It has been more than six months since a Michigan doctor was sentenced to 45 years and admitted to giving his patients “excessive or unnecessary” cancer treatments.

On Sunday evening’s Dateline, NBC’s Lester Holt will speak to 25 people who were given these treatments or lost a loved one who was treated by former oncologist Dr. Farid Fata.

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“When you sit in a room with 25 people talking about the loved ones they’ve lost, about debilitating conditions they have from chemotherapy they didn’t need or you talk to someone who had a death sentence with a false cancer diagnosis hanging over them,” Holt tells PEOPLE, “that’s when it came together for me about how egregious this was.”

More than a year ago, PEOPLE spoke with Patty Hester, a woman who Fata falsely diagnosed with a rare form of blood cancer.

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“It was all a lie,” she says about her false diagnosis. “It’s over three years of my life that I will never get back.”

Holt says it was important to hear from the people who were victims. “It’s a betrayal of trust,” he says.