Distinguishing effects on tumor multiplicity and growth rate in chemoprevention experiments

In some types of cancer chemoprevention experiments and short-term carcinogenicity bioassays, the data consist of the number of observed tumors per animal and the times at which these tumors were first detected. In such studies, there is interest in distinguishing between treatment effects on the number of tumors induced by a known carcinogen and treatment effects on the tumor growth rate. Since animals may die before all induced tumors reach a detectable size, separation of these effects can be difficult. This paper describes a flexible parametric model for data of this type. Under our model, the tumor detection times are realizations of a delayed Poisson process that is characterized by the age-specific tumor induction rate and a random latency interval between tumor induction and detection. The model accommodates distinct treatment and animal-specific effects on the number of induced tumors (multiplicity) and the time to tumor detection (growth rate). A Gibbs sampler is developed for estimation of the posterior distributions of the parameters. The methods are illustrated through application to data from a breast cancer chemoprevention experiment.

Effects of arsenic, cadmium, chromium, and lead on gene expression regulated by a battery of 13 different promoters in recombinant HepG2 cells

Toxic metals occur naturally at low concentrations throughout the environment, but are found in higher concentrations at many of the hazardous waste sites on the EPA Superfund list. As part of the Agency for Toxic Substances and Disease Registry (ATSDR) mandate to evaluate the toxicity of metals and mixtures, we chose four of the high-priority metal pollutants from ATSDR's HAZDAT list, including arsenic, cadmium, chromium, and lead, to test in a commercially developed assay system, CAT-Tox(L) (Xenometrix). This assay employs a battery of recombinant HepG2 cell lines to test the transcriptional activation capacity of xenobiotics in any of 13 different signal transduction pathways. Our specific aims were to identify metal-responsive promoters and determine whether the pattern of gene expression changed with a mixture of metals. Humic acid was used in all assays as a carrier to help solubilize the metals and, in all cases, the cells were exposed to the humic acid-metal mixture for 48 h. Humic acid alone, at 50-100 microM, showed moderate activation of the XRE promoter, but little other notable activity. As(V), at doses of 50-250 microM, produced a complex profile of activity showing significant dose-dependent induction of the hMTIIA, GST Ya, HSP70, FOS, XRE, NFkappaBRE, GADD153, p53RE, and CRE promoters. Pb(II) showed dose-related induction of the GST Ya, XRE, hMTIIA, GRP78, and CYP IA1 promoters at doses in the range of 12-100 microM. Cd(II), at 1.25-15 microM, yielded significant dose-dependent induction of hMTIIA, XRE, CYP IA1, GST Ya, HSP70, NFkappaBRE, and FOS. Whereas Cr(III) yielded small, though significant inductions of the CRE, FOS, GADD153, and XRE promoters only at the highest dose (750 microM), Cr(VI) produced significant dose-related inductions of the p53RE, FOS, NFkappaBRE, XRE, GADD45, HSP70, and CRE promoters at much lower doses, in the range of 5-10 microM. Assays testing serial dilutions of a mixture comprising 7.5 microM Cd(II), 750 microM Cr(III), and 100 microM Pb(II) (the combination of metals most frequently found at National Priority List sites) showed significant dose-dependent induction of the hMTIIA promoter, but failed to show dose-related induction of any other promoter and showed no evidence of synergistic activation of gene expression by the metals in this mixture. Our results thus show metal activation of gene expression through several previously unreported signal transduction pathways, including As(V) induction of GST Ya, FOS, XRE, NFkBRE, GADD153, p53RE, and CRE; Pb(II) induction of GST Ya, XRE, Cyp IA1, and GADD153; Cd(II) induction of NFkBRE, Cyp IA1, XRE, and GST Ya; and Cr(VI) induction of p53RE, XRE, GADD45, HSP70, and CRE promoters, and thus suggest new insights into the biochemical mechanisms of toxicity and carcinogenicity of metals. It is also an important finding that no evidence of synergistic activity was detected with the mixture of Cd(II), Cr(III), and Pb(II) tested in these assays.

