Scans given to women who have a high risk of inheriting breast cancer falsely give a positive result for the disease in five out of six cases, new research shows.
However, ambiguous test results had only a limited effect on whether patients chose to have breasts surgically removed to prevent the disease, the study found.
Researchers found that a high number of women had made up their minds about having a prophylactic mastectomy operation before they had any diagnostic tests.
Women with BRCA1 or BRCA2 gene mutations have an 85% chance of developing breast cancer during their lifetime and therefore are offered intensive screening with both conventional mammography X-rays and magnetic resonance imaging (MRI) scans.
The new research by scientists in the Netherlands published in the cancer journal Annals of Oncology found that MRI scans were good at picking up genuine cancers but also produced a very high proportion of false positive results.
Incorrect or questionable indications of cancer are only revealed after surgically removed tissue samples are examined in a laboratory and show no signs of a tumour developing.