Breast cancer screening for women under 50 should stop in light of growing questions around how beneficial the practice is, health researchers say.

Dr Caroline Shaw and Associate Professor Diana Sarfati from University of Otago Wellington have written blogs that say the benefits of breast cancer screening is much less favourable than previously thought due to over-detection of cancers and other issues.

The screening could detect cancers that would never cause a problem in a person's lifetime, the blog said.

"This is completely counter to how most people think about cancer, but this phenomenon is seen in all cancers (and many other medical conditions) we screen for.


"The problem is that when we identify a breast cancer through screening we can't always tell if it is a cancer that is going to cause a problem or not. So we have to treat them all."

Treatments, such as radiotherapy, could be "mutilating, painful and involve long term side effects", the blog said.

"Thus, any potential benefits of screening have to be weighed up against the harms of unnecessary mastectomies, side effects from the surgery, chemotherapy and radiotherapy. Not to mention stress and anxiety."

Advances in breast cancer treatment since randomised control trials (RCTs) of screening in the 1970s and early 80s had been substantial, and consequently survival and mortality had improved considerably, the researchers said.

The extent of over diagnosis - the key harm of breast cancer screening - was hotly contested, Dr Shaw and Professor Sarfati said.

"It is not disputed that there is over diagnosis and over treatment, but the extent of and best way to measure over diagnosis are not clear."

Screening in women under 50 was not recommended by any of the independent bodies who had reviewed the evidence and no screening programmes in the United Kingdom, Canada or Australia started under 50.

"The balance of benefits and harms in this age group is not favourable. In New Zealand we need to stop screening women under 50 and start being more honest about the lack of clarity about the evidence for screening in older women," Dr Shaw and Professor Sarfati said.

Breast Cancer Foundation chief executive Evangelia Henderson strongly disagreed with the research and said breast cancer detections in women under 50 had saved hundreds of lives.

"In 2012 we had 423 women aged between 45 and 49 diagnosed in New Zealand with breast cancer, and that's invasive breast cancer we're talking about."

The foundation strongly supported screening and promoted the procedure for women aged 40, she said.

"Why not have screening to save a life?"

The five-year survival of a woman whose cancer was diagnosed through a mammogram 95 per cent and the five-year survival rate of a woman who was diagnosed through feeling a lump, was 73 per cent, Ms Henderson said.

"So you can already see what early detection does - it improves your survival."

She acknowledged that the screening was not advanced enough to detect which cancers were going to become potentially deadly and which were not.But the choice was to have screening and treat any cancers detected or take a chance that the cancer was not serious.

"It's all very well being a researcher and being driven by numbers, but actually, people's lives are more important."