Best Practice: CT-guided Adaptive Radiotherapy Reference Planning on the Ethos Platform

Online adaptive radiotherapy (ART) enables treatment plan modifications based on the patient's anatomy at each fraction, addressing limitations of conventional radiotherapy that relies on a static pre-treatment plan. CT-guided ART (CTgART), as implemented on the Varian Ethos platform, has gained increased clinical adoption over the past several years; however, its success is highly dependent on reference plan design due to limited on-table planning flexibility. As a result, clinicians creating and reviewing reference plans will greatly benefit from both understanding how upstream planning decisions affect the downstream adaptive process and proactively incorporating strategies that improve optimization robustness and efficiency on-treatment. We describe a planning framework that emphasizes integration of standard planning while incorporating the temporal workflow considerations. We term this new planning paradigm: Thinking in 4D. In this article, we provide practical recommendations and strategies, gained from high-volume clinical experience adapting hundreds of cases across multiple institutions, to guide the design of robust Ethos reference planning templates. These recommendations are based on accumulated clinical experience and are intended to complement, rather than replace, formal quantitative validation studies. These principles aim to improve adaptive plan quality, efficiency, and safety, while also supporting implementation, training, and routine plan checks for teams transitioning to planning in 4D.