Assessing cancer risks of low-dose radiation

Low-level ionizing radiation from noninvasive cardiac imaging: can we extrapolate estimated risks from epidemiologic data to the clinical setting?

Clinical decision-making regarding the use of low-level ionizing radiation for diagnostic and/or therapeutic purposes in patients with cardiovascular disease must, as in all other clinical scenarios, encompass the broad range of the risk-benefit ratio. Concerns regarding the late carcinogenic effects of exposure to low levels, i.e., <100 mSv, of ionizing radiation stem from extrapolation of exposure-outcome data in survivors of World War II atomic bomb explosions. However, ongoing debate regarding the true incremental risk to subjects exposed to doses currently administered in cardiovascular procedures fails to take into account the uncertainty of the dose-response relationship in this lower range, as well as tissue-specific reparative responses, also manifest at lower levels of exposure. The present discussion draws attention to both of these aspects as they relate to clinical decision-making.

Xrcc2 modulates spontaneous and radiation-induced tumorigenesis in Apcmin/+ mice

XRCC2 has an important role in repair of DNA damage by homologous recombination. Adult Apc(min/+) (min, multiple intestinal neoplasia) mice, wild-type or heterozygous for Xrcc2 deficiency, were sham-irradiated or 2-Gy X-irradiated. Spontaneous mammary and intestinal tumor incidences are lower in Apc(min/+) Xrcc2(+/-) mice than in Apc(min/+) Xrcc2(+/+) mice (mammary tumors: 14% and 38%, respectively, χ(2) P = 0.03; intestinal adenomas in mice reaching full life span: 108.6 and 130.1, respectively, t-test P = 0.005). Following irradiation, the increase in mammary tumors was greatest in female mice heterozygous for Xrcc2 (7.25 ± 0.50-fold in Apc(min/+) Xrcc2(+/-) mice compared with 2.57 ± 0.35-fold in Apc(min/+) Xrcc2(+/+) mice; t-test P < 0.001). The increase in intestinal tumor multiplicity following irradiation was significantly greater in Apc(min/+) Xrcc2(+/-) mice (Apc(min/+) Xrcc2(+/-), 4.14 ± 0.05-fold, versus Apc(min/+) Xrcc2(+/+), 3.30 ± 0.05-fold; t-test P < 0.001). Loss of heterozygosity of all chromosome 18 markers was greater in intestinal tumors from Apc(min/+) Xrcc2(+/-) mice than in tumors from Apc(min/+) Xrcc2(+/+) mice. These findings indicate that Xrcc2 haploinsufficiency reduces spontaneous tumor incidence on an Apc(min/+) background but increases the tumorigenic response to radiation.

Genomic and gene expression signatures of radiation in medulloblastomas after low-dose irradiation in Ptch1 heterozygous mice

Accurate cancer risk assessment of low-dose radiation poses many challenges that are partly due to the inability to distinguish radiation-induced tumors from spontaneous ones. To elucidate characteristic features of radiation-induced tumors, we analyzed 163 medulloblastomas that developed either spontaneously or after X-ray irradiation at doses of 0.05-3 Gy in Ptch1 heterozygous mice. All spontaneous tumors showed loss of heterozygosity in broad regions on chromosome 13, with losses at all consecutive markers distal to Ptch1 locus (S-type). In contrast, all tumors that developed after 3 Gy irradiation exhibited interstitial losses around Ptch1 with distal markers retained (R-type). There was a clear dose-dependent increase in the proportion of R-type tumors within the intermediate dose range, indicating that the R-type change is a reliable radiation signature. Importantly, the incidence of R-type tumors increased significantly (P = 0.007) at a dose as low as 50 mGy. Integrated array-comparative genomic hybridization and expression microarray analyses demonstrated that expression levels of many genes around the Ptch1 locus faithfully reflected the signature-associated reduction in genomic copy number. Furthermore, 573 genes on other chromosomes were also expressed differently between S-type and R-type tumors. They include genes whose expression changes during early cerebellar development such as Plagl1 and Tgfb2, suggesting a recapitulation of gene subsets functioning at distinct developmental stages. These findings provide, for the first time, solid experimental evidence for a significant increase in cancer risk by low-dose radiation at diagnostic levels and imply that radiation-induced carcinogenesis accompanies both genomic and gene expression signatures.

