MEGADETH bassist David Ellefson says that he is not aware of any symptoms that Dave Mustaine had immediately prior to being diagnosed with throat cancer about two months ago.

The 57-year-old MEGADETH leader revealed his cancer diagnosis on June 17 on social media. He said that he has already begun treatment as part of a plan that his doctors feel has a "90% success rate."

Although most of MEGADETH's previously announced 2019 tour dates have been canceled as Mustaine undergoes treatment, the MEGADETH-curated Megacruise — which is set to take place October 13-19 — will still depart as scheduled, and the band will be a part of it in some form.

Asked if he was aware that Mustaine was having health issues just before his diagnosis, Ellefson told SiriusXM's "Trunk Nation" (hear audio below): "No. Not at all. In fact, that's why it was very surprising, even to him. One thing he said [is], 'Hey, you should go checked too.' [But] I don't know how they discovered it… I was in Europe doing stuff [earlier in the year when Mustaine was taking part in the 'Experience Hendrix' tour]. But, obviously, we were communicating; we were working on writing a [new MEGADETH] record; we were sending files back and forth and stuff. For me, and for the [rest of] the band, it really came to light while we were down [in Nashville] working on the record. And once [Mustaine] got a treatment plan in place, that's when the decision was made, 'Clear the decks.' And, really, that wiped the entire fall off the entire calendar — except the [Megacruise]."

Ellefson also talked about the status of the songwriting and recording sessions for MEGADETH's follow-up to 2016's "Dystopia" album. He said: "Right now, we have not actually officially tracked anything. I'd say we're probably two-thirds to three-quarters of the way there with the music. A bunch of lyrics have been written. Obviously, he can't go in and sing anything and start demoing anything now at this point, at least vocally.

"Dave made a comment to me before we all went down [to the studio]. He said, he goes, 'You know, these last bunch of records we've done, we get together, we plug in, we play, then we mix it and then we just go right on tour. We never really get a chance to sit with the songs and really just absorb 'em and take 'em in.' And he said, he goes, 'I really wanna take the time.'

"Dave is not in a hurry with this album, and really hasn't been," Ellefson continued. "And even last year, Kiko [Loureiro, guitar] and Dirk [Verbeuren, drums] had gone down to Nashville to work with Dave a little bit. We were busy doing 'Basstory' [Ellefson solo] stuff, so we were trying to work around the schedule. Then the holiday came up at the end of the year. But it's nice to be together working on it together as a team.

"A couple of the big records, of course, that the fans love — 'Peace Sells', 'Rust In Peace', 'Countdown To Extinction', these things — we were very much a band. We were in a room. We didn't have the luxury of Pro Tools recording, where things could be done very instantaneously. We would sit here [and go], 'Do this part here. Move it up one fret' — very old-school way of doing it. But you really absorb the songs, you live with them, you learn your parts, and you get together day after day, week after week, and the songs start to take shape as a band. And I think you really miss that process. Even with MEGADETH records, where you just come in and you plug and play, you record. 'Okay, the bass is done. Next. Okay, now the guitars are done. Next. Let's sing.' So I like this process that we're doing. Even to this day, I see little things moving around in our Dropbox folder that the guys are working on right now down in Nashville. And it's great, because we can all check back in on it, we can sort of report back in and listen to things. So, as much as we're sidelined from actively out performing, we're very engaged in the actual working on the record right now, as we speak."

Mustaine and Ellefson are the two remaining original members of MEGADETH, which was formed in 1983, following Mustaine's dismissal from METALLICA.