.6). This indicates that MCPyV positive and negative MCC act as two different diseases, and it might be due to different mechanism in the immune response between male and female patients. This could have significance in tailoring treatment and follow-up in MCC patients in the future.

e21047 Background: Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine skin cancer. The pathogenesis is linked to the immune system and Merkel cell polyoma virus (MCPyV) is present in about 80% of MCC. Reports have shown that patients with a virus-negative MCC have a worse prognosis. Adjuvant radiotherapy given after surgery has previously been shown to give a survival benefit in retrospective data. Our aim is to analyze clinical variables and their prognostic impact in a Swedish cohort. Methods: All patients with confirmed diagnosis of MCC referred to Karolinska University Hospital in Sweden from 1989-2018 were included and retrospective data was collected. Results: 113 patients were included. Median age at operation was 75.4 years (range 19-99); 64 (57%) were female. Treatment included surgery with (n = 46) or without adjuvant radiotherapy (RT). 43% of female patients and 37% of male patients received RT. Median overall survival (OS) was 2,6 years (range 0,1-25,8), for female patients 3,4 years (range 0,1-25,8) and for male patients 1,8 years (range 0,1-19,0). Median OS in surgery group was 1,8 years (range 0,4-25,8). With adjuvant RT median OS improved with 1,7 years (p = 0,0001) to 3,5 years (range 0,2-22,1), for female patients 4,0 years (range 0,2-22,1) and for male patients 2,0 years (range 0,3-8,8). MCPyV status was available in 53 patients. 74% were MCPyV positive, 26% negative. MCPyV gave no difference in OS, however when gender was added in to multivariate analysis, female patients with virus negative disease had better outcome than virus negative males (p = 0.03). In virus positive MCC there was no difference in OS between female and male patients. Conclusions: Our data confirms the positive impact of adjuvant radiotherapy on survival and illustrates the difference in outcome between female and male patients, with women having a better outcome. In the MCPyV group the tendency is that men with virus negative MCC has the worst outcome. Our findings indicate that MCPyV positive and negative MCC act as two different diseases and it also raises questions if there is a difference in the disease itself or the immune response towards MCC between male and female patients.