Malignant Mesothelioma of the pleura: current surgical pathology by © Robert C. Byrd Center For Rural Health

The usual gross and microscopic features of malignant mesolthelioma are well described in many standard texts, and require no reiteration in this volume. Instead, this account concentrates on the unusual or controversial, on the still-evolving role of immunohistochemistry for the discrimination between mesotheloma and its look-alikes, and on the differential diagnosis in pleural biopsies.

Malignant pleural mesothelioma

Malignant mesothelioma (MM) has recently attracted the attention of the media because of its relationship with professional and environmental exposure to asbestos. This tumour of the pleura is a disease which has emerged in significant numbers of patients during the last 30 years in the industrialized countries and its increasing incidence makes it of socio-economical interest.

The histological description of MM was first published by E. Wagner in 1870 and later by Klemperer and Rabin. A literature review of pathological cases of lung diseases befor 1940 identified 41 out of 46,000 autopsies as possible MM. In this review they mentioned a report from 1767 by Lieutaud who was the first to describe two possible cases of MM in an autopsy study. Since then, a number of case reports were published in which a relationship with asbestos exposure ws considered important but it was the report of J.C. Wagner in 1960 which identified a clear relationship between exposure to crocidolite mining and the development of MM. From that moment the association between asbestos exposure and MM ws accepted and a beginning was made to abandon the productionand processing of asbestos materials.

Background

The best scientific evidence of human radiation effects initially came from epidemiologic studies of atomic bomb survivors in Hiroshima and Nagasaki. While no evidence of genetic effects has been found, these studies showed a roughly linear relationship between the induction of cancer and extremely high dose-rate single high doses of atomic bomb radiation. This was consistent with the knowledge that ionizing radiation can damage DNA in linear proportion to high-dose exposures and so produce gene mutations known to be associated with cancer. In the absence of comparable low dose effects it was prudent to propose tentatively the no threshold hypothesis that extrapolates linearly from effects observed at very high doses to the same effects at very low doses. It was accepted in 1959 by the International Commission on Radiological Protection (ICRP)1 and afterwards adopted by national radiation protection organizations to guide regulations for the protection of occupationally exposed workers and the public.2 This hypothesis that all radiation is harmful in linear proportion to the dose, is the principle used for collective dose calculations of the number of deaths produced by any radiation, natural or generated, no matter how small. The National Council of Radiation Protection and Measurements Report 121, quot;Principles and Application of Collective Dose in Radiation Protection," summarizes the basis for adherence to linearity of radiation health effects:3 "Taken as a whole, the body of evidence from both laboratory animals and human studies allows a presumption of a linear no threshold response at low doses and low-dose rates, for both mutations and carcinogenesis. Therefore, from the point of view of the scientific bases of collective doses for radiation protection purposes, it is prudent to assume the effect per unit dose in the low-dose region following single acute exposures or low-dose fractions in a linear response. There are exceptions to this general rule of no threshold, including the induction of bone tumors in both laboratory animals and in some human studies due to incorporated radionuclides, where there is clearly evidence for an apparent threshold. However, few experimental studies, and essentially no human data, can be said to prove or even to provide direct support for the concept of collective dose with its implicit uncertainties of nonthreshold linearity and dose-rate independence with respect to risk. The best that can be said is that most [sic] studies do not provide quantative data that, with statistical significance, contradict the concept of collective dose. Ultimately, confidence in the linear no threshold dose-response relationship at low doses is based on our understanding of the basic mechanisms involved. Genetic effects may result from a gene mutation, or a chromosome aberration. The activation of a dominant acting oncogene is frequently associated with leukemias and lymphomas, while the loss of suppressor genes appears to be more frequently associated with solid tumors. It is conceptually possible, but with a vanishing small probability, that any of these effects could result from the passage of a single charged particle, causing damage to DNA that could be expressed as a mutation or small deletion. It is a result of this type of reasoning that a linear nonthreshold dose-response relationship cannot be excluded. It is this presumption [sic], based on biophysical concepts, which provides a basis for the use of collective dose in radiation protection activities." NCRP Report 121 summarizes that while some studies "provide quantitative data that, with statistical significance, contradict the concept of collective dose," "ultimately, confidence in the linear no threshold dose-response relationship at low doses [LNT hypothesis] is based on our understanding of the basic mechanisms involved." Current understanding of the basic biologic mechanisms involved and recent data will be examined after presenting some of the statistically significant epidemiologic data that contradict the LNT hypothesis. The biologic data also contradict "the presumption, based on biophysical concepts, which provides a basis for the use of collective dose in radiation protection activities."

