"I’ve been diagnosed with throat cancer," Mustaine said in a statement. "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," he continued.
His treatment requires the band to cancel most of its scheduled shows this year, Mustaine said, including their Aug. 9 performance on the Illinois Lottery Grandstand Stage.
"While we are obviously disappointed that Megadeath [sic] won’t be playing the Illinois State Fair, our main concern is for Dave Mustaine," State Fair Manager Kevin Gordon said in a statement, adding, "We wish him a full recovery and look forward to having Megadeath back to the Illinois State Fair in the future."
Anyone who purchased tickets through Ticketmaster will automatically receive a refund to the card used, while those who bought them from the Illinois State Fair in person or over the phone need to submit a refund form with their tickets attached.
Mustaine said Megadeth's 2019 "Megacruise" would still happen, with the band a part of it "in some form," and all updated information would be posted to their website.
Mustaine said he and the other band members are in the studio working on their next album, a follow-up to the group's 15th studio album, Dystopia, which was released in 2016. Megadeth has sold 38 million albums and received 12 Grammy nominations in total.
Other performers scheduled to perform at the Illinois State Fair this summer include Pentatonix, Old Dominion, Snoop Dogg, Shinedown and Reba McEntire.
The fair runs from Aug. 8 to 18.