Radiation Biology of Low Doses

All current radiation risk estimates and all radiation-protection standards and practices are based on the so-called “Linear No-Threshold Hypothesis” which states that risk is linearly proportional to dose, without a threshold. This hypothesis therefore predicts that:

• every dose, no matter how low, carries with it some risk

• risk per unit dose is constant, additive, and can only increase with dose

• biological variables are insignificant compared to dose

This talk summarizes results from some of our low dose and/or low dose rate experiments with low LET radiation in human and rodent cells, and in animals, and determines if the results support or reject the LNT hypothesis as it affects the risk of most concern, cancer. It is important to recognize that cancer arises from changes in a single cell and, therefore, this defines the limits of the meaning of “low dose”. Unlike the concept of whole body dose, where dose is averaged over all cells in the body, a single cell is the smallest volume that is relevant for carcinogenic risk. The lowest possible dose is, therefore, that dose which can be deposited in a single cell.

       It is also important to recognize some physical characteristics of radiation:

• radiation deposits energy, and damage, in tracks

• the smallest dose a cell can receive is that deposited by a single track

• at total doses which are less than one track/cell, not all cells are hit, i.e., some cells receive no dose; however, those that are hit still receive the dose deposited by one track.

While the lowest possible dose to a cell is that deposited by one track, the actual dose depends on the nature of the radiation. For example, a single alpha particle track can deposit tens of cGy while a single ⁶⁰Co-g ray will deposit, on average, about 1 mGy.

       When DNA damage is created by radiation in a cell, there are three possible outcomes, an error-free repair which restores the cell to normal, cell death by apoptosis, or error prone repair that creates a mutation and cancer risk. The LNT hypothesis predicts that risk is influenced only by dose, and therefore that the relative proportions of these three biological possibilities must be constant. If they were not constant, then risk would vary with their relative proportions, i.e., not only as a function of dose.

We have tested the influence of prior low doses and low dose rate exposures on the ability of normal human skin cells to repair subsequent acute radiation damage to DNA, which results in breaks in chromosomes. The combined exposure resulted in less broken chromosomes (measured as reduced micronucleus frequency) than the single acute exposure alone. The low dose rate exposure stimulated the cells to increase their ability to repair broken chromosomes, such that the consequences of a second large exposure were reduced. The same result occurred if the initial exposure was 500 mGy, or was 1 mGy, the lowest g dose possible in a single cell (Figure 1).

Figure 1. Ability to repair broken chromosomes in cells adapted by exposure to low doses

This adaptive response to low doses of radiation can be seen in many other situations. For example, the influence of a low dose on cell death by apoptosis has also been tested. Those results show that low doses amplify the probability of apoptotic cell death resulting from a second exposure. This sensitization of cells to radiation-induced cell death increases the probability that a cell will die rather than survive with a mutation, another type of adaptive response that is believed to reduce cancer risk in the whole organism.

As a measure of the overall effect of these processes, we used an assay that measures the frequency at which rodent cells in tissue culture are transformed into cancer cells. We showed that a low dose rate exposure immediately before a large acute exposure did not further increase risk, as predicted by the LNT hypothesis, but actually decreased cancer risk by 2-3 fold (Table 1). In the absence of the second large exposure, an average of one track per cell (1 mGy) reduced the risk of cancer formation below that which occurred spontaneously in the absence of any radiation exposure. Higher doses, up to 100 mGy delivered at a low dose rate, produced the same 2-3 fold reduction in spontaneous transformation risk (Table 2). Since at 1 mGy not all cells actually receive a track of radiation, these results also indicate that some cells are protected in response to signals received from other cells that did receive a radiation track, an example of the bystander effect for adaption to radiation.

Table 1. Reduction in the risk of radiation-induced malignant transformation by a prior chronic exposure

Table 2.    The influence of low doses delivered at low dose rate (2.4 mGy/min) on the risk of spontaneous malignant transformation.

The results show that low dose radiation induces an increase in error-free DNA repair competence. That repair system increases the probability of correctly repairing either radiation-induced or spontaneous DNA damage, or of triggering cell death if the repair is incorrect. This response therefore reduces the overall risk of either radiation-induced or spontaneous transformation to malignancy. It is apparent from these experiments that biological variables are important in determining the consequences of radiation exposures and that the risk of DNA damage is neither constant, nor additive, nor increasing with dose. Low doses or doses delivered at low dose rate reduce rather than increase risk in normal cells. The results contradict the LNT hypothesis.

We have reported the results of similar investigations in mice. In one experiment, low doses of in vivo b-irradiation of mouse skin 24 h prior to treatment with a DNA damaging chemical carcinogen reduced pre-malignant tumor frequency by about 5-fold. This result is consistent with the cell-based studies described above. It implies that the radiation exposure stimulated an error-free DNA repair system that was able to recognize and remove much of the chemically produced DNA damage. In another experiment, a prior low dose exposure delivered at low dose rate delayed the onset of myeloid leukemia induced in genetically normal mice by a subsequent exposure to a large dose (Table 3).

Table 3.     Extension of latency period in mice developing acute myeloid leukemia

                                                         Average                                  Average
                 Treatment                       Lifespan (Days)                  Life Lost (Day

                 Control                                    727                                          0
                 1.0 Gy                                     486                                          241
                 100 mGy, 24h, 1.0 Gy             578                                          149

 

The protective responses observed in mammalian cells and in animals are consistent with those seen in lower eukaryotes, including yeast, indicating that they are evolutionarily conserved and lending credence to the idea that such responses are the normal and expected consequences of low dose exposures.

It seems clear that in normal cells and normal adult animals, low doses and low dose rate exposures to low LET radiation decrease rather than increase cancer risk. However, for human radiation protection, the effects of low doses in two other important situations, exposure of cancer prone individuals and exposure of a fetus, are less clear and we are investigating those problems. We have recently examined cancer risk after low dose, low dose rate exposure in mice that are radiation sensitive and cancer prone due to a genetic defect (heterozygosity for the gene Trp53). We showed that a single low dose (10 mGy) protected these mice against spontaneous cancer formation by increasing tumor latency (Figure 2), the same protective response seen in genetically normal mice (Table 3).

Figure 2. Latency of spinal osteosarcomas in Trp53 +/- mice

The low dose restored about half the life span lost solely as a result of the genetic defect in the absence of radiation, a change similar to that seen in normal mice (Table 3).

Another study examining malformations in irradiated fetal mice showed that low doses can also induce an adaptive response that protects against radiation induced teratogenic effects (shortened tails, Figure 3), although this can occur only at certain times of organ development, and defects in the Trp53 gene can alter that protection (Figure 3).

Figure 3. The influence of a prior low dose on reduced tail length resulting from a high radiation exposure. Fetal mice were exposed to 30 cGy, 24h prior to a 4 Gy exposure on gestational day 11. Tail lengths were measured on gestational day 18. Circles, Trp53 normal; Triangles, Trp53 heterozygous; Closed symbols, 4 Gy; Shaded symbols, 30 cGy + 4 Gy; Open symbols, controls.