Screening for a form of breast cancer has been found to pick up early ones that could become invasive, a new study shows.

There is a controversy over the best way to treat ductal carcinoma in situ (DCIS), which is described as a very early form of breast cancer in which cancer cells are present in milk ducts, but have not yet invaded the surrounding breast tissue.

Some contend that because DCIS rarely turns into invasive breast cancer, mammography screening can lead to overtesting and treatment, resulting in more harm than benefit.

Researchers analyzed data on more than 5.2 million women, aged 50-64, who were screened over a four year period across 84 screening units as part of the in the UK's National Health Service Breast Screening Program.

They found that increased prior screen detection of DCIS was associated with a reduction in invasive cancers occurring in the subsequent three-year interval.

In particularly, they found that for every three screen-detected cases of DCIS, there was one fewer case of invasive cancer in the following three years.

The researchers said the finding suggests that, overall, detection and treatment of DCIS is worthwhile in the prevention of subsequent invasive disease.

They noted, however, that because the data was based on screening unit, and not individual cases, it could not be known for certain what the outcome would have been for any individual woman if the DCIS if it had not been treated.

The study, published in The Lancet Oncology, is the first to explicitly investigate the association between screen-detection of DCIS and subsequent incidence of invasive breast cancer within UK’s national breast screening program.