1088P Efficacy of post-operative radiation therapy in non-metastatic Merkel cell carcinoma: A registry-based analysis

Abstract

Merkel cell carcinoma (MCC) is a rare aggressive neuroendocrine tumor of the skin. The optimal management of non-metastatic MCC is still unclear but it is crucial to decrease relapse risk and improve survival. We sought to assess the efficacy of radiation therapy (RT) of MCC without distant metastases (M0 MCC) by analyzing data from the Surveillance, Epidemiology, and End Results (SEER) registry. We extracted from the SEER Registry data about M0 MCC patients, identified by the 8247 ICD-03 code, irrespective of the site of origin. We collected data about stage at diagnosis, site and size of primary, extent of surgery, lymph node directed surgery, number of involved lymph nodes, RT, and survival outcome. Primary endpoint was overall survival (OS). Of 9773 MCC patients in the SEER registry, 2335 were M0 MCC (63% male, median age 77 years). MCC originated from skin of the limbs in 994 (61.6%), head and neck district in 386 (23.9%), and trunk in 234 (14.5%). Lymph nodes were negative in 1577 patients (67.5%), while among the 687 (32.5%) MCC with nodal involvement this was microscopic in 214, macroscopic in 233, not otherwise specified (NOS) in 240; 71 had in-transit metastases. Post-operative RT was performed in 1212 (51.9%) patients. Overall, median OS of M0 MCC patients was 56 months (95%CI: 50.7-61.3) and was affected by age, site, tumor size, nodal involvement, type of surgery of primary and RT (HR for RT: 0.65 [95%: 0.53-0.79]; p<0.001) at multivariate analysis. In the 1577 with negative lymph nodes, after correcting for confounding factors, OS was affected by age, sex, site, size of primary, extent of surgery and RT (HR for RT: 0.63 [95%: 0.47-0.82]; p<0.001). Among 687 patients with positive lymph nodes (67% male, median age 74), at multivariate analysis OS was affected by age, nodal involvement, number of positive lymph nodes and RT (HR for RT: 0.67 [95%CI: 0.49-0.92]; p=0.012). RT was associated with a decreased risk for death in M0 MCC, irrespective of nodal status.