The 68-year-old, who taught in Shrewsbury and later became chief education advisor with the former Shropshire County Council, is confined to a wheelchair and has been receiving chemotherapy.
Terminally-ill with motor neurone disease and now unable to feed, wash or dress himself, he revealed his latest ordeal in an interview, saying: "I couldn't really visualise the true horror of what was to come and I can't now, with this new diagnosis.
"But then who can really imagine their own dying and death? And, more positively, what is the point of dwelling on what you can't know and can't influence in any way?"
The former Ofsted chief, who regularly hit the headlines with his high-profile and occasionally controversial remarks while in the post, said last week that he was waiting for advice from his consultant and was "travelling, as always, hopefully".
He is being cared for by his wife, Christine, at their home in the Welsh borders.
Sir Chris was diagnosed with motor neurone disease in 2006 and about a year ago found out he had colon cancer, which has now spread to his liver.
"You get used to it," he said. "My reaction when I was told about the MND was fatalistic. This is the pack of cards I've been dealt. I've got to play them as best I can."
Sir Chris, who is campaigning for a change in the law on assisted suicide, said he might travel to the Dignitas clinic in Switzerland to end his life, having previously said he would rather be pushed off a cliff in his wheelchair than "go to Dignitas and sing Beatles songs with bearded social workers".
His change of heart was prompted by a television documentary by Sir Terry Pratchett, the author who has Alzheimer's, in which a motor neurone disease sufferer was filmed ending his life.
A bill to decriminalise helping a terminally ill person to die is going through parliament but Sir Chris fears politicians will not have the courage to change the law.