Background: Precision medicine has changed treatment practices for patients with cancer. Clinical guidelines recommend tumor marker testing for many types of cancer given its benefits. Studies have demonstrated that tumor marker testing increases use of appropriate targeted therapies which is associated with improved survival, particularly among patients with advanced or metastatic cancer. In March 2018, Medicare issued a national coverage determination (NCD) for next-generation sequencing to facilitate tumor marker testing. Methods: We conducted a retrospective study to assess tumor marker testing among Medicare beneficiaries with cancer from 03/01/2016 through 02/28/2020. Data were obtained from the Virtual Data Warehouses of 6 United States (US) healthcare systems in the Cancer Research Network, a well-established distributed data network for cancer research. Together these systems provide care to a diverse population of over 5.5 million people in the US. The index date was the first observed cancer diagnosis date recorded in the tumor registry during the study period. Subgroup analyses included those with lung, breast, colorectal, or prostate cancers, or those with advanced, metastatic or recurrent cancer. This is part of a larger project that aims to advance methods for scalable and rigorous evaluation of outcomes of coverage policies for genetic tests. Results: We report results from one health system (03/01/2016-3/31/2018) including Medicare beneficiaries ≥65 years and ≥ 90 enrollment days after the index date. Among 2,277 Medicare beneficiaries with cancer, mean (SD) age was 74 (6.8) years, 1,065 (46.8%) were women, 199 (9.9%) had a tumor marker test within 90 days of the index date, and 1072 (47.1%) started a cancer drug therapy within 90 days of the index date. Among Medicare beneficiaries with lung (n=352), breast (n=361) colorectal (n=135) and prostate (n=326) cancers, proportions of patients having tumor marker tests ranged from 2.2% to 12.8% within 90 days of the index date. Among 572 Medicare beneficiaries with advanced, metastatic or recurrent cancer, 59 (10.3%) had a tumor marker test within 90 days of the index date. Conclusions: The relatively low tumor marker testing rate among Medicare beneficiaries with cancer in this health system is consistent with the literature. Analyses are underway to examine changes in tumor marker testing among Medicare beneficiaries after the implementation of the 2018 Medicare NCD.