The genetic mutations in cancer cells may vary in every patient, a study found, suggesting that drugs will need to be tailored more finely to small groups, The Wall Street Journal reports. The small study, by doctors at the Brigham and Women’s Hospital in Boston and scientists from a gene-reading unit of Roche Holding AG, is among the first to look comprehensively at the genes in cancerous tumors to find which genes went awry.

The docs examined four patients with a rare and deadly lung-sac cancer called pleural mesothelioma, which strikes about 3,000 people a year. Every patient’s tumor had a different group of mutated genes, and no gene was mutated in more than one patient. That could explain why chemotherapy drugs work well in some patients and not at all in others, but it also means that progress in developing cancer drugs may require targeting them to tiny groups of patients, the paper notes. The study was published online yesterday by the journal Proceedings of the National Academy of Sciences.

This also raises the uncomfortable prospect that cancers in some patients may be so unusual that the cost of developing drugs to treat them might be prohibitively high based on the market that could benefit from them, the Journal writes. Some geneticists say the approach isn’t yet cost-effective.

In the study, it cost more than $100,000 per patient to read out a tumor’s genes and compare them with healthy cells from the same person - and that was using newer, cheaper methods of DNA reading from Roche’s gene-reading division, called 454 Life Sciences. Other methods, considered less reliable, offer similar technology that could, if verified, bring the cost to around $12,000 per patient, the authors of the study say.

“Whether it’s at all clinically useful is way premature,” Bert Vogelstein, a cancer researcher at Johns Hopkins University who published similar research, on breast and colorectal cancers, in 2006, tells the Journal. “The problem is interpreting the results.” Finding a mutation in a tumor doesn’t prove the mutation caused the tumor, he adds.