Testing a BEIR-VI suggestion for explaining the lung cancer vs. radon relationship for U.S. counties

A review of studies of childhood cancer and natural background radiation

PURPOSE

The projected existence and magnitude of carcinogenic effects of ionizing radiation at low doses and low-dose rates is perhaps the most important issue in radiation protection today. Studies of childhood cancer and natural background radiation have the potential to throw direct light on this question, into a dose range below a few tens of mSv. This paper describes the studies that have been undertaken and their context, discusses some problems that arise and summarizes the present position.

CONCLUSIONS

Many such studies have been undertaken, but most were too small to have a realistic chance of detecting the small effects expected from such low doses, based on risk projections from higher exposures. Case-control or cohort studies are to be preferred methodologically to ecological studies but can be prone to problems of registration/participation bias. Interview-based studies of the requisite size would be prohibitively expensive and would undoubtedly also run into problems of participation bias. Register-based studies can be very large and are free of participation bias. However, they need to estimate the radiation exposure of study subjects using models rather than individual measurements in the homes of those concerned. At present, no firm conclusions can be drawn from the studies that have been published to date. Further data and perhaps pooled studies offer a way forward.

Elevated Platelet Count Appears to Be Causally Associated with Increased Risk of Lung Cancer: A Mendelian Randomization Analysis

BACKGROUND

Platelets are a critical element in coagulation and inflammation, and activated platelets are linked to cancer risk through diverse mechanisms. However, a causal relationship between platelets and risk of lung cancer remains unclear.

METHODS

We performed single and combined multiple instrumental variable Mendelian randomization analysis by an inverse-weighted method, in addition to a series of sensitivity analyses. Summary data for associations between SNPs and platelet count are from a recent publication that included 48,666 Caucasian Europeans, and the International Lung Cancer Consortium and Transdisciplinary Research in Cancer of the Lung data consisting of 29,266 cases and 56,450 controls to analyze associations between candidate SNPs and lung cancer risk.

RESULTS

Multiple instrumental variable analysis incorporating six SNPs showed a 62% increased risk of overall non-small cell lung cancer [NSCLC; OR, 1.62; 95% confidence interval (CI), 1.15-2.27; P = 0.005] and a 200% increased risk for small-cell lung cancer (OR, 3.00; 95% CI, 1.27-7.06; P = 0.01). Results showed only a trending association with NSCLC histologic subtypes, which may be due to insufficient sample size and/or weak effect size. A series of sensitivity analysis retained these findings.

CONCLUSIONS

Our findings suggest a causal relationship between elevated platelet count and increased risk of lung cancer and provide evidence of possible antiplatelet interventions for lung cancer prevention.

IMPACT

These findings provide a better understanding of lung cancer etiology and potential evidence for antiplatelet interventions for lung cancer prevention.

Differential expressions of MDM2 and TAP73 in cancer and cancer-adjacent tissues in patients with non-small-cell lung carcinoma

AIM

To investigate the differences in mRNA and protein expressions of MDM2 (mouse double minute 2 homolog) and P73 in cancer and cancer-adjacent tissues in patients with non-small-cell lung carcinoma (NSCLC).

MATERIALS AND METHODS

We compared the protein expressions of MDM2 and P73 in lung cancer and cancer-adjacent tissues in NSCLC patients by IHC (immunohistochemistry) and WB (Western blot). We divided the NSCLC patients into two subgroups, adenocarcinoma and squamous carcinoma. The mRNA expressions of two main isoforms of P73, TAP73 and DNP73, as well as the ratio of DNP73/TAP73 were analyzed by qPCR (quantitative real-time PCR) in the two tissues in all NSCLC patients and in patients with adenocarcinoma or squamous carcinoma, respectively.

RESULTS

WB results did not show significant differences in MDM2 and P73 protein expressions in lung cancer and cancer-adjacent tissues. However, IHC results indicated that MDM2 expression significantly increased in cancer tissues in female patients, but not male patients. In addition, TAP73 mRNA expression significantly increased in cancer tissues in all NSCLC patients (p=0.002) and in patients with adenocarcinoma (p=0.01); while there was no significant difference in DNP73 mRNA expression. Hence the fold-change of DNP73/TAP73 ratio significantly decreased (p=0.0003) in cancer tissues in all NSCLC patients and in patients with adenocarcinoma.

CONCLUSIONS

TAP73 mRNA expression significantly increased in cancer tissues than cancer-adjacent tissues in all NSCLC patients and in patients with adenocarcinoma. Meanwhile, the fold-change of DNP73/TAP73 ratio was similar to TAP73. MDM2 protein expression significantly increased in cancer tissues in female NSCLC patients.

