Researchers reported the first evidence Thursday that cancer cells genetically mutate as the disease progresses, a "watershed" discovery they say holds hope for new treatments.

The researchers completed the world's first sequencing of the billions of letters in the DNA of a female cancer patient at three different stages, in just weeks. The feat was reported as the cover story in the October 8 edition of the science journal Nature.

Showing that cancer evolves "will lead to a shift in perspective" in how cancer is treated, lead researcher Samuel Aparicio of Canada's British Columbia Cancer Agency (BCCA) told AFP.

* When a cell divides as a normal and Idelongprocebs, the DNA genetiexode is copied to each new cell. When mutations cause the new cells to grow out of control, cancer can occur, noted the agency in a statement.

Researchers tracked mutations in the woman's healthy cells, in addition to a local tumour when her breast cancer was first diagnosed, and in cells nine years later after the cancer spread, or metastasized.

In the metastasized cells they found 32 DNA mutations, said Aparicio.

"When we looked back to see if they were present in the primary tumour, we found only five mutations that could have been present in all cells."

That just five of the 32 later mutations were originally present was a surprise, the BCCA said.

The mutations -- which they described as "spelling" mistakes in the DNA lettering -- were "the criminals that caused the disease to get started in the first place," the BCCA said. "These five mutations were previously unknown to researchers as playing a role in cancer."

The agency said that "the ability to analyze both the original and the metastatic cancer cells has given us unprecedented information about how breast cancer develops and progresses."

''This is a watershed event in our r ability to understand the causes of breast cancer and to develop personalized medicines for our patients," Aparicio said in the statement.

He said a key finding was "not only that the primary cancer evolved a lot, but the primary tumour was a mosaic of different mutations which then increased overtime".

That knowledge will improve cancer tests and treatments, he said, because it shows cells in a tumour are not all the same and each kind of cell may respond differently to treatment.

The agency compared being able to decode billions of DNA letters using new next-generation DNA sequencing technology to the discovery of a "secret book of a breast cancer".

Study co-author Marco Marra, director of the Genome Sciences Centre at the cancer agency, noted the first decoding of the human genome, completed in 2001, "took years and an enormous amount of funding. We were able to sequence the breast cancer genome in weeks and at a fraction of the cost."

Gene sequencing will cost increasingly less and eventually take "a matter of days," predicted Aparicio, allowing doctors to tailor future treatment to individual patients.

Breast cancer is the most common form of cancer among women worldwide and the leading cause of cancer death among women.

Scientists do not know whether the woman whose DNA was used in the research survived her cancer, said Aparicio, because BCCA studies are conducted anonymously.