X-rays are used in diagnostic and interventional radiology, and for treatment of certain benign and malignant diseases. Although these procedures utilize only low doses of radiation, the long term effects of exposure to these radiations are not known. Transfer of radiation energy to the cells results in DNA damage, mutations, chromosomal instability, and apoptosis in a dose-dependent manner. When organisms are exposed to ionizing radiation (IR), radiation energy is transferred to the cellular biomolecules, by which they are ionized or excited. This can have adverse effects, like breaks in chemical bonds, production of reactive oxygen species (ROS), and cross-linking of biomolecules, all of which lead to cellular damage. Approximately 60%–70% of these damages are mediated by ROS generated by the radiolysis of water, and remaining 30%–40% are through direct radiation. When cells are exposed to IR, the resultant ROS interact with DNA, causing strand breaks, base oxidation and cross-linking. Formation of 8-hydroxydeoxyguanosine (8-OHdG) is the most common oxidative DNA damage, with a high mutagenic potential. Cells have antioxidant defense mechanisms to neutralize the damaging effects of ROS. Oxidative stress is a state in which ROS production exceeds the capacity antioxidant defense.
8-OHdG produced by ROS, binds to adenine instead of cytosine during the replication and leads to GC-->TA transversion mutation. This damage is repaired by the base excision repair (BER) system, in which, 8-oxo-deoxyguanine DNA glycosylase 1 (OGG1) specifically recognizes and excises 8-OHdG, and this is released into the bloodstream and is excreted through the urine. Increasing evidence suggest that 8-OHdG is involved in the formation of DNA-protein cross-links, and inhibits replication, transcription, and repair. 8-OHdG causes the double-strand breaks in telomeres, leading to their fragmentations and irreversible truncation. The double-strand breaks arise due to excision of 8-OHdG by OGG1 during DNA repair. 8-OHdG has been used as a biomarker of oxidative stress. In the last decade, investigations suggested that 8-OHdG may have some biological functions, as summarized by Marmiy and Esipov. It is believed that 8-OHdG plays a role in regulating gene expression, controlling inflammatory and autoimmune reactions, and activating antioxidant systems. Presence of 8-OHdG on the promoter regions may start the gene expression. It was shown that presence of 8-OHdG in G-quadruplexes in promoters of proto-oncogenes c-Kit and c-Myc destabilizes the structure of these quadruplexes, and transforms them into the duplexes, which makes their transcription easier. 8-OHdG in the gene promoters decrease the methylation of these sites, leading to activation of transcription.
For a long time, radiation was thought to damage only the targeted areas during irradiation. Over time, the bystander effect was identified, which is described as the effects of radiation seen in off target, non-irradiated cells. Bystander effects arise from short-range communication of radiation-induced stress signals through the gap junctions between cells, secretion of ROS, cytokines, and growth factors released from irradiated cells into the bloodstream. These molecules in turn induce ROS and DNA damage, dysregulate epigenetic mechanisms and gene expression in neighboring unirradiated tissues. Low doses of IR induce ROS, but its level may not be enough to cause mutations. However, ROS can modulate gene expression in non-target tissues via epigenetic dysregulation and induce cellular transformations, which can lead to tumor development.
Aberrant DNA methylation refers to widespread demethylation and site-specific gene hypermethylation. Demethylation affects CpG dinucleotides in both repetitive elements and in the CpG islands located in gene-specific promoters. Long interspersed nuclear element 1 (LINE-1) and Alu elements belong to a family of non-long terminal repeat retrotransposons, comprising almost 30% of the human genome. Alterations in the DNA methylation status of these repetitive elements often lead to their activation and retrotransposition. LINE-1 are heavily methylated in normal human tissues, and hypomethylated in many types of tumors. Both genome-wide hypomethylation and gene-specific promoter hypermethylation are frequently detected in cancer. Demethylation of repetitive or transposable elements, or across the genome, leads to genomic instability, increased rate of mutagenesis and development of cancer. Alterations in DNA methylation pattern at CpG islands in promoters may cause aberrant expression of the genes involved in cell proliferation and differentiation, giving rise to tumor. Hypomethylation of the promoters of oncogenes results in their upregulation and over-production of the oncoproteins. In healthy cells, promoters of tumor-suppressor genes have a methylation pattern that allows their expression, to regulate apoptosis and cell survival. Their hypermethylation is an important step in the inactivation of tumor suppressor genes, which may lead to cancer. Aberrant DNA methylation seen in cancer-related genes is presented in Table 1[19-34].
