The Troubled History of Cancer Risk Assessment: The Linear-No-Threshold paradigm, which asserts there are no safe exposure levels, is the product of flawed and corrupted science.

AuthorCalabrese, Edward J.
PositionHEALTH & MEDICINE

When crafting regulations on exposure to carcinogens and other dangers, policymakers often vow to "follow the science" on what is safe and what is unsafe. But what if that science is flawed or grounded in questionable judgments--or worse?

In 1956, a National Academy of Sciences (NAS) panel formally recommended to the U.S. Government that it change how it assesses risk from ionizing radiation. That sounds innocuous enough, but the Biological Effects of Atomic Radiation (BEAR) I Genetics Panel's proposed change was momentous: switching from a threshold model in which exposure is deemed safe if kept below a certain level, to a linear model in which no exposure is considered safe. This recommendation would ultimately be accepted by leading regulatory and advisory bodies in the United States and internationally, and extended to other prospective hazards like chemical carcinogens.

As the saying goes, "As the twig is bent, so grows the tree." All subsequent cancer risk assessments in the United States and throughout the world would inherit the risk assessment paradigm from the NAS BEAR I Genetics Panel. But was this change sound?

X-RAY MUTATIONS

The NAS BEAR I Genetics Panel based its recommendation mainly on a strongly held belief that all radiation-induced mutation was unrepairable, irreversible, cumulative, and linear in the matter of dose response. However, the empirical evidence for this view was weak and equivocal. Yet the recommendation had considerable authority because the panel was deemed by opinion leaders, including the New York Times, as a virtual genetics "dream team" that included a Nobel laureate, a future laureate, and others of high achievement and prestige.

The origin of the Linear-No-Threshold (LNT) belief was borne in the judgment and passion of Hermann Muller, who was the first to claim that X-rays induced gene mutations. Muller had indeed made a momentous breakthrough in late 1926 when he found a way to produce quickly copious transgenerational phenotypic changes (e.g., alterations in size, color, or shape) in fruit flies, which he interpreted as being the result of gene mutation. This was something that no one else had done. Muller believed that he had discovered the long-sought mechanism of evolution, as he claimed that he had produced the "artificial transmutation of the gene." He even introduced the term "point mutation" (i.e., very small mutational gene change) into the geneticist's lexicon.

Muller rushed to publish his discovery after only the first of the three seminal experiments that would ultimately earn him a Nobel Prize. However, the first article, published in the journal Science, offered no data, instead presenting a discussion of his observations. Several months later and with considerable suspense, he unveiled the data at a large conference in Berlin, to great acclaim. (The relevance of all this will be explained below.) His star rose meteorically...

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