Digital Chest Radiography

The results of the LDCT studies described in the previous section have clearly shown the limited ability of conventional chest radiography to detect early lung cancer. Emerging technological advancements in digital chest radiography, including computer-aided diagnosis, temporal subtraction, and dual energy subtraction methods, may significantly improve the ability of chest radiography to detect small lung nodules [44-46]. These techniques are described in further detail in Chapter 13. Once this technology matures, future studies will be necessary to address the ability of digital chest radiography to detect early lung cancer [21].

Table 3 Excerpts from the Consensus Statement of the Society of Thoracic

Radiology

Subject selection

For the general population, an age range should be established. We believe that this should be between 50 and 80 years, depending on the subjects' general health. In high-risk groups such as those occupationally exposed to carcinogens or with a previous NSCLC, selection criteria may vary. Cigarette smoking should be at least 10 or 20 pack-years.

Periodicity

Lacking definitive information, the general trend is to perform annual CT scanning. It has to be understood that some lung cancers will become clinically evident in the periods between screening studies, although how many is yet to be determined.

Screening protocols

Screening protocols will vary with the available imaging technology. It is our opinion that screening be performed with a multirow detector CT so that highresolution scans can be reformatted retrospectively, without the need to use additional radiation.

The entire thorax should be included in the scan field, preferably in a single breathhold. A helical (spiral) mode of operation should be used. Two sample techniques are given below. Multislice protocol

Table feed: 30 mm/sec 120-140 kVp Pitch 3 to 6 20-60 mA

1- to 2.5-mm collimation; with a 1- to 2.5-mm reconstruction interval. Single-slice spiral scanner protocol

Helical mode, 0.8-sec scan time (the shortest possible) 120 kVp Pitch 2 to 1 80 mA

3- to 7-mm collimation; with 2.5- to 3.5-mm reconstruction interval.

It is advisable to train technologists or other observers to detect nodules and calcifications at the time of scanning so that high-resolution 1- to 1.25-mm helical images can be performed through any noncalcified nodules at the same time as the primary screening study. This can obviate the need for a repeat study.

Radiation dose

The effective radiation dose associated with the low-dose screening examination is 0.65 mSv (mRem). The approximate dose for ''conventional'' CT is 5.8 mSv (26). Eliminating the scanogram for the screening CT study can reduce dose. These doses include no high-resolution or follow-up CT studies.

Table 3 Continued

Indeterminate nodules

Indeterminate nodules are solid, smooth-edged, and do not show ''benign calcifications," air bronchograms, or converging vessels. They are not spiculated and are of unknown chronicity. The follow-up interval for indeterminate nodules is often dictated by the individual subject and their physician. Sites experienced in lung cancer screening have adopted the following strategy based on the diameter of the nodule:

<5 mm: high-resolution CT at 3 and/or 6, 12, and 24 months. Consider biopsy/removal for nodules that increase in size (1% malignant in prevalence studies)

5-10 mm: high-resolution CT at 3, 6, 12, and 24 months. Biopsy/removal of nodules that increase in size (25 -30% malignant) >10 mm: consider biopsy of all of these nodules (30-80% malignant). Alternatively, they may be studied with PET scanning or with CT contrast enhancement [27,28].

Physician responsibility

Screening-imposed obligations on the radiologist (similar to mammography) to (1) warn the subject that a negative screen does not preclude the subsequent development of lung cancer, even between scans; (2) ensure the subject knows that some lung cancers may not be amenable to detection by CT screening; (3) ensure that the subject is contacted with results of the CT screening; (4) ensure that appropriate physicians are available to council and treat the patient with a positive result; (5) ensure that patients understand the problem of the number of small lung nodules that are benign and the implications thereof. To ensure the answer to the question of the efficacy of CT screening for NSCLC is made available as soon as possible, it is recommended that all subjects being screened with CT for lung cancer are done as part of a prospective study. Comparable protocols should be used and the recording of results standardized.

Summary of current recommendations

Lung cancer screening with low-dose CT is a complex subject. It is clear that a standard of care cannot be based on currently published prevalence data. However, there are ongoing studies that are generating prevalence data. The appropriate studies which address lung cancer mortality and cure rates need to be performed and the data analyzed and validated before the true utility of this test can be determined. Thus we do not recommend mass screening for lung cancer at this time, but strongly encourage appropriate subjects to participate in trials so that the true effectiveness of lung cancer screening with low-dose helical CT can be determined at the earliest possible time.

Source: www.thoracicrad.org.

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