Ms Thompson and her siblings carry a mutation of the BRCA1 gene, which is known to cause breast and ovarian cancer, and she fears her three daughters will also carry the defective gene.

The siblings were tested after Ms Thompson’s sister developed breast cancer aged 37 – a diagnosis that came after a long family history of cancer including the death of their mother with breast and ovarian cancer. 

Just two weeks after learning she carried the BRCA mutation Ms Thompson, then 33, was diagnosed with breast cancer. She had an immediate mastectomy, and after the birth of her third daughter she had a hysterectomy to reduce her chances of developing ovarian cancer.

Ms Thompson’s eldest daughter is now 18 and adamant she wants to know whether she also carries the faulty gene. “My biggest fear now is for my daughters. I’m not a person who lives negatively, but as a parent I will feel guilty if they carry the gene.”

But she believes ultimately it’s best to know.

Results of a 20-year study of almost 10,000 women with BRCA1 or BRCA2 gene mutations were published yesterday, showing women with BRCA1 mutations have a 72 per cent risk of developing breast cancer by 80 years of age. For BRCA2 mutation carriers the risk is 69 per cent.

Researchers from Peter Mac, University of Melbourne and Cancer Council Victoria also found the lifetime risk of ovarian cancer was 44 per cent for BRCA1 and 17 per cent for BRCA2, according to the study published in The Journal of the American Medical Association.

They found breast cancer risk increased rapidly at a young age and remained high for the rest of a women’s life, peaking in the 30s for BRCA1 carriers and 40s for BRCA2 carriers. Carriers with one or more relatives with breast cancer are at greater risk than carriers with no family history.

The findings could change the way prevention strategies are offered to women with the gene mutations.

“This study provides us with much more precise information about cancer risks for carriers, and will help personalise decision-making about the use and timing of prevention strategies such as medication, surgery and changing lifestyle factors,” said Peter Mac researcher Prof Kelly-Anne Phillips.