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

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

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

By analyzing tumors in unprecedented detail, Were finding that the deeper you go, the more you find, said one study leader, Dr. Charles Swanton of the London Research Institutes Cancer Research UK. Its like going from a black-and-white television with four pixels to a color television with thousands of pixels.

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

The study is reported in Thursdays 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 L. 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 tumors 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 tumor. They also were stunned to see different mutations in the same gene from one part of a tumor to another.

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

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Gene-based cancer research suffers setback, scientists say