Gene Function Tumor type References APC Tumor suppressor Breast,  BIM Pro apoptotic/tumor suppressor in B cell Leukemia  BRCA1 DNA repair Breast  CDH1 Cellular adhesion regulator/tumor suppressor Thyroid  CDKN 2A (p16) Cyclin-dependent kinase inhibitor/tumor suppressor Lung, leukemia, brain [23-25] DAP kinase Promotion of apoptosis Thyroid  DBC1/BRINP1 Cell cycle arrest/tumor suppressor Leukemia  DKK3 Tumor suppressor Leukemia  DPPA2 Oncogene Thyroid  EGFR Cell differentiation and proliferation Breast  GSTP1 Detoxification Breast  hMLH 1 DNA repair Thyroid  INSL4 Oncogene Thyroid  MGEA1 Tumor-specific antigen Brain  NOTCH4 Oncogene Thyroid  O6MGMT DNA repair Leukemia, brain [24,25] PTEN Tumor suppressor Breast, lung, thyroid, brain [22,23,25,30] RARβ2 Retinoic acid receptor Breast, thyroid [22,31] RASSF1A Cell cycle control Breast, thyroid [22,32] RB Cell cycle control Brain  SEPT9 Cell division and migration Breast  SHOX2 Transcriptional regulation Lung  SLC5A8 Sodium-coupled monocarboxylate transporter Thyroid  SOX17 Transcriptional regulation Breast, lung, thyroid [22,23,34] TCF21 Transcriptional regulation Lung  TCL1B Oncogen Thyroid  TIMP3 Metalloproteinase inhibitor/tumor suppressor Thyroid  TP53 Tumor suppressor Brain  TSHR TSH receptor gene Thyroid  Note. APC: Adenomatous polyposis coli, BIM: BH3-only protein, BRCA1: Breast cancer 1, CDH1:Cadherin 1, CDKN2A: Cyclin-dependent kinase inhibitor 2A, DAP-kinase: Death-associated protein kinase 1, DBC1/BRINP1: Deleted in Breast Cancer-1/bone morphogenetic protein/retinoic acid inducible neural-specific, DKK3: dickkopf WNT signaling pathway inhibitor 3, DPPA2: Developmental pluripotency associated 2, EGFR: Epidermal growth factor receptor, GSTP1: Glutathione S-transferase P, hMLH1: Human MutL homolog 1, INSL4: Insulin like 4, MGEA1: melanoma-associated antigen 1, NOTCH4: Notch Receptor 4, O6-MGMT: O6-methylguanine DNA methyltransferase, PTEN: Phosphatase and tensin homolog, RARβ2: Retinoic acid receptor beta 2, RASSF1A: Ras association domain family 1 isoform A, RB: Retinoblastoma, SEPT9: Septin 9, SHOX2: Short stature homeobox 2, SLC5A8: Solute carrier family 5 Member 8, SOX17: Sex related region Y HMG-box 17, TCF21: Transcriptional regulation, TCL1B: T Cell Leukemia/Lymphoma 1B, TIMP3: Tissue inhibitor of metalloproteinases, TP53: Tumor protein P53, TSHR: Thyroid stimulating hormone receptor.