Epidemiologic Studies

What are some of the statistically significant epidemiologic studies that demonstrate risk decrements (hormesis) as predicted by the adaptive responses to low-dose radiation of the DNA damage-control biosystem? 4 For several decades increased longevity and decreased cancer mortality have been reported in populations exposed to high background radiation. Established radiation protection authorities consider such observations to be spurious or inconclusive because of unreliable public health data or undetermined confounding factors such as pollution of air, water and food, smoking, income, education, medical care, population density, and other socioeconomic variables. Recently, however, several epidemiologic statistically significant controlled studies have demonstrated that exposure to low or intermediate levels of radiation are associated with positive health effects. Dr. Zbigniew Jaworowski, past chairman of UNSCEAR, in his current review of hormesis cites recent data showing hormetic effects in humans from the former Soviet Union.5 After radiation exposure from a thermal explosion in 1957, 7852 persons living in 22 villages in the Eastern Urals were divided into three exposure groups averaging 49.6 cGy, 12.0 cGy, and 4.0 cGy and followed for 30 years. Tumor-related mortality was 28%, 39%, and 27% lower in the 49.6 cGy, 12.00 cGy, and 4.0 cGy groups, respectively, than in the nonirradiated control population in the same region. In the 49.6 cGy and 12.0 cGy groups the difference from the controls was statistically significant . Epidemiologic studies showing beneficial effects of low doses of radiation in atomic bomb survivors  and other populations were reviewed by Sohei Kondo, Professor of Radiation Biology, Atomic Energy Research Institute, Kinki University, Osaka, Japan.6 Included are the apparently beneficial effects of low doses of external gamma rays on the life span of radium-dial painters and the significantly lower mortality from cancers at all sites of residents of Misasa, an urban area with radon spas, than residents of the suburbs of Misasa . [INLINE] These beneficial effects are consistent with the findings of B. L. Cohen, Professor of Physics, University of Pittsburgh, that relate the incidence of lung cancer to radon exposure in nearly 90% of the population of the United States.7 The 1601 counties selected for adequate permanence of residence provide extremely high-power statistical analysis. After applying the BEIR IV 8 correction for variations in smoking frequency, the study shows a very strong tendency for lung cancer mortality to decrease with increasing mean radon level in homes, in sharp contrast to the BEIR IV theoretical increased mortality derived by linear no threshold extrapolation of effects in uranium miners exposed to very high radon concentrations. The discrepancy between theoretical and measured slopes is 20 standard deviations. Rigorous statistical analysis of 54 socioeconomic, seven physical, and multiple geographic variables as possible confounding factors, both single and in combination, demonstrates no significant decrease in the discrepancy. The multiple independent requirements that a possible unknown confounding factor must meet, make its existence highly improbable. A reasonable explanation is that stimulated biological mechanisms more than compensate for the radiation "insult" and are protective against cancer in a low-dose, low-dose-rate range. The thirteen-year U.S. Nuclear Shipyard Workers study of the health effects of low-dose radiation was performed by the Johns Hopkins Department of Epidemiology, School of Public Health and Hygiene, reported to the Department of Energy in 1991 9 and reported in UNSCEAR 1994.4 Professor Arthur C. Upton, who concurrently chaired the NAS BEIR V Committee on "Health Effects of Exposure to Low Levels of Ionizing Radiation," 10 chaired the Technical Advisory Panel that advised on the research and reviewed results. The results of this study contradict the conclusions of the BEIR V report 10 that small amounts of radiation have risk - the LNT hypothesis. From the database of almost 700,000 shipyard workers, including about 108,000 nuclear workers, three closely matched study groups were selected, consisting of 28,542 nuclear workers with working lifetime doses 5 mSv (many received doses well in excess of 50 mSv), 10,462 nuclear workers with doses <5 mSv and 33,352 non-nuclear workers. Deaths in each of the groups were classified as due to: all causes, leukemia, lymphatic and hematopoietic cancers, mesothelioma, and lung cancer. The results demonstrated a statistically significant decrease in the standardized mortality ratio for the two groups of nuclear workers for 'death from all causes' compared with the non-nuclear workers. For the 5 mSv group of nuclear workers, the highly significant risk decrement to 0.76, 16 standard deviations below 1.00, of the standard mortality ratio for death from all causes is inconsistent with the LNT hypothesis and does not appear to be explainable by the healthy worker effect 4. The non-nuclear workers and the nuclear workers were similarly selected for employment, were afforded the same health care thereafter, and performed the identical type of work, except for exposure to 60 Co gamma radiation, with a similar median age of entry into employment of about 34 years. This provides evidence with extremely high statistical power that low levels of ionizing radiation are associated with risk decrements. Nevertheless, Professor Arthur C. Upton and others consider the three-country low-dose radiation and cancer study of Cardis, et al11,12, to be the best occupational study of nuclear workers . This study concluded, "There was no evidence of an association between radiation dose and mortality from all causes or from all cancers. Mortality from leukemia, excluding chronic, lymphocytic leukemia (CLL) ...was significantly associated with cumulative external radiation dose (one-sided P value = 0.046: 119 deaths)." The statistical methods used state, "As there was no reason to suspect that exposure to radiation would be associated with a decrease in risk of any specific type of cancer, one-sided tests are presented throughout." The authors' analysis of the 119 deaths from all leukemias except CLL excluded 86 deaths in dose categories 1.3.4, and 6 in which there were fewer deaths than expected. Trend analysis of the remaining 33 deaths in dose categories 2, 5, and 7 for estimated P=0.046 was obtained "using computer simulations based on 5000 samples, rather than the normal approximation."11