X-ray evidence of increased asbestos exposure in the US population from NHANES I and NHANES II, 1973-1978. National Health Examination Survey

OBJECTIVES

Jobs involving heavy asbestos exposure increase risk for lung cancer and mesothelioma substantially, and low-level exposures may carry some risk. At least one indicator of asbestos exposure, mesothelioma, has been increasing in the US for decades. We investigated the prevalence of another indicator, pleural thickening on x-ray, in a defined sample of the US population.

METHODS

Certified physicians read 1060 x-rays from the second National Health and Nutrition Examination Survey (1976-1980) for pleural changes consistent with pneumoconiosis, which are a reasonably specific indicator of asbestos exposure.

RESULTS

Prevalence estimates, in NHANES II, in the age group 35-74 years, are 6.4% (+/- 0.9%) among males, 1.7% (+/- 0.6%) among females, and 3.9% (+/- 0.6%) overall. These prevalences are approximately twice those estimated from NHANES I data (1971-1975).

CONCLUSIONS

X-ray evidence of asbestos exposure was common in the late 1970s and increasing. The increase may be due to occupational asbestos exposure, but it is so large as to suggest some contribution from environmental, non-occupational asbestos exposure.

Unexplained increases in cancer incidence in the United States from 1975 to 1994: possible sentinel health indicators?

To search for unexplained patterns in cancer incidence, we analyzed data from 1975 to 1994 that represent approximately 10% of the population of the United States. Our analysis focused on long-term time trends in incidence and on deviations from those trends attributable to birth cohorts or to calendar periods. On average, cancer incidence rose 0.8% annually in white women and 1.8% in white men. After removing several cancers related to smoking and increased screening, average annual increases fell to 0.1% in white women but persisted at 1.7% in white men. In particular, yearly increases in non-Hodgkin's lymphoma averaged 2.4% in white women and 4.7% in white men. Among men, incidence changes attributable to cohorts grew progressively larger from one cohort to the next. Cancer incidence patterns among black men and women were similar to those among whites despite smaller population sizes. Unexplained patterns of cancer incidence may signal changes in underlying risk factors and highlight the continuing need for research on cancer etiology and prevention.

Hepatocarcinogenicity of chlordane in B6C3F1 and B6D2F1 male mice: evidence for regression in B6C3F1 mice and carcinogenesis independent of ras proto-oncogene activation

Logistic regression analysis of age-specific prevalences for neoplastic and non-neoplastic liver lesions was used to examine treatment responses for B6C3F1 and B6D2F1 male mice continuously exposed to chlordane (55 p.p.m.) and to determine whether neoplasms were dependent on continuous exposure in the B6C3F1 mice. In order to determine if ras oncogene activation plays a role in the carcinogenicity of chlordane and whether the activation is dependent on genetic background, liver tumors from chlordane-treated B6C3F1 and B6D2F1 mice were analyzed for the presence of activating mutations in the ras oncogene. The overall liver tumor prevalence at terminal killing was nearly 100% for both strains; however, the age-specific prevalence increased more rapidly in B6C3F1 mice than in B6D2F1 mice. Tumor-bearing B6C3F1 mice had an average of two or more tumors per liver than B6D2F1 mice at their respective terminal killings (5.4 versus 3.3). When chlordane exposure was discontinued for a group of B6C3F1 mice ('stop' group) at 491 days of age, overall tumor multiplicity significantly decreased by 30% from an average of 4.4 per tumor-bearing-animal at 525 days to 3.1 at terminal killing (568 days). Over the same time period the prevalence of hepatocellular carcinomas significantly decreased from 80 to 54% and adenomas from 100 to 93% by terminal killing in B6C3F1 'stop-group' mice. Chlordane induced diffuse hepatocellular centrilobular hypertrophy, frequent multinucleate hepatocytes, toxic change and hepatoproliferative lesions composed predominantly of acidophilic hepatocytes in nearly 100% of both the B6C3F1 and B6D2F1 mice. The development of histological evidence of toxicity closely paralleled the temporal development of hepatocellular neoplasia and decreased in severity when the tumor burden was maximal. No H- or K-ras mutations were detected in the chlordane-induced hepatocellular tumors in B6C3F1 mice (15 adenomas and 15 carcinomas) or B6D2F1 mice (10 adenomas and 10 carcinomas). In conclusion, chlordane induced liver tumors in both B6C3F1 and B6D2F1 male mice by mechanisms independent of ras oncogene activation and 30% of both benign and malignant liver tumors in the B6C3F1 mice regressed after exposure was discontinued.

