Gary Sinise's son, Mac, died at 33 from a '1 in a million' disease. What to know about chordoma.

On Tuesday, Forrest Gump actor Gary Sinise announced the death of his 33-year-old son, McCanna "Mac" Sinise, who died last month after being diagnosed more than a five years ago with a rare form of cancer called chordoma. In a post on the Gary Sinise Foundation’s website, the CSI: NY star wrote that his son, a musician who sometimes played with his father’s Lt. Dan Band, endured multiple spinal surgeries as well as chemotherapy and radiation to treat the rare cancer.

What is chordoma, and how common is it? Here’s what you need to know.

What is chordoma?

“Chordoma is a rare type of bone cancer,” Dr. Hari Deshpande, a Yale Cancer Center oncologist and researcher, tells Yahoo Life. “They are usually slow growing, but they can cause damage in the area where they grow, which is known as locally aggressive. They are most common in the bones at the top of the spine and adjacent skull bone as well as the bones in the sacrum but can occur in other bones in the spine.”

Chordoma is extremely rare — in the United States, the incidence is about 1 in a million. It does not appear that there are any risk factors for the disease. “It happens often in older patients, usually in their 50s, but it can happen any age — even in children,” Dr. Juan Carlos Fernandez-Miranda, professor of neurosurgery at Stanford Medicine and surgical director of the Stanford Brain Tumor, Skull Base and Pituitary Centers, tells Yahoo Life. “I’ve operated on children as young as 4 years old. However, at 33, it is very, very uncommon.”

Because of the placement of the tumors, Deshpande says that symptoms are typically related to pressure of the tumor on nerves or other structures close to where it is growing. This can cause pain such as a headache if it is near the skull, or back pain if it is in the lower spine. The tumors can also damage nerves, causing loss of sensation, function or movement in that area. That may include a hoarse voice, trouble swallowing or weakness in the legs or bladder problems.

What is the treatment for chordoma? Can it be cured?

In some cases, “chordomas can be cured," says Fernandez-Miranda. “Many patients can live for a decade or more. However, some chordomas are very aggressive, and someone can die in just a few years.”

Fernandez-Miranda says that “it’s extremely important that patients find the right team” when someone learns they have chordoma.“The first treatment for chordoma is surgery, which can be curative in some cases,” he says. “Finding the right surgical team to treat you is very important, as it’s rare to have expertise in this unusual tumor.”

Managing chordomas can be challenging because of their proximity to the spinal cord, brain, nerves or arteries, and the sooner a patient is able to receive treatment, typically, the better the prognosis. A common treatment for chordoma, beyond surgery, includes radiation therapy, which is sometimes used before surgery to shrink the tumor so it's easier to remove.