How Whistle-Blower Helped Expose Michigan Cancer Doctor Who Mistreated Patients Dr. Farid Fata was sentenced to 45 years after misdiagnosing over 550 patients.

 -- An office manager for Dr. Farid Fata, the Michigan oncologist convicted of misdiagnosing hundreds of patients to defraud insurance companies, revealed details of how staffers helped expose the doctor’s crimes.

George Karadsheh, who worked for Dr. Farid Fata’s Crittenton Cancer Center in Rochester Hills, Michigan, said he started asking questions after staffers kept leaving the practice. When Karadsheh learned that a different oncologist in the practice was quitting, he said the doctor told him it was because Fata was administering chemotherapy to patients who didn’t need it.

“[This doctor] explained that Dr. Fata was actually administering chemotherapy to patients without need,” Karadsheh told ABC News "Nightline." “He was also explaining that patients who were on hospice were taken off hospice and put on chemotherapy, put back on chemotherapy. He also pointed out that patients who were receiving chemotherapy without disease were receiving it to the very last day of life. So at that point I discovered that there may be some issues there.”

Fata was a trusted oncologist in the community. He was trained at Memorial Sloan-Kettering in New York and founded Michigan Hematology and Oncology, Inc., the state's largest private cancer practice at the time of his arrest.

"It was so difficult to believe," Karadsheh said. "Here is a physician whose background is second to none in Michigan... I had never had a patient complain that there was a problem with a reaction to a drug or whether it was necessary."

But in 2013, after Karadsheh found out what was going on, he spoke to the U.S. Department of Justice. On Aug 6, 2013, FBI agents arrested Fata and stormed the Crittenton Cancer Center.

Fata, 50, was sentenced to 45 years in federal prison today after pleading guilty last year to fraud, money laundering and conspiracy charges.

“I’m relieved that Dr. Fata has been held accountable,” Karadsheh said. “But I’m also very sad because I feel the patients endured pain and suffering that’s going to last far longer than the sentence that was imposed upon Dr. Fada.”

At least 553 victims had been identified, with Fata often prescribing treatments for cancer they didn’t have.

“I have never seen a case anywhere close to this,” Barbara McQuade, the U.S. Attorney for the Eastern District of Michigan told ABC News. “We have a lot of cases involving Medicare fraud here where doctors submit fraudulent billings but those cases really are all about money. This case was unique in that it was really about patient harm and really harm that was unimaginable.”

In the Detroit federal courthouse Fata sobbed as he spoke prior to sentencing. Speaking to victims of his treatment and their families in attendance, Fata said he had violated the Hippocratic oath and caused "anguish, hardship and pain."

"I misused my talents, yes, and permitted this sin to enter me because of power and greed," Fata told the court.

Earlier this week numerous former patients spoke against the doctor. Some of the victims who spoke never had cancer, others were over-treated and some had treatment for different cancers than the ones for which they were diagnosed because it brought in more money.

One of those victims was 53-year-old Monica Flagg.

“I was feeling fine,” Flagg told "Nightline." “I continued to feel fine until I started some of my treatments with him.”

Flagg said Fata diagnosed her with myeloma, a cancer of the plasma cells in bone marrow, in July 2012. She said she endured multiple rounds of unnecessary testing, bone marrow biopsies, medication and one chemo treatment under his care.

“Other than the physical, or the emotional stress, I don’t know what side effects are anymore,” she said. “It’s very disturbing.”

When Fata gave her the diagnosis, Flagg said he reassured her that they had caught the cancer early but that she would need “a lifetime maintenance dose of chemotherapy.”

“Everybody we knew loved him and I just, from the day I met him, did not like him,” she said.

Flagg said she continued treatment with Fata over several months. On July 1, 2013, the same day Flagg said she had her first and only chemo treatment, she said she tripped over a suitcase on her bedroom floor and broke her leg, which landed her in the hospital. Another doctor in Fata’s practice happened to be doing rounds on Fata’s patients that same Fourth of July weekend and reviewed Flagg’s chart. That’s when he told Flagg she had never had cancer.

“He read my chart, looked at me very strangely, I’ll never forget the look that he had when he looked at me,” Flagg said. “He came back the following day and said to me, ‘I want you to know, I work for Dr. Fata. You’re 51, you’re extremely active, you’re a professional, you don’t have cancer and you need to not ever go back to Dr. Fata.”

After months of feeling like something was wrong, Flagg said she was “ecstatic” to hear that she was cancer-free, and then angry to learn that she had been healthy all along. There are many nights, she said, that she still gets upset thinking about her ordeal.

“It’s just heartbreaking,” she said. “I did one chemo treatment. The others did so many that they’re practically dead anyway.”

When asked if he feels like a hero, Karadsheh said the patients are the real heroes.

“They had to endure incredible, horrific treatments that left them incapable of having normal lives and some actually did not recover,” he said.

ABC News' Ron Claiborne, Brandon Baur, Catherine Cole and Gillian Mohney contributed to this report