A comparison of tumour incidence analyses applicable in single-sacrifice animal experiments

Monte Carlo methods are used to evaluate the operating characteristics of several trend tests for comparing incidence rates of occult tumours. The candidate tests are applicable in long-term animal experiments that have just one sacrifice time and no information on cause of death. When survival decreases with dose and the tumour is lethal, the tests that do not adjust for survival or that assume tumours are non-lethal behave conservatively, whereas the test that assumes tumours are rapidly lethal rejects the hypothesis of equal incidence rates too frequently. The only test which consistently operates at the correct level is one that specifies a constant difference between the death rates for animals with and without the tumour. Surprisingly, the test that specifies a constant ratio of these death rates often is conservative. Finally, a test based on a simple modification of the unadjusted test, which accounts for differential mortality by scaling down the size of the risk set, performs reasonably well in many cases. Among the tests that operate at the proper level, the constant risk difference test consistently exhibits high power across a wide range of situations.

Decreasing cardiovascular disease and increasing cancer among whites in the United States from 1973 through 1987. Good news and bad news

OBJECTIVE

Trends in cancer mortality, cardiovascular mortality, and cancer incidence are assessed among US whites to determine whether aging of the population and smoking patterns completely account for increased cancer rates from 1973 through 1987.

DESIGN

For mortality, percentage changes in age-specific rates were calculated. For cancer incidence, trends in age-specific rates across time periods and birth cohorts were assessed for several sites.

MAIN OUTCOME MEASURES

National US cardiovascular and cancer mortality rates and incidence rates for smoking-related cancer, breast cancer, and all other types of cancer in 10% of the US population covered by the National Cancer Institute's Surveillance, Epidemiology, and End Results Program were analyzed.

RESULTS

From 1973 through 1987, cardiovascular mortality decreased 42% in the age group 0 to 54 years and decreased 33% in the age group 55 to 84 years; concurrently, cancer mortality decreased 17% in the younger group but increased 12% in the older group. By 1987, even though proportionally fewer people in the older age groups died, relatively more of them died of cancer. Men born in the 1940s had twice as much cancer as those born in 1888 through 1897 and more than twice as much cancer not linked to smoking; women born during this period had 50% and 30% more of these same cancers, respectively. Rates of smoking-related cancers in recent cohorts of women were five to six times greater than in those born in 1888 through 1897, while rates in men declined. Recent cohorts of women also had more than twice as much breast cancer as those born in 1888 through 1897.

CONCLUSIONS

In recent US birth cohorts, our model found that increases in cancer have occurred that are not solely linked to aging of the population and smoking patterns. In light of these results and similar findings in Sweden, changes in carcinogenic hazards in addition to smoking are likely to have occurred and need to be studied further.

Evaluating constraints that allow survival-adjusted incidence analyses in single-sacrifice studies

Ideally, analyses of tumor incidence data from long-term animal experiments should allow the incidence rates to vary with age, without restricting tumor lethality, without requiring data on cause of death, and without assuming independence of competing risks. This article focuses on nonparametric methods that not only satisfy these conditions, but also accommodate studies having just one sacrifice time, by constraining the effects of age on one of the three functions used to characterize the likelihood. Several constrained analyses were evaluated using data on over 18,000 mice from the ED01 study. Of the five constraints considered, only the assumption of constant differences between the death rates for animals with and without the tumor of interest produced reasonable results across a wide range of tumor lethalities.

