BOSTON - Scientists are reporting what could be very bad news for efforts to customize cancer treatment based on each person's genes.

They have discovered big differences from place to place in the same tumour as to which genes are active or mutated. They also found differences in the genetics of the main tumour and places where the cancer has spread.

This means that the single biopsies that doctors rely on to choose drugs are probably not giving a true view of the cancer's biology. It also means that treating cancer won't be as simple as many had hoped.

By analyzing tumours in unprecedented detail, "we're finding that the deeper you go, the more you find," said one study leader, Dr. Charles Swanton of the Cancer Research UK London Research Institute in England. "It's like going from a black-and-white television with four pixels to a colour television with thousands of pixels."

Yet the result is a fuzzier picture of how to treat the disease.

The study is reported in Thursday's New England Journal of Medicine.

It is a reality check for "overoptimism" in the field devoted to conquering cancer with new gene-targeting drugs, Dr. Dan Longo, a deputy editor at the journal, wrote in an editorial.

About 15 of these medicines are on the market now and hundreds more are in testing, but they have had only limited success. And the new study may help explain why.

The scientists used gene sequencing to a degree that has not been done before to study primary tumours and places where they spread in four patients with advanced kidney cancer. They found that two-thirds of gene mutations they detected were not present in all areas of the same tumour. They also were stunned to see different mutations in the same gene from one part of a tumour to another.

That means a single biopsy would reveal only a minority of mutations. Still, it's not clear whether doing more biopsies would improve accuracy, or how many or how often they should be done.

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Gene study suggests treating cancer is more complex than many had hoped