Gellar posted a photo of herself and Doherty -- who recently shaved her head in anticipation of her fight against the aggressive ailment -- and wrote in the caption, "My #mondaymuse (and really every day #muse) is my brave friend @theshando."
"I've always said she is one of the most loyal friends I've ever known, and now while she is facing the toughest journey of her life, she is sharing it with friends she has never met in the hopes of helping other people who are going through the same struggle," Gellar continued. "That's a true friend #F**kCancer."
Doherty has been very open about her struggles with breast cancer since receiving the diagnosis in February 2015. Last month, she documented shaving her head on Instagram, which she called an "impossibly tough day," for the benefit of others going through the same difficult journey.
Recently, Doherty revealed to ET that the disease has spread to her lymph nodes. So far, she's undergone three out of eight rounds of chemotherapy and had a single mastectomy back in May.
The 45-year-old actress got candid with ET about the effect her cancer has had on her relationship with her husband, Kurt Iswarienko, sharing, "It's made my marriage a thousand times stronger." Check out the video below to hear more.