Nearly 11,000 people in the UK are thought to be unaware they have breast cancer after the pandemic disrupted diagnostic screenings.
Routine mammograms have been paused at times amid the ongoing coronavirus outbreak. Now running, safety measures mean appointments are still being scheduled at a reduced rate.
Mammograms help spot tumours that are too small to see or feel, enabling an earlier diagnosis and greater hopes of treatment success.
Nearly 1.2 million fewer women had the screening between March and December 2020 compared to the same nine months in 2019, according to Breast Cancer Now.
As a result, the charity estimates 10,700 fewer people across the UK have been diagnosed with the disease.
"The tragic cost of almost 11,000 missing breast cancer diagnoses is in the worst cases, women could die from the disease," said Baroness Delyth Morgan, chief executive of Breast Cancer Now.
"Looking ahead, while we cannot know the full impacts of the pandemic, what we do know now is over the coming years the number of women coming forward could overwhelm our already over-stretched workforce."
NHS breast cancer screening services are running, however, the health service is prioritising certain individuals, like those considered at very high risk of the disease or who needed additional tests after their last mammogram.
The risk of developing breast cancer increases with age. In the UK, women who are registered with a GP are automatically invited for a screening every three years from age 50 to their 71st birthday.
One in seven women in the UK will statistically develop breast cancer at some point in her life.
Mammograms, an X-ray, are said to prevent around 1,300 women from dying with the disease every year in the UK.
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Breast Cancer Now analysed the number of people who started their first breast cancer treatment within the 31-day wait target between March and December 2020, compared to data from the same nine months in 2019.
It also looked at urgent referral information, the average number of women screened for breast cancer each month, the length of time services were paused and the fact mammograms have been operating at around 60% capacity since restarting.
Broken down across the UK, England is expected to have had 8,900 fewer breast cancer diagnoses between March and December 2020, while Scotland missed 890 cases, Wales 687 and Northern Ireland 248.
In England alone, an estimated 90,000 fewer referrals where made to a specialist for suspected cancer patients between March and December 2020.
Women have also told Breast Cancer Now they were reluctant to attend medical appointments for fear of catching the coronavirus or further straining the NHS.
"We hear first-hand from women who are worried about the risk of COVID-19 so are reluctant to attend medical appointments and who don't want to bother their GP during the pandemic," said Jane Murphy, clinical nurse specialist at the charity.
"The pandemic has thrown us all into unprecedented times and it's natural people will have concerns.
"But the sooner breast cancer is diagnosed, the better the chances of treatment being successful, which makes it vital women continue to check their breasts regularly and get any new or unusual changes checked with the GP, and continue to attend breast screening appointments when invited.
"The NHS wants people to attend their appointments and report symptoms to their GP. They have measures in place to keep staff and patients safe."
Scientists from Cardiff University recently reported more than two in five people with suspected cancer symptoms during the UK's first lockdown did not seek medical help.
"It is vital those patients who missed screening last year, or who did not see their GP if they had possible symptoms, come forward," said Dr Jeanette Dickson, president of The Royal College of Radiologists.
"The NHS is open for business and the sooner we can diagnose cancer, the sooner we can treat it."
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Breast Cancer Now has warned the imaging and diagnostic workforce that is now under unprecedented pressure was already chronically under-resourced pre-pandemic.
"Screening teams are getting back up to speed, but breast imaging services were in a precarious position going into the coronavirus pandemic and those resourcing challenges are still there," said Dr Dickson.
"Many breast units have vacancies and there is a looming shortage of breast radiologists due to retirements.
"The backlog of cases waiting will put even more pressure on stretched diagnostic teams and the cancer teams then responsible for tailoring and delivering treatment.
"The diagnostic and treatment workforce caring for breast cancer patients desperately needs more investment to ensure our future patients get the speedy care they deserve."
The charity is calling on the government to invest in long-term breast cancer solutions and take a strategic approach to addressing the problems facing the disease's workforce.
"Only then will we be giving women the best chance of an early breast cancer diagnosis which we know is critical to their chances of survival," said Baroness Morgan.
Breast cancer symptoms
Common signs of the disease include:
Lumps or swelling in the breast, upper chest or armpit
Changes to the skin, like puckering or dimpling
Changes to the colour of the breast, which may become red
Changes to the nipple, like being inverted
Rashes or crusting around the nipple
Unusual discharge from the nipple
Changes to the size or shape of the breast
Pain all or most of the time. On its own, discomfort in the breasts is not usually a sign of cancer