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* From the Department of Medicine, Duke University Medical Center, Durham, NC.
Correspondence to: Michael J. Kelley, MD, Hematology/Oncology (111G), Durham VAMC, 508 Fulton St, Durham NC 27705; e-mail: kelleym{at}duke.edu
| Abstract |
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Design, setting, and participants: Systematic searches of MEDLINE, HealthStar, and Cochrane Library databases to July 2001 and print bibliographies. For chemoprevention studies, we considered only randomized controlled trials (RCTs) with lung cancer incidence as an end point. For studies of smoking avoidance or cessation, we selected systematic reviews and meta-analyses, and searched for individual RCTs only where high-quality and current reviews and meta-analyses were not available.
Measurement and results: Chemoprevention of lung cancer has been studied in five RCTs of primary prevention, no RCTs of secondary prevention, and five RCTs of tertiary prevention. None of these trials has shown evidence for efficacy of any agents tested, including retinol (vitamin A), ß-carotene, N-acetylcysteine, and selenium. There is a great deal of evidence about a wide variety of clinician-based and community-based efforts at smoking avoidance or cessation. Certain approaches have been shown to be effective (eg, mass media public education campaigns, direct restrictions on smoking, clinician-based approaches ranging from brief clinician advice to more in-depth sessions, and "life-skills training" in schools). Some approaches have intermediate or short-term effectiveness (ie, youth access restrictions and school-based interventions), and others have been shown to be ineffective (ie, acupuncture and provider education) or have been insufficiently studied (ie, provider feedback).
Conclusions: There are no agents that have been proven to be effective for preventing lung cancer. Several clinician-based and community-based interventions show promise for reducing lung cancer incidence through smoking avoidance and prevention.
Key Words: carotenoids chemoprevention health education lung neoplasms primary prevention smoking cessation vitamin A
| Introduction |
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Chemoprevention is the use of specific natural or synthetic chemical agents to inhibit the development of invasive cancer by blocking the DNA damage that initiates carcinogenesis or by reversing or arresting the progression of premalignant cells.2 Chemoprevention strategies can be applied to the prevention of lung cancer in those persons with known risk factors (primary chemoprevention), those persons with disease precursors (secondary chemoprevention), or those persons with a prior cancer that had been treated with curative intent (tertiary chemoprevention). As a strong and prevalent risk factor for lung cancer, tobacco smoking has been the target for the prevention of lung cancer and other smoking-related diseases.
| Materials and Methods |
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For chemoprevention studies, we considered only RCTs with lung cancer incidence as an end point. For studies of smoking avoidance or cessation, we selected systematic reviews and meta-analyses, and we searched for individual RCTs only where high-quality and current reviews and meta-analyses were not available.
| Results |
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-Tocopherol, ß-Carotene (ATBC) Lung Cancer Prevention Study3
4
5
and the ß-Carotene and Retinol Efficacy Trial (CARET).8
9
A similar trend was observed in a small study comparing ß-carotene and retinol (vitamin A) in asbestos workers,7
although the difference was not statistically significant.
A closer examination of these studies is instructive for the design of future studies. The ATBC trial3
4
5
examined the effect of
-tocopherol (vitamin E) and ß-carotene on the incidence of lung cancer in 29,133 Finnish male smokers using a 2 x 2 factorial design. The selection of these agents was based almost exclusively on epidemiologic studies linking a vegetable-rich diet (high in ß-carotene and vitamin E) with a decrease in the risk of lung cancer.11
ß- carotene and vitamin E have antioxidant properties in vitro. Subjects were recruited from 1985 to 1993 and took supplements (ie,
-tocopherol, 50 mg/d, and ß-carotene, 20 mg/d) for 5 to 8 years (mean, 6.1 years) for a total follow-up of 169,751 subject-years. No effect of
-tocopherol (vitamin E) on lung cancer incidence was observed (relative risk [RR], 0.98; 95% confidence interval [CI], 0.86 to 1.12). However, in the ß-carotene arms, the RR of lung cancer incidence was 1.18 (95% CI, 1.03 to 1.36). This effect was most pronounced in those who persons who smoked
20 cigarettes a day and in those with higher alcohol intake.5
The CARET study8
9
was a large two-arm study that was designed to compare the effects of a combination of ß-carotene and retinol to those of placebo on lung cancer incidence in subjects who were at high risk for the development of lung cancer. The rationale for the study was based on the results of observational epidemiologic studies that had demonstrated a statistically significantly decreased RR of lung cancer between the extreme quintiles or quartiles of dietary intake and serum levels of ß-carotene and vitamin A.11
12
13
In addition, vitamin A analogs were demonstrated to have potential utility in preventing cancer in animal models. A pilot study14
of 1,029 high-risk subjects (ie, those persons with
20 pack-years of cigarette smoking who were currently smoking or had quit within the previous 6 years) demonstrated the safety and tolerability of ß- carotene, 50 mg daily, retinol, 25,000 IU daily, and the combination of the two. The completed study enrolled a total of 18,314 subjects, including current and former smokers of both sexes and male asbestos workers (ie, those persons who had worked with asbestos at least 15 years prior to study enrollment who had evidence of asbestosis on chest radiogram), with a total of 73,135 person-years of follow-up and a mean length of follow-up of 4.0 years. The active intervention was continued throughout this period of time. A planned interim analysis demonstrated a statistically significantly increased RR for the development of lung cancer (RR, 1.28; 95% CI, 1.04 to 1.57), death from any cause (RR, 1.17; 95% CI, 1.03 to 1.33), and death from lung cancer (RR, 1.46; 95% CI, 1.07 to 2.00), resulting in early termination of the active intervention arm. It has been suggested15
16
that higher concentrations of ß-carotene resulting from supplementation may have pro-oxidant effects, inducing DNA damage and membrane instability.
