The stem cells were harvested from patients at Children's Hospital Los Angeles (CHLA) before they underwent chemotherapy and radiation treatments. They were put in long-term storage in case their cancer ever returned.
"One of the freezer's sensors failed and the notification process ... failed and so we lost those specimens," said Dr. James Stein, CHLA's Chief Medical Officer.
All but one patient had gone through initial therapy, the Los Angeles Times reported. The loss of stem cells hasn't jeopardized any of the children's health, the hospital said.
The hospital also bungled the notification of families. A letter breaking the news was accidentally addressed to the children and not their parents.
Sean Anderson Corona, 13, described the news as "painful" and broke into tears.
"I got almost to the very bottom [of the letter] and I just started crying," he told KABC-TV.
Corona underwent stem cell therapy after being diagnosed with Stage 4 Neuroblastoma. The grueling process required him to be hooked up to machines for hours. His blood was then reinfused back into his body once the stem cells were harvested.
“I would try to sleep and my body would just start shaking, and then I just started to freak out and I’d start crying,” said Corona, who will have been cancer-free for three years in November.
Stein said patients can always have the stem cells harvested again. In a statement, the hospital apologized for the mishap.
"We are very sorry that this loss occurred," it read. "We apologize for any distress or confusion that this has caused our patients and their families."
It said the freezer was replaced and the system that monitors the sensors has been upgraded.