Seeing is not always relieving
Mar 8th 2007
From The Economist print edition Screening for lung cancer may do more harm than good
SOMETIMES you can know too much. The aim of screening healthy people for cancer is to discover tumours when they are small and treatable. It sounds laudable and often it is. But it sometimes leads to unnecessary treatment. The body has a battery of mechanisms for stopping small tumours from becoming large ones. Treating those that would have been suppressed anyway does no good and can often be harmful.
Take lung cancer. A report in this week’s Journal of the American Medical Association, by Peter Bach of the Memorial Sloan-Kettering Cancer Centre in New York and his colleagues, suggests that, despite much fanfare around the use of computed tomography (CT) to detect tumours in the lungs well before they cause symptoms, the test may not reduce the risk of dying from the disease at all—indeed, it may make things worse.
以肺癌为例。纽约纪念斯隆-凯特灵癌症研究中心（Memorial Sloan-Kettering Cancer Centre）的彼得•巴赫（Peter Bach）及其同事在本周出版的《美国医学会杂志》（Journal of the American Medical Association）上发表的一项报告中指出，用于症状出现前肺部肿瘤检测的计算机断层扫描技术（computed tomography，CT）虽深受吹捧，但它可能根本无法降低肺癌死亡率，相反会“雪上加霜”。
The story begins last year, when Claudia Henschke of Cornell University and her colleagues made headlines with a report that patients whose lung cancer had been diagnosed early by CT screening had excellent long-term survival prospects. Her research suggested that 88% of patients could expect to be alive ten years after their diagnosis. Dr Bach found similar results in a separate study. In his case, 94% of patients diagnosed with early-stage lung cancer were alive four years later.
Survival data alone, though, fail to answer a basic question: “compared with what?” People are bound to live longer after their diagnosis if that diagnosis is made earlier. Early diagnosis is of little value unless it results in a better prognosis.
Dr Bach, therefore, interrogated his data more thoroughly. He used statistical models based on results from studies of lung cancer that did not involve CT screening, to try to predict what would have happened to the individuals in his own study if they had not been part of that study. The results were not encouraging.
Screening did, indeed, detect more tumours. Over the course of five years, 144 cases of lung cancer were picked up in a population of 3,200, compared with a predicted number of 44. Despite these early diagnoses, though, there was no reduction in the number of people who went on to develop advanced cancer, nor a significant drop in the number who died of the disease (38, compared with a prediction of 39). Considering that early diagnosis prompted a tenfold increase in surgery aimed at removing the cancer (the predicted number of surgical interventions was 11; the actual number was 109), and that such surgery is unsafe—5% of patients die and another 20-40% suffer serious complications—the whole process seems to make things worse.
Dr Bach’s conclusion is that many of the extra cancers picked up by CT screening would never have caused clinical disease, while the most aggressive tumours—those that cause most of the 160,000 lung-cancer deaths in America each year—grow too quickly to be found early, even with annual CT screening. The situation resembles prostate-cancer screening, which relies on a blood test for a molecule secreted by prostate tumours. In prostate screening, a lot of disease is identified, but there is great doubt over the number of lives this saves. Dr Bach’s research also resembles an earlier attempt to deal with lung cancer, in which researchers uncovered 20% more tumours in groups that underwent screening using chest X-rays than in those who did not. Then, too, the frequency of death from the disease did not differ between the two groups. Both Dr Bach and Dr Henschke had hoped that by using CT, which is better than X-rays at picking up small tumours in the lungs, they might have changed this outcome.
Dr Bach was comparing data from screened people with a model of what would have happened to an unscreened group. The final word on CT screening for lung cancer will have to wait for the results of a proper experiment that compares screened and unscreened groups. These are expected in the next year or two. The omens, however, are bad. What you do know can hurt you.