He resigned from the company in 1983 after being diagnosed with Hodgkin's disease, a similar form of cancer to the kind he is now battling for the second time. Tumors develop from lymphocytes, which are a type of white blood cell.
There are several options for treatment, including chemotherapy, radiation therapy and drug treatments.
Allen was treated for the disease in 2009, and had been in remission.
He says a lot has happened in medicine since he overcame the disease almost a decade ago and that he and his doctors are optimistic that he will see good results from the latest therapies.
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'I will continue to stay involved with Vulcan, the Allen Institutes, the Seahawks and Trail Blazers, as I have in the past.
"A lot has happened in medicine since I overcame this disease in 2009", Allen said in a statement.
"I have confidence in the leadership teams to manage their ongoing operations during my treatment", he said.
"I am very grateful for the support I've received from my family and friends", Allen wrote on his website.
Vulcan and Seattle's Cinerama Theater were among the many Allen-backed ventures returning their leader's cheer. "The 12s are behind you and we believe there is no better fighter for this challenge".