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Cancerous breast lumps are less likely to be detected in overweight or obese women before the tumour becomes large, a Swedish study has found.

These women may need more frequent mammograms to help spot early tumours, say researchers, but experts say more evidence is needed.

In the UK, women aged 50-70 are invited for screening every three years.

Some women judged to be at higher risk of breast cancer are already offered more frequent screening.

This might be a woman with a strong family history of breast cancer, for example.

Being overweight also increases a woman's risk of developing breast cancer, but it is not currently considered for setting breast screening intervals.

Obesity risk

The Karolinksa Institute study involved 2,012 women who developed breast cancer between 2001 and 2008.

The women had been receiving mammograms every 18 months to two years, as standard in Sweden.

The researchers looked at how large the tumours were at diagnosis, as well as the women's body mass index (BMI), a measure of obesity.

The team found women who were overweight were more likely to have a larger tumour when detected either at their mammogram or between screenings.

This might be because their breasts were larger and therefore the tumour was harder to find, or because their tumours grew at a faster rate, the study's lead author Fredrik Strand told the BBC.

Larger tumours tend to carry a worse prognosis.

More frequent screenings

Dr Strand said: "Our study suggests that when a clinician presents the pros and cons of breast cancer screening to the patient, having high BMI should be an important 'pro' argument.

"In addition, our findings suggest that women with high BMI should consider shorter time intervals between screenings."

But Sophia Lowes, from Cancer Research UK, said the study, which is being presented at the annual meeting of the Radiological Society of North America, did not provide enough evidence to support changing how often women are screened.

"Breast screening has harms as well as benefits.

"It saves lives by helping detect breast cancer at an early stage, but harms include some women being diagnosed with a cancer that would never have caused them problems in their lifetime.

"The time between screening is designed to help the benefits outweigh the harms overall."