Patients who are found to have the earliest form of breast cancer have a higher risk of invasive breast cancer and dying from the disease, a new study suggests.

Ductal carcinoma in situ (DCIS) is non-invasive breast cancer, which means it has not spread beyond the milk duct.

Diagnoses have increased substantially and now account for around a fifth of new diagnoses of breast cancer detected through screening.

But the authors of a new study into the condition said its links to a subsequent diagnosis of invasive breast cancer or death due to breast cancer are not well understood.

Their study, published in The BMJ, set out to track women diagnosed with DCIS to see whether they were subsequently diagnosed with breast cancer, or died from breast cancer.

They analysed data on 35,000 women in England diagnosed as having DCIS by the NHS Breast Screening Programme from its start in 1988 until March 2014.

By December 2014, 13,606 women had been followed for up to five years, 10,998 for five to nine years, 6,861 for 10-14 years, 2,620 for 15-19 years, and 939 for at least 20 years.

Among these women, 2,076 developed invasive breast cancer – more than double that expected from national cancer incidence rates.

And 310 women died from breast cancer – 70% higher than that expected from national breast cancer mortality rates.

Increased risk lasts for at least 20 years after diagnosis, the authors suggested.

Women who had more intensive treatment for their DCIS had lower rates of invasive breast cancer – women who had breast conserving surgery had a lower rate of invasive breast cancer if they also had radiotherapy, and the lowest rate was in women who had mastectomy, the authors said.

The authors concluded: “To date, women with DCIS detected by screening have,on average, experienced higher long term risks of invasive breast cancer and death from breast cancer than women in the general population during a period of at least two decades after their diagnosis. More intensive treatment and larger final surgical margins were associated with lower risks of invasive breast cancer.”

Professor Sarah Darby, from the Nuffield Department of Population Health at the University of Oxford, who led the research, said: “This is the first time we have been able to show that women with DCIS have more than double the risk of developing invasive breast cancer and dying from the disease, even up to 20 years after being diagnosed.

“While this is concerning, understanding more about this risk puts us in a better position to make informed decisions about how to treat and monitor women with DCIS to give them the best possible care and save lives.”