The Physicians Health Study6 was a randomized, double-blind, placebo-controlled trial of aspirin (325 mg every other day) and ß-carotene (50 mg every other day) conducted using a 2 x 2 factorial design among 22,071 male physicians in the United States. Only 11% of participants were current smokers and 39% were former smokers at study entry. The study began in 1982, and the aspirin arms were terminated in 1988 when a significant reduction in first myocardial infarction incidence was noted. The primary end points were cardiovascular disease and cancer incidence. An analysis of the cancer incidence in patients in the ß-carotene arms at the end of the study in 1995 showed no significant difference in the incidence of total cancers, lung cancer, or any other cancer. The study has not been analyzed for an effect of aspirin on lung cancer incidence.17
The Womens Health Study10 randomized nearly 40,000 women who were at least 45 years of age in a 2 x 2 x 2 factorial design to measure the effect of ß-carotene, aspirin, and vitamin E on cancer and cardiovascular disease incidence. Thirteen percent of enrolled women were current smokers. Following the publication of other studies, the ß-carotene arm was closed. After supplementation for 2.1 years with ß-carotene and an additional median follow-up of 2.0 years (median total follow-up, 4.1 years), there was no difference in cancer incidence or death with respect to ß-carotene supplementation. There were 30 cases of lung cancer in the ß-carotene arms and 21 cases in the placebo arms (RR, 1.42; 95% CI, 0.88 to 2.49), a result that is not statistically significant. The early termination of the study limits the ability to detect a significant effect of the ß-carotene supplementation on lung cancer incidence or death from lung cancer.
A South African study7 of 1,024 asbestos workers (92% male) randomly assigned subjects in equal proportions to either ß-carotene (30 mg/d) or retinol (25,000 IU/d) from 1990 to 1995. Twenty-one percent of participants were current smokers, and 52% were former smokers. After a median intervention and follow-up time of 4.5 years, no difference in lung cancer incidence was found (RR, 0.66; 95% CI, 0.19 to 2.32), although the small number of lung cancers (10) severely limited the power of this statement. The incidence of malignant mesothelioma was statistically significantly lower in the retinol group compared to the ß-carotene group (RR, 0.24; 95% CI, 0.07 to 0.86).
No ongoing large-scale RCTs for the primary prevention of lung cancer in high-risk individuals were identified. The recently begun Selenium and Vitamin E Cancer Prevention Trial18 will examine the effect of selenium and vitamin E on lung cancer incidence as a secondary end point in low-risk men.
Secondary Chemoprevention Interventions
Persons with identifiable precursor lesions are appropriate for possible secondary prevention interventions. In lung cancer, there appears to be an orderly histologic progression of changes in the epithelium of the large airways from normal epithelium to metaplasia, increasing degrees of dysplasia, carcinoma in situ, and invasive squamous cell cancer.19
These histologic changes are more pronounced in the proximal than in the distal airways and are more common with increased cigarette usage, reaching their maximum in the airways uninvolved with lung cancer in patients who died of lung cancer.20
21
In addition, the frequency of carcinoma in situ increases with the number of cigarettes smoked per day and is highest in patients who died of lung cancer.22
23
Bronchial epithelial histologic changes can be identified in directed or blind endobronchial biopsy samples and as atypia in sputum samples. In the periphery of the lung, atypical adenomatous hyperplasia has been proposed as a precursor lesion of invasive adenocarcinoma,19
the most common histologic subtype of non-small cell lung cancer (NSCLC).24
Angiogenic squamous hyperplasia is a recognized lesion that also may be a malignant precursor.25
Neither of these latter morphologic lesions is frequently identified in nonsurgical biopsy specimens.
We identified no RCTs of secondary prevention in subjects who had been selected for the presence of precursor lesions that used lung cancer incidence as an end point. At least five small randomized studies of secondary chemoprevention have been performed with retinoids, ß-carotene plus retinol, and the combination of vitamin B12 and folate administered for up to 6 months.26 However, all of these studies used either bronchial epithelial metaplasia or sputum atypia as a surrogate end point. None used lung cancer incidence as a primary or secondary end point.
Tertiary Chemoprevention Interventions
Patients with a previous incidence of cancer of the upper aerodigestive tract (ie, lung, head and neck, and esophagus) have the highest rate of development of lung cancer of any population. Patients who have undergone resection for early-stage NSCLC develop a second primary lung cancer at a rate of approximately 2% per subject per year.27
This population has been used to test chemoprevention strategies in five RCTs28
29
30
31
32
that had primary end points of second primary cancer (of the lung) [Table 2
].
