What are the current official guidelines regarding managing patients during COVID-19?

Here are some guidelines and FAQ from professional societies:

NCCN: https://www.nccn.org/covid-19/default.aspx

ASTRO FAQ: https://www.astro.org/Daily-Practice/COVID-19-Recommendations-and-Information/COVID-19-FAQs

ASCO Coronavirus Resources: https://www.asco.org/asco-coronavirus-information

ASCO:https://www.asco.org/asco-coronavirus-information/care-individuals-cancer-during-covid-19

ASTRO:https://www.astro.org/Daily-Practice/COVID-19-Recommendations-and-Information

ESTRO:https://www.estro.org/About/Newsroom/News/Radiotherapy-in-a-time-of-crisis

EMCrit:emcrit.org/ibcc/covid19/

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The Belgians have an excellent clinical guidance document.