The 77-year-old TV star - who made her name writing The Sun's agony aunt column Dear Deidre since the 1980s - has been diagnosed with aggressive ductal carcinoma and is due to undergo surgery to remove the malignant cells on Saturday 1 October.
Sanders explained: "I happen to be among a cohort of several thousand women who, by error, did not get invited for a mammogram when they turned 70.
"We were offered it a couple of years later, but by then I reckoned I must be too old to need it any more as the NHS stops inviting you for a screening after that age."
But after suffering back pain the agony aunt visited her GP and was quickly referred for a mammogram and ultrasound scan through the NHS within two weeks.
She said: "I tend to have tense shoulders but I realised the ache seemed to be spreading into my right breast. I must have let at least a month pass while I wondered, 'Is it anything to worry about? Is it just muscular? I am 77, after all, is it just the aches and pains of getting older?'
"I finally got round to calling our GP practice a fortnight before I was due to go on holiday. She saw me in person that afternoon, thought she could feel a difference in the right breast and put me on the NHS two-week cancer pathway."
Sanders is now urging other women to make sure they attend their routine mammograms.
The TV star said: "I think people panic at the word cancer often and they can be scared of even going to a doctor with their initial worry because they're terrified of getting a diagnosis even though getting the diagnosis is what leads to treatment.
"We can be so scared about that word so I am really happy to bring it out into the open."
Sanders retired from writing the Dear Deidre column in 2020 and was replaced by Sally Land.
She still appears regularly on This Morning answering calls from viewers about their problems.
Watch: Alison Hammond celebrates 20 years on This Morning