螺旋电脑断层用于肺癌筛选（Spiral CT Scans for Lung Cancer Screening）

肺癌通常是由吸烟引起的，是美国的主要癌症死亡原因。2006年，美国有162,460人死于肺癌。

螺旋电脑断层是一种在吸烟者和以前的吸烟者中进行肺癌筛选的新的试验性方法。有一些研究显示，电脑断层可以发现小肺癌。然而，到目前为止，这种新的方法作为常规筛选的好处和危险性还有待进一步的探讨。

美国国立癌症研究所正在进行一个大型的研究，将会对螺旋电脑断层或胸片能否降低肺癌死亡做出一个明确的答案。

大约有25到60%的吸烟者和以前的吸烟者会在电脑断层中显示非癌症的异常病变。

85%的肺癌是由吸烟引起的。降低患肺癌危险的唯一有效的方法是不吸烟。对吸烟的人群来说，戒烟数年后，将会大大降低得肺癌的危险。

Lung cancer, which is most frequently caused by cigarette smoking, is the leading cause of cancer-related death in the United States, claiming almost 162,460 lives in 2006. Spiral computed tomography (CT or CAT) scans are being tested as a new way to find early lung cancer in smokers and former smokers. At present, however, questions remain about the technology's risks and benefits as a screening tool.

Promising evidence from several studies shows that the scans can detect small lung cancers. But detecting these early tumors has not been proven to reduce the likelihood of dying from lung cancer, the gold standard for any cancer screening test. The National Cancer Institute (NCI) has designed a large study that should conclusively answer if either spiral CT or chest X-ray can reduce lung cancer deaths. Other recent studies have looked at survival rather than mortality, which can be misleading because screening generally does increase survival rates (the proportion of patients alive at some point after the diagnosis of their cancer) but may not decrease mortality rates.

While spiral CT scans may eventually prove to be an effective lung cancer screening tool, they can trigger unnecessary invasive testing or even chest surgery that may potentially lead to decreased pulmonary function or death. Scarring from smoking and other non-cancerous changes in the lungs can mimic tumors on CT scans, challenging the radiologists who read them. Interpretations of the scans can vary, leading to confusion about recommendations for follow-up care.

About 25 percent to 60 percent of CT scans of smokers and former smokers will show abnormalities that are not cancer. When these suspicious areas, or nodules, are found, the physician may recommend waiting several months to a year before a repeat scan to see if the nodule has grown.

The physician may also advise an immediate lung biopsy, a potentially risky procedure that involves the removal of a small amount of tissue, either through a scope fed down the windpipe (bronchoscopy) or with a needle through the rib cage (CT-directed needle biopsy). Possible complications from biopsies include partial collapse of the lung, bleeding, infection, and pain and discomfort.

Depending on the size and location of the nodule, chest surgery (thoracotomy) to obtain a larger biopsy may be recommended. Thoracotomy is a major surgery that removes substantial amounts of lung tissue; the procedure can damage nerves in the chest and may lead to chronic pain, as well as result in decreased pulmonary function or death.

Sixty percent of the hospitals in the United States own a spiral CT machine. These machines are routinely used for staging lung and other cancers to determine how advanced the cancer is after diagnosis. But recently, some hospitals have begun promoting spiral CT scans to smokers for early detection of lung cancer, despite the lack of evidence that such scans can decrease mortality. Each scan costs $300 to $1,000.

Some experts worry that this marketing may lead smokers to falsely believe that they can continue smoking without increasing their risk of dying from lung cancer. Eighty-five percent of all lung cancers are caused by smoking, and the only proven way to reduce the risk of lung cancer is not to smoke. For people who do smoke, quitting reduces the risk of lung cancer considerably over the course of several years.

Research has shown that high-risk individuals say they would participate in a study comparing spiral CT to chest X-rays, even if the individuals were selected to receive another intervention instead of spiral CT. This research served as a preliminary study in which about 3,000 smokers were recruited over several months to receive either a CT scan or a chest X-ray. The study provided important information on how much follow-up (additional scans, biopsies, surgery, etc.) is needed after each type of lung cancer screening. In addition, this short-term, feasibility study determined that the willingness of high-risk people to participate in such a trial translated into actual participation. Medical centers that are part of the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial began this feasibility study in September 2000 and the results were published in 2004.

This feasibility study led the way to the NCI-sponsored National Lung Screening Trial (NLST), which is now tracking over 53,000 smokers and former smokers to see if those who are screened with spiral CT scans have a lower mortality rate than those who receive a chest X-ray.

The PLCO also is separately examining whether annual chest X-rays, which are easier to perform than spiral CT scans, can reduce mortality from lung cancer. The PLCO trial began in 1994 and is following nearly 155,000 men and women. People participating in the PLCO Cancer Screening Trial cannot participate in the National Lung Screening Trial.

How spiral CT works: Spiral CT uses X-rays to scan the entire chest quickly, in 12 to 20 seconds, during a single breath-hold. Throughout the procedure, the patient lies very still on a table. The patient passes through the X-ray machine, which is shaped like a doughnut with a large hole. The machine rotates around the patient and a computer creates images from the scan, which can be reconstructed into a three-dimensional model of the lungs.