Seventy nine women have been offered breast clinic appointments at Sherwood Forest Hospitals NHS Foundation Trust to receive an apology and a review of their treatment after failings in their breast cancer tests.
Between 2004 and 2010 1,400 women were diagnosed at the Trust and are unaffected.
But for patients already diagnosed with breast cancer, the "oestrogen receptor" test is shown to have been faulty. The result helps to predict how likely it is that hormone therapy will be an effective treatment.
During this time the machine used showed more patients to be "negative" than "positive". Had they been shown as positive it is more likely hormone therapy would have assumed to be helpful.
The trust is offering a hotline for any concerned patients - 01623 672371.