It can be hard to spot breast cancer in mammography scans for certain types of women, an international study shows.
Some scans were harder to read
Women on hormones or those who are thin or have had previous breast surgery are less likely to have their cancer picked up by these scans, say experts.
The research, published in the British Medical Journal, looked at more than 120,000 women aged between 50-64.
Experts stressed mammography was still the best way to screen for this cancer and urged women to attend for scans.
Among the 122,355 women followed up for a year, 726 developed breast cancer.
Of these, only 629 had any sign of their cancer picked up by routine mammography.
In the remaining 97 the screen had been negative.
Also, 3,885 of the other women with a positive scan result, which would suggest they might have cancer, did not develop any tumour.
When the researchers looked at the pattern of these 'false' results, they found they were more likely to occur among certain groups of women.
Lead researcher Dr Emily Banks, from the Australian National University, said: "Our results suggest that mammography may be less efficient in users of hormone replacement therapy, in women with previous breast surgery, and in thin women, compared with other women."
Other researchers have similarly found it is harder for doctors to interpret breast scans of women taking HRT or those with breast implants.
In these instances the breast tissue is denser, which makes it difficult to read using mammograms, scientists believe.
But it is not clear whether such problems lead to cancer reaching a more advanced stage before it is picked up, and hence more deaths.
Be 'breast aware'
Samia al Qadhi, from Breast Cancer Care, said: "The results of this study highlight that some women are diagnosed with breast cancer between their screening appointments which means that being breast aware throughout your life is vital.
"Breast awareness is knowing how your breasts look and feel normally so that you notice any change that might be unusual for you and reporting any changes to your doctor immediately."
Dr Michelle Barclay of Breakthrough Breast Cancer said the results were interesting but longer follow-up of these women was needed.
She urged women to continue attending mammography scans.
"It is the most effective method of detecting breast lumps in women over 50 and women should continue to attend their screening appointments when invited - early breast cancer detection is vital in maximising the chances of beating this disease.
"Any woman concerned about the factors in this research should raise them at their screening appointment," she said.
In the UK, women aged between 50 and 64 are routinely invited for breast screening with mammography every three years.