Epidemiologic Studies  (cont.)

The Canadian Breast Cancer Fluoroscopy Study13 reports the observations of the mortality from breast cancer in a cohort of 31,710 women who had been examined by multiple fluoroscopy between 1930 and 1952. The observed rates of mortality are related to breast radiation doses and presented only in tabular form. The authors compare linear and linear-quadratic dose-response models fit to the data and conclude, "that the most appropriate form of dose-response relations is a simple linear one, with different slopes for Nova Scotia and the other provinces." On the basis of this linear model that includes only non-significant data and excludes the data with the highest confidence limits , the authors predict the lifetime excess risk of death from breast cancer after a single exposure at age 30 to 1 cGy(1r) to be approximately 60 per million women or 900 per million women exposed to 15 cGy. The observed data, however, demonstrate with high statistical confidence, a reduction of the relative risk of breast cancer to 0.66 (P=0.05) at 15 cGy and 0.85 (P=0.32) at 25 cGy. The second author, in his 1996 revision of this study, removed this highly significant contradiction of the LNT hypothesis by lumping all low-dose data into a single 1-49 cGy category.14 The study actually predicts that a dose of 15 cGy would be associated with 7,000 fewer deaths in these million women. Lauriston S. Taylor, past president of the NCRP, considered application of LNT theory for calculations of collective dose as, "deeply immoral uses of our scientific heritage"15. METABOLIC AND RADIATION DNA DAMAGE CONTROL During the past decade rapid advances in our knowledge of molecular biology and cell function enable us to understand why low-dose radiation is associated with positive health effects in contrast to the carcinogenic effect of high-dose radiation. Our understanding is based upon current, cellular molecular biology observations. Estimates are based on published data and recent personal communications: * Two to three percent of all metabolized oxygen is converted to free radicals (reactive oxygen species),16 10E10/cell/d, that produce about 10E6 DNA oxidative adducts/cell/d.7, 8 These include about 0.5 double strand breaks/cell/d.17 In addition, a relatively small number of metabolic DNA alterations are produced by DNA replication and thermal instability.19 By comparison, 1 cGy low LET radiation produces 20 DNA oxidative adducts/cell that include an average of 0.4 double strand breaks/cell.18,19 * Over eons of time, as multicellular animals developed and metabolized oxygen, a complex DNA damage-control biosystem evolved .17 The damage corresponding to 10E10 free radicals/cell/d is largely prevented by antioxidants that scavenge approximately 99% of these free radicals. The resultant 10E6 DNA oxidative adducts/cell/d are reduced by enzymatic repair to about 10E2 mis/unrepaired DNA alterations/cell/d. Apoptosis, differentiation, necrosis, and the immune system remove approximately 99% of these mis/unrepaired DNA alterations so that an average of 1 mutation/cell/d (possibly up to 2-3) accumulates during the lifetime of a stem cell to decrease DNA damage-control capability with associated aging and malignant growth .17 Cancer increases as the third to fifth power of age. This remarkably efficient biosystem prevents precocious aging and malignancy unless impaired by genetic defects, or damaged by high doses of radiation or other toxic agents. 9, 16-19, 22-33 * How does background radiation add to the metabolic accumulation of mutations? A much larger fraction of double strand breaks occurs in DNA oxidative adducts produced by radiation than in those produced by metabolism (2x10E-2 vs 5x10E -7).17,21 The mis/unrepaired fraction of these double strand breaks is also much larger than that of other metabolic DNA oxidative adducts (10E-1 vs 10E-4). Nevertheless, the number of metabolic DNA oxidative adducts (10E6/cell/d) is so much greater than the number of oxidative adducts from low LET background of 0.1 cGy/y (5x10 -3/cell/d), that an average of only 10E -7 radiation mutation/cell/d is added to 1 metabolic mutation/cell/d .17 