Exploring time trends in cancer incidence

We examined incidence time-trends for lung, stomach, intestinal, prostate, and breast cancer among Whites diagnosed in the United States between 1973 and 1987. For each sex and five-year age group, we modeled cancer incidence as a log-linear function of diagnosis-year to permit extrapolation over time and simple summarization of trends. Comparisons with nonparametric estimates show that, except for breast cancer, the model performs well. Plots of the annual percent change in incidence cf age illustrate the way in which time trends depend on age. Between 1973 and 1987, stomach cancer incidence decreased by about two percent per year. The annual change in lung cancer incidence progressed from a two to three percent decrease in persons under age 40 to an increase of two percent in men and eight percent in women by age 80. Intestinal cancer incidence decreased annually by as much as three percent in persons under age 50, remained constant in women aged 50 to 74, and otherwise increased about one percent per year. The annual increase in prostate cancer incidence declined from about six percent in men under age 40 to about two percent in men over age 80. After a surge in female breast-cancer diagnoses in 1974, the annual increase in incidence between 1980 and 1987 stabilized at four to six percent.

Constant risk differences in the analysis of animal tumorigenicity data

In a typical tumorigenicity study, most tumors are not observable in live animals and only a single (terminal) sacrifice is performed. This paper proposes a nonparametric, survival-adjusted analysis for these data that focuses on tumor incidence and yet does not require data on cause of death or assumptions about the tumor's lethality. The tumor onset/death process is most naturally characterized in terms of the tumor incidence rate and the death rates for tumor-free and tumor-bearing animals. The proposed approach, however, reparameterizes the problem in terms of the incidence rate, the death rate for tumor-free animals, and the difference between the death rates for tumor-free and tumor-bearing animals (i.e., the risk difference). The advantage of this alternative formulation is that a full likelihood analysis is possible with as few as one sacrifice time if the risk difference is assumed to be constant with respect to time. Data from the large ED01 study suggest that reasonable results can be obtained under the assumption of constant risk differences.

Using mortality data to estimate radiation effects on breast cancer incidence

In this paper we combine Japanese data on radiation exposure and cancer mortality with U.S. data on cancer incidence and lethality to estimate the effects of ionizing radiation on cancer incidence. The analysis is based on the mathematical relationship between the mortality rate and the incidence and lethality rates, as well as on statistical models that relate Japanese incidence rates to U.S. incidence rates and radiation risk factors. Our approach assumes that the risk of death from causes other than the cancer does not depend on whether or not the cancer is present, and among individuals with the cancer, the risk of death attributable to the cancer is the same in Japan and the U.S. and is not affected by radiation exposure. In particular, we focus on the incidence of breast cancer in Japanese women and how this incidence is affected by radiation risk factors. The analysis uses Japanese exposure and mortality data from the Radiation Effects Research Foundation study of atomic bomb survivors and U.S. incidence and lethality data from the Surveillance, Epidemiology, and End Results Registry. Even without Japanese incidence data, we obtain reasonable estimates of the incidence of breast cancer in unexposed Japanese women and identify the radiation risk factors that affect this incidence. Our analysis demonstrates that the age at exposure is an important risk factor, but that the incidence of breast cancer is not affected by the city of residence (Nagasaki versus Hiroshima) or the time since exposure.

Estimating tumor incidence rates in animal carcinogenicity experiments

Tumor incidence is the primary measure of carcinogenesis. This article focuses on estimating time-dependent incidence rates in animal experiments with few sacrifices. When the context of observation is known for none or all of the animals dying with the tumor of interest, previous results are obtained under relaxed assumptions. The link with existing semiparametric and nonparametric procedures based on latent failure times is exploited by using these methods to compute maximum likelihood estimates of the incidence rates without introducing latent random variables. Nonparametric estimators that are appropriate when all contexts of observation are known are generalized to the case in which the contexts of observation are unknown for a subset of the tumor-bearing animals.

Semiparametric analysis of tumor incidence rates in survival/sacrifice experiments

This paper addresses the problem of comparing treatment groups with respect to the rate of tumor development for animals in a survival experiment with some serial sacrifices. The analysis specifies a parametric model for the tumor incidence function, but places no parametric restrictions on the death rates. The procedure is feasible with as few as two sacrifice times and requires no individual data on cause of death. Other diseases need not act independently of the tumor of interest, nor are any restrictions imposed on tumor lethality or the relationship between the onset and death times for tumor-bearing animals. The proposed methods are illustrated with some survival/sacrifice data.

