A doctor at the National Cancer Center in Tokyo mistakenly drew twice as much bone marrow from a donor while preparing for a transplant earlier this month, hospital officials said.
The donor was discharged and has not experienced any complications, the hospital said.
The doctor drew 800 milliliters of bone marrow fluid from a woman in her 30s in mid-April. the usual amount taken from a healthy donor is supposed to be around 400 milliliters.
Hospital chief Ryosuke Tsuchiya said the physician's unfamiliarity with the filtering equipment, which is designed to remove bone fragments and other materials, might have caused the error. It was only the third time the hospital had used the equipment, which was approved for domestic use by the Health, Labor and Welfare Ministry in February.
"We are truly sorry for putting the extra burden on the donor. We'd like to thoroughly investigate the case and we will disclose the result of our investigation," said Tsuchiya, adding that the donor has accepted the hospital's explanation.