Lung Cancer Testing: The Present
Light-induced fluorescence endoscopy (LIFE)
A number of endobronchial treatment modalities, including cryotherapy, laser therapy, and photodynamic therapy, have been developed to target small intraepithelial (or preinvasive) neoplastic lesions. Unfortunately, identifying people at risk for and then localizing the endobronchial lesions can present a challenge. When occult lung cancer is detected by sputum cytology alone without radiographic abnormalities, conventional bronchoscopy is the only way to identify the lesion and does so successfully in only 29% of cases.
A difference in fluorescence between normal and neoplastic tissue can be used to enhance the ability of bronchoscopy to identify intraepithelial neoplasia.[32-34] In a multicenter trial, the addition of LIFE bronchoscopy to conventional white-light bronchoscopy improved the sensitivity (detection of at least one lesion) from 37.3% to 75%, but there was no improvement in positive predictive value. While this modality may be superior at localizing tracheobronchial dysplasia, sputum cytology, a study not shown to improve mortality, is needed to identify those harboring dysplastic or malignant lesions.
Airway epithelial markers
Based on previous studies that demonstrated airway epithelial cell injury and various genetic mutations in the large airways of current and former smokers without lung cancer,[36-38] DNA microarrays were used to describe the smoking-induced changes in the gene expression of large-airway epithelial cells obtained during bronchoscopy. Spira et al took this work further and performed genetic expression profiling on specimens obtained from the large airways of patients undergoing bronchoscopy to follow up a clinical suspicion of lung cancer. A training set consisting of 77 samples was used to identify an 80-gene biomarker that distinguished between smokers with and without lung cancer. In a test set of 52 samples, the accuracy, sensitivity, and specificity were 83%, 80%, and 84%, respectively. These results were then validated with 35 samples collected prospectively, with similar results. When combined with cytopathologic analysis of airway brushings obtained during bronchoscopy, the biomarker had an increased sensitivity of 95% and a 95% negative predictive value.
The airway epithelial gene expression biomarker was later applied in a clinicogenomic prediction model that combined gene expression and clinical factors. The prediction model was used to predict cancer in three different sample sets and was compared to a clinical prediction model. The combined clinicogenomic prediction model had a sensitivity and negative predictive value of 100% and resulted in a higher specificity and positive predictive value than other models. Currently, there is a large multicenter trial underway to assess the role of this epithelial gene expression model in patients with and without lung cancer. One could envision this technology being useful for helping to predict malignancy in non-calcified pulmonary nodules.
Under normal conditions, the immune system is self-tolerant, thereby preventing reactions directed toward the host. Aberrations in this self-tolerance lead to inflammation and/or tissue destruction, as seen in autoimmune diseases. A similar phenomenon is seen in cancer, in which an immune response is initiated by alterations in the tumor itself; this results in the presence of circulating serum antibodies to autologous cellular antigens, also known as tumor-associated antigens (TAA). Several antigens expressed in tumor cells but not at all in normal cells have been identified and likely function as TAAs capable of priming the immune system to recognize the tumor cells. These antibodies have the potential to be detected much earlier than the underlying tumors and therefore hold promise as a target for a diagnostic serologic test.
Recently, a panel of six TAAs was validated in three groups of patients with newly diagnosed lung cancer. These proteins, which include p53, NY-ESO-1, CAGE, SOX2, and Annexin 1, were selected to be part of the panel because of their roles in inducing the production of autoantibodies or immune biomarkers in lung cancer.
Circulating autoantibodies to Annexin 1, a glycoprotein expressed diffusely in lung cancer cells, have been described in patients with NSCLC. Similarly, SOX2, a transcription factor, has been described as inducing an autoantibody response in small-cell lung cancer. NY-ESO-1 is a protein expressed by a number of solid tumors, including those of the lung, liver, and breast. Its function is unknown; however, antibodies to NY-ESO-1 can be detected in serum samples. Increased levels of antibodies to CAGE, a protein thought to act as an oncogene, have also been identified in solid tumors, including gastric, endometrial, and lung cancers. Finally, the first autoantibodies targeting p53, a tumor suppressor gene that is often mutated in a wide variety of cancers, were described in breast cancer.
Serum samples were collected after the diagnosis of lung cancer was made but prior to therapy initiation. The panel was tested in three groups of patients and demonstrated an overall sensitivity of 37% and a specificity of 90%. There was no difference between lung cancer stages. The poor sensitivity suggests that this test may be best used as an adjunct in evaluating high-risk patients.
Volatile organic compounds
There is a growing body of literature to support the use of exhaled breath analysis for the diagnosis of lung cancer. The majority of exhaled breath is composed of nitrogen, carbon dioxide, oxygen, water, and inert gases. The latter trace components are volatile substances that are a result of the body’s cellular biochemical processes or absorbed from the environment. The measurement of volatile organic compounds (VOCs) is noninvasive, can be repeated in short intervals, and therefore has potential as a screening test.
