The 60-year-old, who rose to fame alongside magician husband Paul Daniels, had two tumours removed last week after cancerous tissue was found in her left breast.
The TV and radio star found fame as Daniels's assistant before they married in April 1988.
The couple were married for 28 years before Daniels died of a brain tumour at the age of 77 in 2016.
McGee says the stress of losing her husband may have brought on the disease, which was diagnosed at an early stage in October.
"Stress, I think caused mine. I've never been through the stress I've been through since I lost Paul," she told The Sun newspaper.
"Grief hits you in so many ways you're not expecting."
McGee said her appearance on Strictly Come Dancing in 2017 made her "genuinely happy" for the first time since her husband's death.
Speaking at the time, she said: "The last time I danced was over 35 years ago.
"I've never had any experience in this type of dancing, but I learnt ballet and modern tap when I was a child and I did dance for a few years - but that was 35 years ago, so now it's like starting from scratch."
Her foray into the show followed her husband's in 2010, and she reached the final with professional partner Giovanni Pernice after impressing the public.
At the age of 16 McGee won a place at the Royal Ballet School before joining the Iranian National Ballet Company in Tehran.
She became part of the Corps de Ballet and later a soloist, but her ballet career was brought to a halt by the Iranian Revolution.
McGee first met Daniels after being chosen to perform in his 1979 summer show in Great Yarmouth, Norfolk, and returned for a second summer show in Bournemouth.
The West End followed and the couple performed together in It's Magic which ran for more than a year in the early 1980s.
It was then that the producer of The Paul Daniels Magic Show asked her to consider becoming his full-time assistant.