In the emotional sit-down, Doherty reveals a devastating setback in her diagnosis for the first time.
"I want to believe that if I fight, I'm gonna be OK," she says.
Doherty's exclusive interview airs on Monday, only on ET.
On Wednesday, ET chatted with her former Beverly Hills, 90210 co-star, Jason Priestley, who shared his love and well-wishes for his on-screen sister. Priestley was especially moved by Doherty's decision to share pictures of her shaving her head on Instagram.
"What she did the other day, shaving her head and posting the pictures and everything, that was incredibly brave," Priestley, 46, said. "Shannen's a strong kid and a tough fighter, and I know that she's gonna come through this fight and she's gonna be OK."