Logistic regression analysis of incidental-tumor data from animal carcinogenicity experiments

Survival differences can have a substantial impact on the statistical comparison of tumor development in control and treated animals and thus should be taken into account routinely in the analysis of carcinogenicity data from laboratory experiments. However, the appropriate survival adjustment depends on whether the tumor of interest is fatal or incidental. The usual analysis of incidental tumors, which adjusts for survival by stratifying the animals according to age at death, has various shortcomings. Alternatively, logistic regression methods allow a continuous survival adjustment and furnish a convenient framework for solving many of the problems associated with the age-stratified approach of grouping the data into time intervals. Logistic regression substitutes modeling the prevalence function for the arbitrary choice of time intervals, providing a survival adjustment (when the model holds) even when differential mortality might increase the bias or decrease the sensitivity of interval-based methods. The logistic analysis also can incorporate covariables which, if ignored, might confound the interpretation of the data. Several examples illustrate these potential advantages of basing the analysis of incidental tumors on logistic regression techniques.

Testing for a trend in tumor prevalence rates: I. Nonlethal tumors

In the analysis of animal carcinogenicity studies, the standard survival-adjusted test for a dose-related trend in the prevalence of nonlethal tumors is the Hoel-Walburg test, which stratifies on age at death by grouping survival times into intervals. An alternative analysis assesses trend on the basis of the likelihood score test under a logistic model for the prevalence function, which adjusts for survival by including age at death as a continuous regression variable. Extensive simulations demonstrate that the test based on modeling the prevalence log-odds as a linear function of age is more powerful than the Hoel-Walburg test, regardless of the intervals used by the latter to stratify the data. Without incorporating a continuity correction, the size of each test often exceeds the nominal level, especially when the mortality patterns differ across dose groups. Corrected versions of the tests operate at conservative levels, where the degree of conservatism varies with the distribution of the data. When the mortality patterns for the dose groups are similar, both tests have essentially the same power to detect a trend in tumor prevalence rates. However, when mortality varies with dose, the logistic regression test with a linear age term is more powerful than the Hoel-Walburg test, and this gain in power increases as the dose-specific mortality patterns become more disparate.

Occupational lead neurotoxicity: improvement in behavioural effects after reduction of exposure

To evaluate critical exposure levels and the reversibility of lead neurotoxicity a group of lead exposed foundry workers and an unexposed reference population were followed up for three years. During this period, tests designed to monitor neurobehavioural function and lead dose were administered. Evaluations of 160 workers during the first year showed dose dependent decrements in mood, visual/motor performance, memory, and verbal concept formation. Subsequently, an improvement in the hygienic conditions at the plant resulted in striking reductions in blood lead concentrations over the following two years. Attendant improvement in indices of tension (20% reduction), anger (18%), depression (26%), fatigue (27%), and confusion (13%) was observed. Performance on neurobehavioural testing generally correlated best with integrated dose estimates derived from blood lead concentrations measured periodically over the study period; zinc protoporphyrin levels were less well correlated with function. This investigation confirms the importance of compliance with workplace standards designed to lower exposures to ensure that individual blood lead concentrations remain below 50 micrograms/dl.

A randomized comparison of cyclophosphamide, Adriamycin, and 5-fluorouracil with triethylenethiophosphoramide and methotrexate, both as sequential and as fixed rotational treatment in patients with advanced ovarian cancer

