Gary Sinise’s only son, McCanna “Mac” Sinise, died last month after a 5 1/2-year fight with a rare bone cancer, the veterans activist and “Forrest Gump” actor announced Tuesday on his foundation’s website.

“Like any family experiencing such a loss, we are heartbroken and have been managing as best we can,” the actor wrote. “As parents, it is so difficult losing a child. My heart goes out to all who have suffered a similar loss, and to anyone who has lost a loved one.”

Mac Sinise was 33 when the cancer chordoma took his life Jan. 5, his father wrote. After being diagnosed in August 2018 — the same year his mother, Moira, was diagnosed with and treated for breast cancer — Mac faced five spine surgeries in less than two years, Sinise said, in addition to radiation and chemotherapy treatments.

“Chordoma is a one in a million cancer,” Gary Sinise wrote Tuesday, sharing information about the disease. “Originating in the spine, Chordoma affects, on average, only 300 people in the U.S. per year. In 70% of the cases the initial tumor can be removed, and it is cured. But in 30% of the cases, perhaps about 90 people per year, the cancer returns.”

Mac — a drummer, pianist and University of Southern California Thornton School of Music graduate who sometimes filled in on drums in his dad’s Lt. Dan Band — had been working for the Gary Sinise Foundation since 2017. He resigned in 2020 to focus on recovery and rehab between his fourth and fifth surgeries, and in early 2023 he set to work on finishing a piece of music he had started while in college, his father said.

“The cancer had paralyzed him from the chest down, but he still had limited use of his right arm, and fingers on his left hand,” Sinise wrote. “Being right-handed, he would strap a stylus to his right hand, and he could punch letters and notes into his phone or iPad. He also had a small keyboard he laid on his hospital bed table that he used to work on his music.”

Mac worked with members of his father’s band as well as a friend from college, composer and arranger Oliver Schnee. Unable to play drums or piano anymore, Mac learned to play harmonica, Sinise said, and the project grew to include more songs and more collaborators. Recording sessions were held in July at Sunset Sound in Hollywood and in November at Blackbird Studio in Nashville.

But in December, Sinise wrote, “We had to take Mac to the emergency room for what would be his final trip to the hospital. He was having trouble getting his breath and after stabilizing him, he was admitted. I stayed with him as I had done many times before. During the first few days, I thought this would be another trip where we get things under control and head home. … But the days got tougher, and on January 5th, with the family all around him, he let go.”