Devon Still -- Crucial Day For Leah ... Starts New Treatment This A.M.

Big day for Leah Still -- because her father told us the 4-year-old starts a new round of chemotherapy this morning that will hopefully crush her disease so badly, she can finally begin stem cell therapy.

Cincinnati Bengals defensive lineman Devon Still is out in AZ -- and told us that while the cancer is still in Leah's body, she's fighting like hell to beat it.

"We're just hoping for the best and just trusting in the doctors to their best for her," Still told us.

Still also told us why he and Leah decided to write a children's book about Leah's battle with pediatric cancer -- and it's truly inspiring stuff.