The actress was diagnosed in March 2015 and has been open with her illness, even posting photos of herself shaving her own head on Instagram.
Shannen played Brenda Walsh on the hit ‘90s TV show, with her onscreen twin brother Jason Priestley recently speaking about how brave the 45-year-old has been throughout.
Now Ian, who played Steven Sanders on the show, has also praised his former co-star.
"I gave her a call last week and just let her know that I was thinking about her," he told ET. "She's one tough cookie. If she could handle the press outlets that were roasting her (during her time on Beverly Hills, 90210), then I'm sure (she can beat) cancer.
"She can stand up to that, if anybody can."
Shannen took to her social media accounts last week (end24Jul16) to document her hair transformation in a series of black and white photos. She also thanked her family and friends for their continued support.
"Thank you to these three who helped me thru (sic) an impossibly tough day and continue to be there every minute supporting and loving me. (Husband Kurt) @kurtiswarienko was in Mexico working but knew he left me in good hands.@annemkortright @chriscortazzo#mamarosa I love you. #mycrew#thisisasquad," she posted.