Michael Landon, who starred in "Bonanza" and "Little House on the Prairie," died in 1991 at age 54. The cause of death was pancreatic cancer. He died three months after announcing his diagnosis.

Melissa Gilbert opens up to PEOPLE about witnessing how “brutal” Michael Landon’s and Patrick Swayze’s battles with pancreatic cancer were and how that inspired her new partnership with PanCAN, a charity for the disease.

While Michael Landon had nine children of his own, he was a father figure to many others, including his Little House on the Prairie costar Melissa Gilbert. So, it was just as hard for her when Landon was diagnosed with pancreatic cancer and died three months after announcing his diagnosis to the world.