The epidemic status and risk factors of lung cancer in Xuanwei City, Yunnan Province, China

Xuanwei City (formerly known as Xuanwei County) locates in the northeastern of Yunnan Province and is rich in coal, iron, copper and other mines, especially the smoky (bituminous) coal. Unfortunately, the lung cancer morbidity and mortality rates in this region are among China's highest, with a clear upward trend from the mid-1970s to mid-2000s. In 2004-2005, the crude death rate of lung cancer was 91.3 per 100,000 in the whole Xuanwei City, while that for Laibin Town in this city was 241.14 per 100,000. The epidemiologic distribution (clustering patterns by population, time, and space) of lung cancer in Xuanwei has some special features, e.g., high incidence in rural areas, high incidence in females, and an early age peak in lung cancer deaths. The main factor that associates with a high rate of lung cancer incidence was found to be indoor air pollution caused by the indoor burning of smoky coal. To a certain extent, genetic defects are also associated with the high incidence of lung cancer in Xuanwei. Taken together, lung cancer in this smoky coal combustion region is a unique model for environmental factor-related human cancer, and the current studies indicate that abandoning the use of smoky coal is the key to diminish lung cancer morbidity and mortality.

Smoking and hormesis as confounding factors in radiation pulmonary carcinogenesis

Confounding factors in radiation pulmonary carcinogenesis are passive and active cigarette smoke exposures and radiation hormesis. Significantly increased lung cancer risk from ionizing radiation at lung doses < 1 Gy is not observed in never smokers exposed to ionizing radiations. Residential radon is not a cause of lung cancer in never smokers and may protect against lung cancer in smokers. The risk of lung cancer found in many epidemiological studies was less than the expected risk (hormetic effect) for nuclear weapons and power plant workers, shipyard workers, fluoroscopy patients, and inhabitants of high-dose background radiation. The protective effect was noted for low- and mixed high- and low-linear energy transfer (LET) radiations in both genders. Many studies showed a protection factor (PROFAC) > 0.40 (40% avoided) against the occurrence of lung cancer. The ubiquitous nature of the radiation hormesis response in cellular, animal, and epidemio-logical studies negates the healthy worker effect as an explanation for radiation hormesis. Low-dose radiation may stimulate DNA repair/apoptosis and immunity to suppress and eliminate cigarette-smoke-induced transformed cells in the lung, reducing lung cancer occurrence in smokers.

Test of the linear-no threshold theory: rationale for procedures

A tightly reasoned justification is presented for the procedures used in our test of the linear-no threshold theory of radiation carcinogenesis by comparing lung cancer rates with average radon exposure in U.S. counties. A key point is its dependence on ecological variables rather than on characteristics of individuals and the principal problems involve treatment of potential confounding factors (CF). The method of stratification is introduced and shown to be preferable to multiple regression for evaluating effects of confounding. Utilizing numerous available CF reduces the problem of representing a complex confounding relationship by the average value of a single CF. The requirements on a CF for affecting the results are quantified in terms of its correlations with lung cancer rates and radon levels and it is shown that the existence of an unknown confounder satisfying these requirements is highly implausible. Effects of combinations of confounding factors are treated and shown not to be important. The problem of confounding factors on the level of individuals is resolved. Consideration of plausibility of correlations is used in several applications, including treatments of uncertainty in smoking prevalence, within county differences in radon exposure between smokers and non-smokers, variations in intensity of smoking, differences between measured radon levels and actual exposures, etc. Examples are presented for all applications. The differences between our study and case-control studies, and the advantages of each for testing the linear-no threshold theory, are discussed.

Residential radon exposure and lung cancer risk: commentary on Cohen's county-based study

The large United States county-based study () in which an inverse relationship has been suggested between residential low-dose radon levels and lung cancer mortality has been reviewed. While this study has been used to evaluate the validity of the linear nonthreshold theory, the grouped nature of its data limits the usefulness of this application. Our assessment of the study's approach, including a reanalysis of its data, also indicates that the likelihood of strong, undetected confounding effects by cigarette smoking, coupled with approximations of data values and uncertainties in accuracy of data sources regarding levels of radon exposure and intensity of smoking, compromises the study's analytic power. The most clear data for estimating lung cancer risk from low levels of radon exposure continue to rest with higher-dose studies of miner populations in which projections to zero dose are consistent with estimates arising from most case-control studies regarding residential exposure.