Mac Sinise -- son of film star, veterans' advocate Gary Sinise -- dies of cancer at 33

Feb. 27 (UPI) -- Actor Gary Sinise -- a tireless advocate for U.S. veterans -- announced Tuesday his son McCanna "Mac" Sinise has died of cancer at the age of 33.

Starting in 2017, Mac served as assistant manager of education and outreach for the Gary Sinise Foundation, hosting events, leading tours, working with donors and reaching out to military service members, veterans, first responders and families who were recipients of the organization's support, according to an obituary posted on Gary's website.

Mac also filled in for drummer Danny Gottlieb when Gotlieb was unavailable to play Gary's Lt. Dan Band music shows.

"Those were some great times, father and son rockin' out together for the troops," Gary said.

Mac was diagnosed with a rare cancer called Chordoma at the same time Mac's mother Moira was battling breast cancer.

While Moira's treatment was successful and, after treatment, she is now cancer-free, but Mac succumbed to his disease on Jan. 5, despite years of surgery, treatment and rehab.

He was laid to rest on Jan. 23.

"Like any family experiencing such a loss, we are heartbroken and have been managing as best we can," Gary said.

"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. We've all experienced it in some way. Over the years I have met so many families of our fallen heroes," he added.

"It's heartbreaking, and it's just damn hard. Our family's cancer fight lasted for 5 1/2 years, and it became more and more challenging as time went on. While our hearts ache at missing him, we are comforted in knowing that Mac is no longer struggling, and inspired and moved by how he managed it. He fought an uphill battle against a cancer that has no cure, but he never quit trying."

Gary, 68, is known for his roles in Forrest Gump, Apollo 13, The Green Mile, The Stand and CSI: NY.