Gary Sinise's son McCanna dies at 33 after rare cancer battle

Actor Gary Sinise on Tuesday announced that his son, McCanna Anthony "Mac" Sinise, died last month at age 33 after battling a rare form of cancer.

Days after being taken to an emergency room on Dec. 30, McCanna "was smiling and filled with joy when he would hear (hospital staff) sharing how moved they were by the music" on his YouTube channel that he had shared with them, according to a heartfelt obituary authored by Gary Sinise and published on his foundation's website. "But the days got tougher, and on January 5th, with the family all around him, he let go. His battle with Chordoma was over and he was at peace."

The National Cancer Institute describes chordoma as a slow-growing cancer of tissue found inside the spine. One in 1 million people worldwide are diagnosed with the disease each year.

"Our story is not unique," Gary Sinise wrote. "No one escapes cancer. It affects us all, as every one of us in this life knows someone, personally or otherwise, who has faced this awful disease."

The obituary included photos of the father and son together on the road at the foundation's Soaring Valor events which honor WWII heroes, and the organization's Invincible Spirit Festivals, where Sinise's Lt. Dan Band -- named for his well-known character in the movie "Forrest Gump" -- uplifts wounded personnel at military hospitals across the U.S.

"Like any family experiencing such a loss, we are heartbroken and have been managing as best we can," the elder Sinise wrote in the tribute. "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. It's heartbreaking, and it's just damn hard."

He noted that his son's battle with cancer lasted five and a half years, adding that "it became more and more challenging as time went on."

The actor said his son was diagnosed in 2018, the same year his wife Moira Harris was diagnosed with stage 3 breast cancer, but she is now cancer-free after months of treatment.

"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 continued. "He fought an uphill battle against a cancer that has no cure, but he never quit trying."

"I am so blessed, fortunate, and proud to be his dad," he added.

Also known as notochordal sarcoma, chordoma is most often found near the tailbone (called a sacral tumor) or where the spine meets the skull (called a clival tumor). The average survival is around 10 years after diagnosis depending on where the tumor is and how much can be removed by surgery, according to the NCI.

In his tribute this week, Gary Sinise also spoke about Mac Sinise's zest for life, his contribution to his foundation -- which works to create and support programs that help "defenders, veterans, first responders, their families, and those in need," according to its website -- and his son's passion for music (he completed a full album titled "Mac Sinise: Resurrection and Revival," prior to his death).

Gary Sinise said he also hopes that by sharing his family's story, it will "shine a little bit of light on what has been a difficult time for us," adding that Mac Sinise "was truly a light for all of us."

"An incredible inspiration to those who knew and loved him, he faced his battle with grace, courage, and love," he continued. "Even with one setback after another, he never stopped living and learning, creating, and giving, and loving."

Gary Sinise concluded the message with a note to his son, writing, "We were blessed to have you in our lives as son, brother, and friend...and we will miss you and love you for eternity."

In addition to Mac, Gary Sinise is also a father to two other children, daughters Sophie and Ella.

ABC News contributed to this report.