Sarah Michelle Gellar [1] has a lot of admiration for her longtime friend Shannen Doherty. On Wednesday, the Buffy the Vampire Slayer [2] actress celebrated Shannen's 46th birthday with a heartfelt post on Instagram [3]. "Where do I even start with this birthday girl?!?" she wrote alongside a throwback picture of them sharing a laugh. "She's everything. She's inspired me . . . well since . . . even before I called her a friend. But over the last year, I have watched her inspire and give hope to the world," she continued, referring to Shannen's decision to document her courageous battle with breast cancer [4] since revealing her diagnosis in August 2015.

"I thought I knew how tough she was, but I didn't even have a clue to her power," she added. "When I think twice about leaving the house without makeup, Shannen shaved her head on the public stage. While I fiercely guard my privacy, she opened her life, her struggles, her fears and her journey to help people she has never even met. She is a true friend to everyone. Shannon also has the greatest laugh in the world, which is why I chose this picture. To the fiercest, most loyal friend I know....let's party like it's your birthday!!! I love you."