Alex Trebek believes in the power of positive thinking — and he's vowing to beat the odds as he battles stage 4 pancreatic cancer.

In this week's People magazine cover story, on newsstands, Friday, the beloved “Jeopardy!” host, 78, revealed he's responding so well to chemotherapy treatments, some of his tumors have already shrunk by more than 50 percent. 

"The doctors said that they hadn't seen this kind of positive result in their memory," he shared. "It's kind of mind-boggling. I've already gone from where I was to this. The doctors are so excited, just beside themselves with joy."

"I got a little emotional," he added. "But these were tears of joy — not tears of great depression."

Trebek, who revealed his cancer diagnosis in a video message to fans on March 6, stayed on the job as he began chemotherapy, taping five episodes of "Jeopardy!" twice each week until the show went on summer hiatus in April.

During that time, he suffered through chemo's side effects, including hair loss and the loss of his sense of taste.

Through it all, he's stayed upbeat thanks in no small part to the steadfast support of his wife of 29 years, Jean Currivan, and their children, Matthew, 28, and Emily, 25.

"They're very positive and until I demonstrate otherwise, they're going to continue in that vein," he shared.