"It's extremely regrettable," Lars-Göran Holtby, chief doctor at the Gävle hospital, told the local Arbetarbladet newspaper.
The woman, who is in her sixties, had previously battled breast cancer and after complaining of back pains, doctors feared the cancer had spread, as x-rays indicated she had tumours in her liver and other vital organs.
A rigourous treatment programme was started, including chemotherapy, radiation, and cortisone.
The treatment, which continued for two years, caused a number of debilitating side effects, including brittle bones and compressed vertebrae.
Doctors also discovered the treatment had damaged parts of the woman's brain, reducing her dexterity and her ability to control her hands, the newspaper reported.
A subsequent examination revealed that doctors had been wrong in diagnosing the woman with cancer, realizing that what they had seen on the x-rays two years prior weren't actually tumours.
However, the unnecessary treatment has left the woman with permanent injuries, prompting the hospital to file a report with the National Board of Health and Welfare (Socialstyrelsen).
"We're very sorry for this and wish it could be undone," Holtby told Arbertarbladet.