Enhancing the HemOnc Knowledgebase of Chemotherapy Drugs and Regimens - Project Summary Systemic cancer treatments have transformed the landscape for many patients with cancer, yet are toxic and often given in complicated combinations and protocols. In 2011, we began to build the HemOnc.org website as a free and open resource for hematology and oncology healthcare professionals. HemOnc.org contains granular information on anti-cancer drugs, regimens, and guidelines, and is now the largest resource of its kind. In 2020, there were >250,000 unique visitors and >1,000,000 pageviews from 181 countries. Users are primarily split between clinicians and pharmacists, with utilization by trainees as well as experienced practitioners. Translational researchers also utilize the site as a source of domain knowledge. Beginning in 2017, we formalized the structure of much of the content on HemOnc.org, enabling its transformation into a structured vocabulary, HemOnc. As of January 2021, HemOnc comprises 83,879 concepts across 30 concept classes, and 223,039 relationships across 45 relationship types. Explicit mappings exist between HemOnc and RxNorm, CanMED, SNOMED-CT, OncoTree, and the NCI Thesaurus. Several organizations and projects actively utilize or are evaluating HemOnc, including AACR Project GENIE, the ITCR-funded DeepPhe project, the Variant Interpretation for Cancer Consortium, and cBioPortal. In addition to maintaining the steady growth in content that has taken place over the past nine years, we aim to enhance the HemOnc.org website and HemOnc ontology through the following five specific aims: 1) present interactive ranked regimen graphs on the website; 2) allow for dynamic temporal exploration of regimens, studies, and authors with an interactive Regimen Browser; 3) grow and refine the HemOnc ontology; 4) build structured summaries and narrative synopses for the most important anticancer regimens, including identification of regimens specifically evaluated in underrepresented populations; 5) cultivate a robust user community. This work will be undertaken by an accomplished team of clinicians and translational researchers.