The test is being developed at Llanelli's Prince Phillip Hospital with the help of Singleton Oncology and the University of Swansea.
During recent trials, nearly half of patients who were initially recommended for chemotherapy and endocrine therapy, had their treatment changed to endocrine therapy alone.
This test will have a major impact on the lives of many women, allowing them to get on with their lives following breast cancer surgery rather than having to go through four months of chemotherapy."
The 'Oncotype DX' test, which is only for women with early breast cancer, will give clinicians a clearer idea of the risk and benefit of chemotherapy.
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Today will be dry for most of the time with bright spells. It will feel pleasant again, though it will feel less warm than recently.
Figures from the ONS show we are more likely to be a victim of fraud than any other crime.
Outbreaks of rain and drizzle will clear tonight with some late sunshine expected.