e22063 Background: Merkel Cell Carcinoma (MCC) is a highly aggressive neuroendocrine skin cancer that affects the elderly. Most cases are associated with the Merkel cell polyomavirus. It has previously been shown in clinical trials that MCC respond to immunotherapy with PD1 and PDL1-inhibition. PD1-inhibitiors have been used to treat MCC at Karolinska University Hospital in Sweden since 2016. The aim of this study is to describe the PD1-MCC-cohort at Karolinska. Methods: Patients with inoperable MCC that were treated with PD1-inhibitor at the oncology dept between August 2016 and February 2020 at Karolinska were included in the study, and response data was assessed retrospectively. Response was assessed clinically and through CT-scans. Results: Among 12 treated pts, median age was 75 yrs, 8 male and 4 female, 10 were treatment naive, 2 with prior chemotherapy. 10 pts received pembrolizumab and 2 pts nivolumab. No pts discontinued treatment due to side-effects. Comorbidities seen in the cohort were amongst others Atrial fibrillation (4 pts), Systemic Lupus Erythematosus (1), Hypertension (6), COPD (1), Aortic stenosis (2) and congestive heart failure (1). Follow-up time was median 10 months (range 2-43). First evaluation varied between 2-4 months with the majority at ~3 months. At first evaluation ORR was 75% and PFS and OS was 75% and 100% respectively. At 10 months, PFS and OS rates were 58% and 67% respectively. At study end mean PFS was 15,4 months and mean OS 17,8 months. 6 pts (50%) were still in response. Conclusions: PD1-inhibitors in clinical use in this cohort induced rapid and durable tumor responses in patients with advanced MCC, and the treatment was well tolerated. Our conclusion is that a PD1-inhibitor is a feasible and effective treatment for advanced Merkel Cell Carcinoma in the clinical setting.

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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.