Michael Strahan came on his own show to share that his daughter was recently diagnosed with a rare brain cancer ... and it explains why he'd taken off so much time from his TV jobs back in the fall.
The 'GMA' host sat for a segment on the ABC program Thursday opposite his coworker Robin Roberts -- where he and his 19-year-old, Isabella, revealed she's been receiving treatment lately for a malignant tumor doctors found in her brain in October.
The cancer is known as medulloblastoma, and in the package they aired ... Roberts goes into detail about the size of the tumor that was discovered by doctors -- noting it was about as big as a golf ball and lodged toward the base of her skull.
Once they discovered the tumor, Isabella underwent emergency surgery at Cedars-Sinai in L.A. ... which is where Michael's daughter was attending college at USC.
Once her father caught wind of everything, he says he booked it to the West Coast -- Mike's normally working in NYC -- and he was by Isabella's side for a good long while ... which now explains his absence from 'GMA' and Fox NFL Sunday around that time.
The procedure to remove the mass appears to have been a success -- with the operation happening right around her 19th birthday -- and her recovery was long and arduous.
Now, though ... Isabella says she's doing much better as she continues to receive treatment -- including radiation, which has caused her to lose her hair. In February, Michael says she'll begin chemo as well. Of course, the whole journey has been stressful on the family.
Strahan says the health scare has given him a new POV on what's important in life -- and Isabella is looking forward to getting back to normal once her treatments are wrapped.