The blunder, revealed by Le Parisien newspaper, dates back to November and occurred at a hospital in Creteil, a suburb to the south east of Paris.
According to the report the woman underwent 22 sessions of radiotherapy to treat her breast cancer.
The only problem was the treatment was directed at the wrong breast. The error was only picked up when the husband became worried about reddening skin on the healthy breast and pointed it out to doctors.
The hospital said that a “combination of errors led to the mistake”.
“The team was deeply upset by what happened,” said a hospital manager in charge of cancer treatment. “We talked a lot to understand what happened."
Thankfully the patient finished her course of treatment in the correct breast in the same hospital, without apparently having suffered any adverse consequences apart from the skin lesions.
France's Nuclear Safety Authority, which regulates radiation protection, is to carry out an inspection of the hospital in the coming weeks.
They said the incident was level two on a scale of eight in terms of gravity, which means “significant” but without serious consequences.
The hospital stressed that it carries out 20,000 radiation treatments annually.