Patients are holding out hope that someday soon, they hope physicians will be able to personalize medical treatment more precisely than theyve been able to in the past. For people with cancer, this might mean taking a quick biopsy, studying the genetic profile of a tumor and then tailoring interventions to target the cancer effectively, with as few side effects as possible.

But a study published in the New England Journal of Medicine on Wednesday underscores why the vision remains a challenge. Cancer researchers in England showed that individual kidney tumors and their metastases had different mutations in different locations and that those mutations, in turn, affect the biology of those tumors in varying ways in different locations.

A single tumor-biopsy-specimen reveals a minority of genetic aberrations that are present in an entire tumor, wrote Dr. Marco Gerlinger of the Cancer Research UK London Research Institute and co-authors.

For example, the scientists found that one region of a renal carcinoma could display gene expression signatures associated with a good prognosis, while signatures in another region of the same tumor could be associated with a poor prognosis.

The basic insight that a single cancer can contain a number of mutations isnt entirely new, but the teams genetic analysis helps demonstrate why it probably wont be possible to devise targeted, patient-specific treatment strategies by looking at minimally invasive biopsies collected from a single site, wrote Dr. Dan Longo of the National Institute on Aging in an editorial accompanying the study.

A new world has been anticipated in which patients will undergo a needle biopsy of a tumor in the outpatient clinic, and a little while later, an active treatment will be devised for each patient on the basis of the distinctive genetic characteristics of the tumor, he wrote. But a serious flaw in the imagined future of oncology is its underestimation of tumor heterogeneity.

The Los Angeles Times has reported on tumor genetics in the past. In April 2011, writer Amber Dance described efforts to catalog the mutations that cause cancer. Earlier that year, Thomas H. Maugh II explained how researchers sequenced the genomes of prostate cancers in seven different men.

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Personalized cancer treatment: Genetic differences abound in tumors

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