The combinations of triethylenethiophosphoramide and methotrexate (TM) and cyclophosphamide, Adriamycin (doxorubicin), and 5-fluorouracil (CAF) were compared, both as sequential and fixed rotational treatments for advanced ovarian cancer, with L-phenylalanine mustard (L-PAM). Treatment with CAF produced a higher response rate (25% complete responses plus 31% partial responses) than treatment with L-PAM (15% complete responses plus 18% partial responses). A fixed rotation of TM and CAF resulted in longer survival (median of 15 months and 75th percentile of 27 months) than sequential treatment with TM initially, followed by CAF upon failure (median of 12 months and 75th percentile of 22 months). The fixed rotation of TM and CAF also increased progression-free survival (median of 12 months and 75th percentile of 24 months) over that achieved by initial treatment with TM (median of 6 months and 75th percentile of 15 months) or L-PAM (median of 9 months and 75th percentile of 21 months). Most patients (96%) on the fixed rotation were treated with both TM and CAF. Fewer patients (62%) on the sequential schedule with TM actually received both combination regimens, and even fewer patients (37%) beginning on CAF ever crossed over to TM. Patient age of 50 years or younger was a favorable prognostic factor for response, survival, and time to first treatment failure (progression-free survival). Disease Stage IIIA or IIIB, surgery including a bilateral salpingo-oophorectomy plus hysterectomy, and treatment within 6 months of initial diagnosis were favorable predictors for both survival and time to first treatment failure. Ambulatory performance status and well-differentiated disease were favorable prognostic factors for survival. Patients with unevaluable disease failed later than those with evaluable disease who, in turn, failed later than patients with measurable disease.

Occupational lead neurotoxicity: a behavioural and electrophysiological evaluation. Study design and year one results

To evaluate the effects of chronic lead exposure on the nervous system in adults, a set of neurobehavioural and electrophysiological tests was administered to 99 lead exposed foundry employees and 61 unexposed workers. Current and past blood lead concentrations were used to estimate the degree of lead absorption; all previous blood lead concentrations had been less than or equal to 90 micrograms/100 ml. Characteristic signs (such as wrist extensor weakness) or symptoms (such as colic) of lead poisoning were not seen. Sensory conduction in the sural nerve was not affected. By contrast, various neurobehavioural functions deteriorated with increasing lead burden. Workers with blood lead concentrations between 40 and 60 micrograms/100 ml showed impaired performance on tests of verbal concept formation, visual/motor performance, memory, and mood. Thus impairment in central nervous system function in lead exposed adults occurred in the absence of peripheral nervous system derangement and increased in severity with increasing lead dose.

Monitoring neurotoxins in industry: development of a neurobehavioral test battery

To facilitate the early detection of neurotoxicity in workers exposed to hazardous substances, we assembled a set of noninvasive neurobehavioral tests designed to assess memory, verbal concept formation, visuomotor performance, and mood. The tests were administered to an unexposed reference population and multivariate prediction equations were developed to control for effects of age, gender, and education. To define the briefest test set with appropriate sensitivity and specificity, we applied factor analysis and stepwise logistic regression to data from our referents and a group of lead-exposed workers. As a result, we have identified a comprehensive set of tests that can be administered in less than 40 minutes by trained medical technicians on plant premises and used to detect the early effects of neurotoxic substances on exposed workers. Guidelines for applying this technique to common work-place exposures, including other metals and organic solvents, are discussed.

Nonparametric estimation of lifetime and disease onset distributions from incomplete observations

In this paper we derive and investigate nonparametric estimators of the distributions of lifetime and time until onset associated with an irreversible disease that is detectable only at death. The nonparametric maximum likelihood solution requires an iterative algorithm. An alternative though closely related pair of estimators for the lifetime and onset distributions exists in closed form. These estimators are the familiar Kaplan-Meier estimator and an isotonic regression estimator, respectively. First-order approximations provide variance estimators. The proposed methods generalize and shed additional light on the constrained estimators presented by Kodell, Shaw and Johnson (1982, Biometrics 38, 43-58). Data from an animal experiment illustrate the techniques.

Nonparametric estimation for partially-complete time and type of failure data

Many statistical models focus on a random variable that represents time until failure and an indicator variable that denotes type of failure. When censoring mechanisms are introduced, an incomplete observation on the failure time often precludes observation of the indicator. In addition to conventional outcomes, for which observations on the time until failure and the type of failure are both complete or both incomplete, this paper considers partially-complete outcomes, for which only one of the random variables if fully observed. An iterative algorithm yields distribution-free estimates of the joint law governing this random pair; these estimates converge to the maximum likelihood solution. Recent developments permit approximations to the information and covariance matrices. Several special cases lead to closed-form estimates of the underlying distribution. Data from two recent clinical trials are used to illustrate the proposed techniques.