Background: Thymic epithelial tumors (TETs) are rare tumors originating from the epithelial cells of the thymus. Thymomas tend to be slowly growing, whereas thymic carcinomas are more aggressive and often metastasize wildly. TETs have a very low tumor mutational burden (TMB). cfDNA has been used in several tumor types to describe the molecular characteristics and select treatment options, especially in absence of tissue availability. There is no information on the cfDNA detected in TETs. The purpose of this study was to identify common genomic alterations occurring in circulating tumor DNA (ctDNA) in patients with advanced TETs, detected using a cfDNA assay. Methods: We retrospectively evaluated 157 TET samples from the Guardant Health database between November 2017 – November 2020. The cfDNA analysis interrogated single nucleotide variants (SNV), fusions, indels and copy number variations (CNV) of up to 83 genes using a commercially available liquid biopsy assay (Guardant360; Guardant Health, Redwood City, CA). We evaluated the frequency of genomic alterations based on diagnosis, age, and sex. Results: In this cohort, 66% of the patients had thymic carcinoma and 34% had thymoma. The median age was 60 years, and 59% of patients were male. 126 patients (80%) of this cohort had ≥1 somatic alteration detected. The most prevalent mutations detected are TP53 (55%), KIT (13%), EGFR (12%), BRCA2 (11%), PIK3CA (10%), ARID1A (10%), ATM (10%), KRAS (9%), APC (9%), and BRAF (9%). Mutations were more commonly observed in thymic carcinomas than thymomas, but statistical significance was not reached due to the small sample size. Frequencies of the observed genomic alterations are shown in the table below. Conclusions: This study confirms that advanced stage TETs shed tumor DNA into the circulation that can be picked up in the majority of patients, using a solid tumor platform, despite the low TMB typically observed in these tumors. This assay can potentially be used to monitor response to therapy. A more targeted gene panel, enriched for genes commonly mutated in TETs (e.g. GTF2I, BAP1, CYLD) might provide further insights in the future in the management of TETs.