VOCs were first described in lung cancer in 1985 by Gordon and colleagues using a gas chromatography–mass spectrometry (GC-MS) system. The breath of 12 lung cancer patients was analyzed and compared with that of controls. A significant difference in the detected level of three VOCs allowed for a model that had 93% accuracy in identifying persons with lung cancer. Since this first study, a number of large-scale multicenter studies have been conducted that demonstrate varying sensitivities and specificities for the use of VOCs to detect lung cancer. Two of these studies conducted by the same group applied a model of exhaled VOCs for lung cancer screening.[51,52] The first study compared 87 patients with lung cancer to healthy volunteers using nine VOCs and demonstrated a sensitivity of 89.6% and a specificity of 82.9%. The next evaluated 193 patients with lung cancer, again comparing results of 16 exhaled VOCs to results in healthy volunteers, with a sensitivity of 84.6% and specificity of 80%. Importantly, smoking status did not affect the results, nor was there a difference in results by stage of lung cancer.
There are two main devices available that have been studied to analyze VOCs in exhaled breath, each with advantages and disadvantages. GC-MS is very sensitive and can detect and measure specific VOCs. However, GC-MS devices are more expensive and require more expertise than gaseous chemical sensing devices. Gaseous chemical sensing devices (also known as electronic noses) have a high sensitivity, are easy to use, and are portable. They do not, however, obtain quantitative data and cannot be calibrated.
A colorimetric sensor array using exhaled breath has also been evaluated for the detection of lung cancer. This sensor uses a disposable cartridge that has 36 spots impregnated with different chemical compounds that change color when contact is made with an inducing chemical. A total of 143 persons, including both healthy persons and persons with lung cancer, were evaluated with the sensor, which demonstrated a sensitivity of 73.3% and a specificity of 72.4%.
Interestingly, VOCs can be recognized by dogs. In a double-blinded study, dogs were first trained to distinguish between the exhaled breath of persons with lung cancer from those without lung cancer. The dogs then sniffed breath samples from patients that they had not been exposed to previously. The sensitivity of canine scent detection of lung cancer was 99%, with a specificity of 99%.
Cost-Effectiveness of Screening Procedures
Various models of cost-effectiveness analysis of CT scanning for lung cancer screening have produced results ranging from highly cost-effective to not cost-effective (estimates range from $2,500 to $2 million per life-year gained).[57-59] Since the release of the NLST, a microsimulation model that simulated cohorts of individuals representative of the US population (not calibrated to the NLST results) suggested that the cost-effectiveness of screening with CT scanning would be strongly influenced by specific eligibility criteria and the rate of smoking cessation in screening participants. It should be noted, however, that the only randomized study to assess enrollment in smoking cessation programs at the time of lung cancer screening showed that screening did not improve rates of cessation.
Cost-effectiveness analyses from the NLST are currently underway for the use of LDCT for lung cancer screening. The estimates will vary based on the reproducibility of the study findings in general practice settings. For example, LDCT will be less cost-effective in populations in which screening results in more surgery, or in populations that have a higher morbidity or mortality for the work-up of benign disease. Cost-effectiveness will also be lower if scans performed to follow up screen-detected nodules are obtained at more frequent intervals than recommended. Alternatively, cost-effectiveness will be improved if follow-up is conservative, if the morbitity and mortality of surgery remain low, and if high-risk populations such as current smokers with COPD are screened.
Should screening using LDCT be deemed cost-effective, the question will remain of how to financially support this technology while continuing to fund programs that have already been shown to be cost-effective ($5,000 per quality-adjusted life-year to implement the Agency for Healthcare Research and Quality [AHRQ] smoking cessation guidelines). Reducing smoking initiation or increasing cessation rates will have benefits in reducing the rate of not only lung cancer but also coronary artery disease, vascular disease, and the 11 or so other cancers associated with smoking. An additional consideration is the incorporation of any of the new testing procedures discussed above into screening strategies. A new technology might further reduce the cost of screening by preventing the need for surgery and other invasive procedures were it able to identify malignancy in those with screen-detected pulmonary nodules.
Lung Cancer Testing: The Future
While the NLST showed an improvement in mortality of 20% in those screened for lung cancer, the number of false-positive results was staggering. It may be that new testing techniques, such as exhaled breath VOCs, the airway epithelial gene expression biomarker, or serum sampling for antibodies, will be included as part of a screening algorithm for lung cancer. For example, one can foresee how a simple breath test could be performed on a high-risk individual, and if positive would raise the possibility of lung cancer and lead to chest CT screening. A negative breath test might prevent CT scans from being performed in those without lung cancer, but allow the screening to miss very few patients with lung cancer.
Similarly, in patients who undergo LDCT for screening and are found to have solitary pulmonary nodules, it might be advantageous to examine exhaled VOCs to determine which patients require further invasive work-up.
While it is recognized that smoking is a major risk factor for lung cancer, only a minority of smokers develop lung cancer—and in patients with lung cancer, approximately 15% are never-smokers. This suggests that to improve our ability to detect lung cancer in those who have it and to avoid testing in those who don’t, we need to better identify those at the highest risk for developing this disease. This may include some combination of airway genetic testing, serum biomarker analysis, and exhaled VOCs, as well as factors yet to be elucidated. The early detection of lung cancer holds great promise, but clinicians and scientists should not consider this issue in a vacuum. Without continued efforts at tobacco control and smoking cessation, significant reductions in the mortality from lung cancer and other smoking-related diseases will not be realized.
Financial Disclosure: Drs. Tanner and Silvestri are currently involved in trials sponsored by Allegro Diagnostics that evaluate airway epithelial gene expression in the diagnosis of lung cancer. The authors have no other significant financial interest or other relationship with the manufacturers of any products or providers of any service mentioned in this article.