Deciding when to get routine mammograms is confusing. Some health groups recommend women begin at age 40 or 45, while another recently opted for age 50. They also differ on whether yearly or every other year is best.

The conflict is partly because guidelines for breast cancer screening are designed for women at average risk and with no possible cancer symptoms. But breast cancer is so common that it is hard to know who is really “average”, or how to balance the pros and cons of screening.

“Breast cancer is not one disease,” says Dr Laura Esserman of the University of California, San Francisco, in the US. “So how in the world does it make sense to screen everybody the same when everyone doesn’t have the same risk?”

Esserman is leading research to better understand the nuances of who is low or high risk or in between.

More than 320,000 women in the US are expected to be diagnosed with breast cancer in 2026, according to the American Cancer Society. Death rates have been dropping for decades, thanks to better treatments, but it is the second-most common cause of cancer death in US women – and diagnoses are inching up.

In Hong Kong, more than 5,500 new cases of female breast cancer are diagnosed annually, rising at an average of about 2.5 per cent per year over the past decade. It is the most common cancer among women in Hong Kong, accounting for 28.9 per cent of all new female cancer diagnoses, with one in 13 women at risk of developing breast cancer before age 75. It is the third-leading cause of cancer death in Hong Kong women.