Response to Low-Dose Radiation

The activity of the DNA damage control biosystem is decreased by high-dose radiation, but adaptively responds with increased activity to low-dose radiation.

9, 22,23,26-33 The efficiency of this biosystem is increased by the adaptive responses to low-dose ionizing radiation .This is well documented in UNSCEAR 1994:4 "There is substantial evidence that the number of radiation-induced chromosomal aberrations and mutations can be reduced by a small prior conditioning dose in proliferating mammalian cells in vitro and in vivo. There is increasing evidence that cellular repair mechanisms are stimulated after radiation-induced damage... Whatever the mechanisms, they seem able to act not only on the lesions induced by ionizing radiation but also on at least a portion of the lesions induced by some other toxic agents. As to the biological plausibility of a radiation-induced adaptive response, it is recognized that the effectiveness of DNA repair in mammalian cells is not absolute...

An important question, therefore, is to judge the balance between stimulated cellular repair and residual damage." This statement applies not only to the mutations produced by radiation and other toxic agents, but also to the unmentioned enormous number of daily metabolic mutations. The operative effect of reducing metabolic mutations by the adaptive response of the DNA damage-control biosystem to low-dose radiation is the critical factor, not reduction of the relatively negligible number of mutations produced by low-dose radiation. This critical factor must be considered, "to judge the balance between stimulated cellular repair and residual damage." Assuming a 20% increased efficiency of biosystem control in response to a tenfold increase of annual background radiation from 0.1 cGy/y, to 1 cG/y, radiation mutations would indeed increase from 1x10-7/cell/d to 8x10-7/cell/d but metabolic mutations would decrease from 1/cell/d to 0.8/cell/d (Figure 11).17

"The balance between stimulated cellular repair and residual damage" is a decrease of mutations from an average of 1 mutation/cell/d to 0.8 mutation/cell/d UNSCEAR did not consider that the increase of radiation mutations is negligible compared to the operative effect of the adaptive response to low-dose radiation upon the high background of metabolic mutations. The biologic effect of radiation is not determined by the number of DNA mutations it creates, but by its effect on the biosystem that controls the relentless enormous burden of oxidative DNA damage.

High-dose radiation impairs this biosystem with consequent significant increase of metabolic mutations and corresponding risk increments. Low-dose radiation stimulates the DNA damage-control biosystem with consequent significant decrease of metabolic mutations and corresponding risk decrements (Figures 8-11).35 This reduction of gene mutations in response to low-dose radiation provides a biological explanation of the statistically significant observations of mortality and cancer mortality risk decrements, and contradicts the biophysical understanding of the basic mechanisms upon which, ultimately, the NCRP's confidence in the LNT hypothesis is based. This article represents the views of the author and not necessarily those of the U.S. Nuclear Regulatory Commission.

