TOKYO — The Chiba Cancer Center announced that a woman in her 30s had a breast removed unnecessarily after it mixed up test samples from two patients.

The woman had only early stage cancer, and did not require immediate surgery.

The two samples were collected at the hospital in Chuo Ward, Chiba, on the same day in mid-October, one from the woman in her 30s and the other from a woman in her 50s. Both from Chiba Prefecture underwent needle biopsies, in which cellular tissue is collected by piercing the breast with a needle.

The mix-up resulted in the younger woman being told she had advanced cancer. The patient and her family agreed to surgery, and in early December a total mastectomy of the right breast was performed.

However, when the removed portion was examined on Dec. 15, only early stage cancer was found.

On Dec. 17, the hospital performed genetic tests on the samples from the two women, which revealed the mix-up.

“The woman in her 30s did not need a total mastectomy at the time of the initial needle biopsy,” a spokesperson for the hospital said.

For the older woman, cancer cells from the needle biopsy in mid-October were not detected because of the sample mix-up.

However, her external appearance strongly indicated breast cancer, so later that month another needle biopsy was performed, which confirmed the presence of breast cancer. She then began treatment immediately.

According to the hospital, needle biopsy samples are first collected by a doctor, and a nurse labels the sample container with the patient’s name. The sample is then taken to the pathology department by a nursing assistant, where the cellular tissue is stained by a clinical technologist so it can be examined by a pathologist.

The hospital said the mix-up likely occurred somewhere in this process. An investigative commission that includes outside experts was established on Dec. 18 to discover what happened.

The hospital is also considering compensating the woman in her 30s.

Last year, it was revealed that 11 of the hospital’s patients had died from 2008 to 2014 after undergoing laparoscopic surgery. The hospital has been working on reforms to prevent a repeat of such incidents.

At a press conference Friday, hospital director Matsuo Nagata said: “We solemnly acknowledge that this accident occurred in the midst of our reforms. We want to devise countermeasures so it never happens again.”