Strong association between cancer and genomic instability

After a first wave of radiation-induced chromosomal aberrations, a second wave appears 20-30 cell generations after radiation exposure and persists thereafter. This late effect is usually termed "genomic instability". A better term is "increased genomic instability". This effect has been observed in many cell systems in vitro and in vivo for quite a number of biological endpoints. The radiation-induced increase in genomic instability is apparently a general phenomenon. In the development of cancer, several mutations are involved. With increasing genomic instability, the probability for further mutations is enhanced. Several studies show that genomic instability is increased not only in the cancer cells but also in "normal" cells of cancer patients e.g. peripheral lymphocytes. This has for example been shown in uranium miners with bronchial carcinomas, but also in untreated head and neck cancer patients. The association between cancer and genomic instability is also found in individuals with a genetic predisposition for increased radiosensitivity. Several such syndromes have been found. In all cases, an increased genomic instability, cancer proneness and increased radiosensitivity coincide. In these syndromes, deficiencies in certain DNA-repair pathways occur as well as deregulations of the cell cycle. Especially, mutations are seen in genes encoding proteins, which are involved in the G(1)/S-phase checkpoint. Genomic instability apparently promotes cancer development. In this context, it is interesting that hypoxia, increased genomic instability and cancer are also associated. All these processes are energy dependent. Some strong evidence exists that the structure and length of telomeres is connected to the development of genomic instability.

Low-dose cancer risk modeling must recognize up-regulation of protection

IONIZING RADIATION PRIMARILY PERTURBS THE BASIC MOLECULAR LEVEL PROPORTIONAL TO DOSE, WITH POTENTIAL DAMAGE PROPAGATION TO HIGHER LEVELS: cells, tissues, organs, and whole body. There are three types of defenses against damage propagation. These operate deterministically and below a certain impact threshold there is no propagation. Physical-static defenses precede metabolic-dynamic defenses acting immediately: scavenging of toxins; - molecular repair, especially of DNA; - removal of damaged cells either by apoptosis, necrosis, phagocytosis, cell differentiation-senescence, or by immune responses, - followed by replacement of lost elements. Another metabolic-dynamic defense arises delayed by up-regulating immediately operating defense mechanisms. Some of these adaptive protections may last beyond a year and all create temporary protection against renewed potentially toxic impacts also from non-radiogenic endogenous sources. Adaptive protections have a maximum after single tissue absorbed doses around 100 to 200 mSv and disappear with higher doses. Low dose rates initiate maximum protection likely at lower cell doses delivered repetitively at certain time intervals. Adaptive protection preventing only about 2 - 3 % of endogenous life-time cancer risk would fully balance a calculated induced cancer risk at about 100 mSv, in agreement with epidemiological data and concordant with an hormetic effect. Low-dose-risk modeling must recognize up-regulation of protection.

The dose window for radiation-induced protective adaptive responses

Adaptive responses to low doses of low LET radiation occur in all organisms thus far examined, from single cell lower eukaryotes to mammals. These responses reduce the deleterious consequences of DNA damaging events, including radiation-induced or spontaneous cancer and non-cancer diseases in mice. The adaptive response in mammalian cells and mammals operates within a certain window that can be defined by upper and lower dose thresholds, typically between about 1 and 100 mGy for a single low dose rate exposure. However, these thresholds for protection are not a fixed function of total dose, but also vary with dose rate, additional radiation or non-radiation stressors, tissue type and p53 functional status. Exposures above the upper threshold are generally detrimental, while exposures below the lower threshold may or may not increase either cancer or non-cancer disease risk.