Low-dose spiral computed tomography (CT) scanning is a noninvasive medical imaging test that has been used for the early detection of lung cancer for over 16 years (Sone et al. 1998; Henschke et.al. 1999). A low-dose spiral chest CT differs from a full-dose conventional chest CT scan primarily in the amount of radiation emitted during CT scans. Chest CT, in general, requires less radiation exposure than other CT procedures because the air-filled tissues of the lungs are not as dense as the tissues of other organs (i.e., less x-ray radiation is needed to penetrate the lung). Radiation dose can be further reduced with lung cancer screening due to the naturally high contrast between low-density, normal lung tissue and lung nodules that could be suspicious for lung cancer (International Commission on Radiological Protection [ICRP] 2007, Naidich et.al. 1990). The amount of radiation of the low-dose chest CT scan (an estimated average of 1.5 mSv, for the majority of former worker participants) is over five times lower than that absorbed during a full-dose diagnostic chest CT scan (an estimated average of 8 mSv). As further comparison, the estimated average annual radiation exposure from natural sources is 3.1 mSv.
The low-dose spiral chest CT scan is offered only to those workers who are determined to be at elevated risk for lung cancer. In addition, a participant must also be medically eligible to be enrolled in the lung cancer screening program (i.e., having sufficient lung function to withstand chest surgery).
The low-dose spiral chest CT scanning procedure requires that the patient lie down on an exam table as it moves through a sophisticated x-ray machine. The x-ray equipment is housed inside a doughnut-shaped tube. As the table moves through the "doughnut," the x-ray beam inside the tube rotates around the patient, taking more than 100 pictures in sequence. Because the continuous movement of the x-ray tube within the doughnut is combined with continuous movement of the table throughout the scan, the x-ray beam forms a spiral path - hence, the term "spiral" or "helical" CT. A special computer program processes this large volume of data and produces two-dimensional cross-sectional views of the chest, which are then displayed on a monitor. If needed, three-dimensional images of internal structures or abnormalities within the lung can also be created. During the procedure, the patient is asked to be as still as possible and to hold his or her breath for about 11 seconds.
Any screening test may detect some abnormalities that may appear to represent early signs of possible illness, but studies show that most of these abnormalities turn out to be false alarms. This is also true of screening for lung cancer. The initial low-dose CT scan may show a white spot in the lung, called a nodule. Most nodules detected in the Early Lung Cancer Detection (ELCD) program are actually small areas of scar tissue or healed infection. In most cases, this can be determined on the initial low-dose CT scan.
However, in other cases, the nodules are indeterminate; that is, it is not immediately clear whether the spot is benign or something of concern. For this type of nodule, the best course of action is to look for any changes over time. Therefore, if an indeterminate nodule is found, the patient will be invited back for a second low-dose CT scan three or six months after the initial scan.
If a nodule increases in size, the patient will be advised to follow up immediately with his/her personal doctor, because this nodule would now be considered suspicious for lung cancer. In this case, his/her personal doctor will arrange for the necessary consultations and procedures to find out whether or not the nodule is a lung cancer and, if so, to treat it appropriately. The most common follow-up procedures are further imaging studies, such as a positron emission tomography (PET) scan. Some people may undergo a lung tissue biopsy, which involves removing a piece of lung tissue from the lung and examining it under a microscope.
Most of the nodules followed as part of the ELCD program do not increase in size and are not cancerous. Many nodules may actually shrink on follow-up scans. If the nodule is unchanged or smaller, it is unlikely to be malignant, and the patient will be invited to return for an annual repeat low-dose CT scan one year from his/her baseline CT scan.
The annual scan is offered to everyone except those diagnosed with either lung cancer or a major illness (including other cancers) after enrolling in the ELCD program (that is, after completing the initial CT scan). The program offers the initial and annual CT scan and follow-up CT scans at no cost to the participant. The ELCD program does not provide or pay for any diagnostic evaluation and treatment of abnormalities discovered on CT scan, but these costs are normally covered by health insurance policies.
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References:
Henschke CI, McCauley DI, Yankelevitz DF, et al. Early Lung Cancer Action Project: overall design and findings from baseline screening.
Lancet 1999; 354(9173):99-105. ICRP, 2007. Managing Patient Dose in Multi-Detector Computed Tomography (MDCT). ICRP Publication 102. Ann. ICRP 37 (1).
Naidich DP, Marshall CH, Gribbin C, Arams RS, McCauley DI. Low-dose CT of the lungs: preliminary observations. Radiology 1990; 175(3):729-31.
Sone S, Takashima S, Li F, et al. Mass screening for lung cancer with mobile spiral computed tomography scanner. Lancet 1998; 351(9111):1242-5.