Here is what Dave Mustaine posted on his official Twitter

I’ve been diagnosed with throat cancer. It’s clearly something to be respected and faced head on – but I’ve faced obstacles before. I’m working closely with my doctors, and we’ve mapped out a treatment plan which they feel has a 90% success rate. Treatment has already begun.
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Unfortunately, this requires that we cancel most shows this year. The 2019 Megacruise will happen, and the band will be a part of it in some form. All up to date information will be at megadeth.com as we get it. Megadeth will be back on the road ASAP.
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Meanwhile, Kiko, David, Dirk and I are in the studio, working on the follow up to Dystopia – which I can’t wait for everyone to hear.
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I’m so thankful for my whole team – family, doctors, band members, trainers, and more.
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I’ll keep everyone posted.
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See you soon,
Dave Mustaine

The reactions and support online have been amazing

As scary as this must be for Dave we I know he’s gonna kick cancers ass! Thought and Prayers Dave!