Doctors have reported a "complete and durable regression" of an advanced breast cancer after treating a woman with pioneering therapy using her own immune system.
Judy Perkins is free of advanced drug-resistant breast cancer two-and-a-half years after cells were used to target her deadly tumours.
The 52-year-old structural engineer had a mastectomy after she was diagnosed in 2003, but a decade later, she found out the cancer had spread and was given a few years to live.
After enrolling for new trial in 2015, doctors in the US adopted an experimental approach combining two different forms of immunotherapy after conventional hormone treatments and chemotherapy failed.
The outcome led to "complete durable regression" of the rapidly spreading cancer, the medical team said.
The case was detailed in the journal Nature Medicine.
Judy Perkins told the BBC: "About a week after (the therapy) I started to feel something, I had a tumour in my chest that I could feel shrinking.
"It took another week or two for it to completely go away."
Experts have described the study as "fascinating and exciting" even though it reported on just one patient.
Peter Johnson, Professor of Medical Oncology at Southampton General Hospital, said: "This is another piece of evidence confirming that some cancers are recognisable by the body's immune system and that if this can be stimulated in the right way, even cancers that have spread to different parts of the body may be treatable."
The US team led by Dr Steven Rosenberg, from the National Cancer Institute in Bethesda, Maryland, identified immune system T-cells within the cancerous breast tissue that were able to recognise and target four mutant tumour proteins.
The immune cells, known as tumour-infiltrating lymphocytes (TILs), were removed from the patient, multiplied under laboratory conditions and injected back into the bloodstream in large numbers.
In addition the scientists deployed one of a range of new immunotherapy drugs called "checkpoint inhibitors", designed to overcome a cancer's ability to shield itself from the immune system.
Previously in clinical trials, checkpoint inhibitors have proved ineffective against most breast cancers.
Dr Simon Vincent, director of research at the charity Breast Cancer Now, warned more patients will have to be treated to assess how effective the therapy is.
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He said: "This is a remarkable and extremely promising result, but we need to see this effect repeated in other patients before giving hope of a new immunotherapy for incurable metastatic breast cancer."
This week, a trial showed that men given just months or weeks to live after being diagnosed with prostate cancer are surviving for more than a year thanks to a breakthrough in immunotherapy treatment.