Each of these studies was limited by the use of a clinicopathologic definition of the term second primary cancer. Molecular genetic analysis can assess more definitively the clonality of metachronous tumors of the upper aerodigestive tract.33
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A randomized trial of selenium in patients who had undergone resection for early-stage NSCLC was recently begun in North America.34 The basis for this study, as well as that for the ongoing Selenium and Vitamin E Cancer Prevention Trial18 mentioned earlier, includes observational studies showing lower serum levels of selenium in lung cancer patients compared with control subjects, antioxidant properties of selenium as essential cofactor for glutathione peroxidase, decreased cancer incidence following selenium supplementation in Linxian, China,35 and decreased lung cancer incidence in an RCT of selenium supplementation to prevent skin cancer.36 Tin miners in the Yunnan province of China have a high incidence of lung cancer (ie, > 1% per year). A small (n = 40) blinded feasibility study37 in this population demonstrated that selenium supplementation with malt cakes was well-tolerated, increased serum and tissue levels of selenium, increased glutathione peroxidase levels, decreased lipid peroxide levels, and improved DNA repair in response to ultraviolet or benzo-a-pyrene damage. This population continues to be studied.
Smoking Prevention (Primary Smoking Prevention) and Cessation (Secondary Smoking Prevention)
An overwhelming body of evidence links cigarette smoking to lung cancer in a causative relationship. The cessation of smoking results in a slow decline in the risk of cancer development, but this risk remains elevated compared to never-smokers > 15 years after smoking cessation.38
The relative benefit of smoking cessation appears to be greater for those persons with shorter smoking histories.38
39
Thus, the prevention of smoking initiation among nonsmokers and the encouragement of smoking cessation among smokers lead to a decline in lung cancer incidence and mortality, which recently has been seen in states instituting aggressive antismoking campaigns. Statewide comprehensive tobacco control programs, which typically involve some mix of public education, print media campaigns, prevention of youth access to tobacco, restriction of advertising, creation of smoke-free environments, work site antismoking programs, health professional training on cessation techniques, and school-based smoking prevention curricula including a "life-skills training" approach, have had the most success. A distinguishing feature of these comprehensive tobacco control programs is their focus on changing smoking behavior through strategies that alter the social environment where smoking occurs. Successful school-based programs have included interventions that teach social reinforcement (ie, dealing with peer pressure), social norms (ie, increasing self-esteem), and developmental orientation (ie, development of decision making and interpersonal skills).40
To determine whether community-based or clinician-based smoking prevention interventions are effective in reducing smoking, we sought to identify existing systematic reviews and meta-analyses to summarize an enormous body of literature. The following two major efforts have covered most of the available evidence on these topics: the US Public Health Service Tobacco Use and Dependence Clinical Practice Guideline41 and the Task Force on Community Preventive Services.42 Evidence for the effectiveness of clinician-based interventions has been described by the Tobacco Use and Dependence Clinical Practice Guideline panel, staff, and consortium representatives in two reports, the first published by Agency for Health Care Research and Quality in 1996,43 and the second, an update, by the US Department of Health and Human Services in 2000.41 44 The Task Force on Community Preventive Services focused on interventions intended to achieve tobacco use prevention and control in the general population but excluded interventions that targeted only high-risk individuals (eg, cessation interventions for smokers with coronary artery disease, cessation programs conducted entirely in hospital settings, or interventions to reduce exposure to environmental tobacco smoke in homes with asthmatic children). This task force reviewed interventions at the health-care system level (eg, provider education or provider performance feedback), but not interventions delivered by clinicians to patients in a clinical setting.
Table 3 and 3A identifies, for each topic covered, the most up-to-date and complete systematic review. We describe the conclusions reached, the number of studies cited, and the quality of the supporting evidence according to the scale used by the Task Force on Community Preventive Services42 (ie, "strong," "sufficient," or "insufficient"). The systematic reviews, in turn, identify the individual studies cited and, in the case of Hopkins et al,42 provide detailed descriptions of each study in an appendix.
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| Discussion |
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There is a great deal of evidence about a wide variety of clinician-based and community-based efforts at smoking avoidance or cessation. Certain approaches have been shown to be effective (eg, mass media public education campaigns, direct restrictions on smoking, clinician-based approaches ranging from brief clinician advice to more in-depth sessions, and "life-skills training" in schools). Some approaches have intermediate or short-term effectiveness (ie, youth access restrictions, school-based interventions), and others have been shown to be ineffective (ie, acupuncture and provider education) or have been insufficiently studied (ie, provider feedback). Thus, although there has been little progress in chemoprevention, several clinician-based and community-based interventions show promise for reducing lung cancer incidence through smoking avoidance and prevention.
| Footnotes |
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-Tocopherol ß-Carotene Lung Cancer Prevention Study; CARET = ß-Carotene and Retinol Efficacy Trial; CI = confidence interval; NSCLC = non-small cell lung cancer; RCT = randomized controlled trial; RR = relative risk This research was supported by a contract from the American College of Chest Physicians.
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