Mesothelioma: Occupational and Enviornmental Medicine

My comments are being made on the behalf of the Art and Creative Materials Institute, a non-profit trade organization that represents the major manufacturers and importers of art materials in the United States. Talc is a common component of  these art materials. I would like to address several issues discussed in the draft Report on Carcinogens: Background Document for Talc. Asbestiform and Non-Asbestiform. These comments are offered to the Report on Carcinogens Subcommittee with the expectation that this report can be strengthened if it addresses certain issues in more detail. I will comment on both on studies concerning both asbestiform and non- asbestiform talc.

Asbestiform Talc

Definition: The draft report discusses the definition of asbestiform fibers. It would be strengthened if it includes NIOSH's definition of these fibers:. NIOSH ( Kullman, et al. 1995) defines asbestiform habit as:

 "a specific type of mineral fibrosity in which the growth is primarily in one dimension and the crystals form naturally as long, flexible fibers. Fibers can be found in bundles that can be easily separated into smaller bundles or ultimately into fibrils."

This definition is important since many of the fibers in asbestiform talc are cleavage fragments. NIOSH's definition for asbestiform habit contrasts with their definition for the nonasbestiform habit :

"These minerals have . crystal habits where growth proceeds in two or three dimensions instead of one dimension. When milled, these minerals do not break into fibrils but rather into fragments

resulting from cleavage along the two or three growth planes. Particles formed by the comminution of these minerals are referred to as cleavage fragments."

Respirable fiber size:  Although the draft report notes that a respirable fiber has a diameter of 3-4 m m this is for fibers with a density of 1. Talc has a specific gravity of 3 and, consequently the equivalent aerodynamic diameter of respirable talc fibers would be 1/3 of this, on the order of 1 m m (Wylie, et al. 1993).  This finding is particularly important in that the fibers in asbestiform talc are primarily wider than 1 m m with only 10-11% of fibers in commercial talcs being <1 m m in diameter.

Fiber size and cancer risk:   There are excellent animal models for the relationship between fiber dimension and risk of both mesothelioma and lung cancer. For mesothelioma risk, fibers with a dimension of £ 0.25 m m in diameter and >8 m m long appear to present the greatest risk (Stanton, et al., 1981; Oehlert, 1991) with almost no risk presented by short fibers (Davis, et al. 1986). Most amphibole fibers in a asbestiform talc mine are shorter than 10 m m (Kelse and Thompson, 1989) and would not be expected to present a risk of mesotheliomas.  Similarly, lung cancer risk also depends on fiber dimensions. Based on asbestos inhalation studies, Berman et al (1995) found that potency for lung cancer rested with fibers that were longer than 10 m m and less than 0.3 m m in diameter. Their model found that fibers that were <10 m m long and had widths from 0.3-5.0 m m were not associated with a lung cancer risk. Lippmann (1988) performed as similar analysis. He found that fiber retention drops rapidly as fiber diameter increases from 0.8 to 2.0 m m. No lung cancer risk was associated with fiber length less than 5 m m. Lung cancer risk was associated with fibers with a diameter of 0.3-0.8 m m and a substantial fraction >10 m m in length.

Animal Studies:  Although IARC considered a number of studies involving the carcinogenicity of talc in experimental animals, they did not have access to identification information concerning several of the fibrous talcs.  This is particularly important because talcs form the Grouvenor Talc Company (GTC), the mine most studied for cancer risk, have been examined in a number of animal models and have been found to be non-carcinogenic. Stanton, et al. (1981) examined two asbestiform talcs from the Grouvenor talc district including one from GTC (Stanton talc #6) in their pleural implantation rat model. Neither of these talcs induced mesotheliomas although based on particle dimensions, a 60% incidence of mesotheliomas would have been expected with the GTC talc. Oehlert (1991) re-analyzed the Stanton data, breaking out potency assessments not only by particle size but by mineral type. When compared to asbestos, the author found that talcs were 1/135,000 as potent for causing pleural tumors. This re-analysis included both the asbestiform talcs and 5 non-asbestiform talcs studied by Stanton, et al.

Smith, et al. (1979) also studied one GTC talc (FD14) in their hamster pleural mesothelioma model. This talc, as well as another talc containing amphibole fibers, was negative in their model.

Wylie, et al. (1997) studied the FD14 talc from the Smith et al. study in an in vitro system. It was not cytotoxic and did not induce cell proliferation. Talc samples not containing quartz were not cytotoxic where asbestos was both cytotoxic and induced proliferation.