DETROIT (AP) _ Patients of a Detroit-area doctor received “stunning” doses of a powerful, expensive drug, exposing them to life-threatening infections, an expert testified Monday as a judge heard details about a cancer specialist who fleeced insurance companies and harmed hundreds of people.

Dr. Farid Fata is headed to prison for fraud and other crimes. But U.S. District Judge Paul Borman first is hearing from experts and former patients about the extent of his scheme to reap millions of dollars from Medicare and other health programs.

Nearly three dozen ex-patients and family members, many dressed in black, chartered a bus to attend the hearing, which could last days. Some will testify Tuesday.

“This is a small fraction of the people this guy has hurt,” said Terry Spurlock, 52, of Holly, who had three more years of treatments after a tumor on his neck disappeared. “He gave me so much treatment, it stopped my immune system.”

Fata, 50, pleaded guilty last fall to fraud, money laundering and conspiracy. The government is seeking a 175-year prison sentence, while the Oakland County man is asking for no more than 25 years.

The government said 553 people have been identified as victims, along with four insurance companies. There were more than 9,000 unnecessary infusions or injections.

“There is an aggressive approach to treating cancer. This was beyond. This was over the top,” said Dr. Dan Longo, a Harvard medical professor and deputy editor at the New England Journal of Medicine, who testified Monday as an expert for prosecutors.

Longo was asked about patients who were given a drug called Rituximab, which can weaken the immune system if overused. It’s typically given eight times for aggressive lymphoma but one patient got it 94 times. Another got it 76 times.

“It’s a stunning number of injections of that drug. … That creates a susceptibility to infectious agents of various sorts,” Longo said.