Table 1. Aberrant DNA methylation in cancer-related genes
Exposure to low dose of IR promotes changes in both global and gene-specific promoter methylation. In experimental animal models, both direct and bystander exposure to radiation showed dose-dependent, sex- and tissue-specific effects on DNA methylation patterns. A single dose of radiation is not enough to cause a significant long- or short-term changes in global DNA methylation patterns. However, chronic exposure to even low doses of IR is a more potent inducer of epigenetic effects and a more effective genome destabilizer. Increased global DNA methylation was observed in human B lymphoblast cell line HMy2 as an adaptive response to long term exposure to low dose of IR. Global DNA hypermethylation was accompanied by increases in DNMT1 and MeCP2 expression, and heterochromatin formation in these cells. On the other hand, Wang et al. detected global as well as specific hypermethylation in 811 regions of genome in mice after chronic whole body exposure to low dose of IR. Genomic hypomethylation, detected in blood 2 h post-irradiation, was associated with downregulation of DNMT1 and MeCP2 in a tissue-specific manner, but was not retained at one month post-irradiation. Hypermethylated regions were more prevalent in gene involved in DNA repair, cell cycle, apoptosis, hippo signaling pathway, GTP catabolic process, and intracellular transport. One of the hypermethylated genes reported in this study was Rad23b, involved in DNA damage recognition and repair. Product of another hypermethylated gene reported in this study, DNA-damage-inducible transcript 3 (Ddit3), plays a key role in the regulation of cell cycle by inhibiting the G1 to S phase transition. Interestingly, both these genes displayed tissue-specific hypermethylation. This study suggested that promoter hypermethylation, rather than global hypomethylation, is more stable, and able to dysregulate the expression of important genes and influence IR-induced pathogenesis. IR-induced methylation changes in the genome are shown in Table 2.
Table 2. IR-induced methylation changes in genome
Alterations in DNA methylation patterns may not be isolated events, but may also be associated with histone methylation. Trimethylations at histone H3 lysine 9 (H3K9), histone H3 lysine 27 (H3K27) and histone H4 lysine 20 (H4K20) are responsible for the formation of transcriptionally silent heterochromatin structure. Tumor cells show some characteristic changes in histone methylation. Recent studies revealed that exposure to IR, even at low doses, alters histone methylation status. For example, histone H3K9 and histone H4K20 trimethylation are regulated negatively after exposure to both low- and high-dose of IR.
Taken together, aberrant DNA methylation in cancer-related genes is a common feature of tumors. Exposure to low dose of IR promotes aberrant DNA methylation, and chronic exposure is an effective inducer of changes in DNA methylation. This may be the missing link between medical radiation exposure and cancer development.
Exposure to x-rays in diagnostic procedures constitutes a significant part of the annual radiation exposure from all sources. Although doses of single procedures are low in standard radiographic examinations, patients who may need repeated examinations to follow their cardiac, urinary, pulmonary, orthopedic conditions, may receive relatively high cumulative doses. Specific doses from various medical imaging techniques are shown in Table 3[40-43]. Computed tomography (CT) examinations use radiations in a narrower range but have relatively higher average effective doses. Cancer risk is reported to be higher in individuals who have undergone repeated CT examinations as compared with age-matched controls. Exposure of the breast tissues to radiation during the chest CT scanning has critical importance, especially in girls and young women. Increased use of CT in pediatrics has enhanced the concerns about cancer risk, because children are more sensitive to the carcinogenic effects of IR and have many years of life for cancer development. The lifetime risks of cancer that may be attributed to childhood CT scans is shown in Table 4[46-54].
Item Typical effective dose value (mSv) CT Abdomen-pelvis 8 Angiogram aorta (chest, abdomen, pelvis-rule out dissection or aneurysm) 24 Angiogram of thorax (rule out pulmonary embolism) 15 Chest 5.1 Chest (pulmonary embolism) 10 Head 2.3 Lower limbs (excluding pelvis) 0.6 Lung V/Q scan 2.2 Multiphase abdomen and pelvis 31.0 Neck 2.2 Sinuses 0.6 Spine 6 Trauma CT 'pan scan' (head, neck, chest, abdomen pelvis) 34 Radiography Abdomen radiograph (anteroposterior) 1.8 Cervical spine radiograph 0.2 Chest radiograph (PA and lateral) 0.1 Extremities radiograph 0.005 Knee radiograph 0.005 Hip radiograph 0.7 Lumbar spine radiograph 0.6 Pelvis radiograph 0.6 Shoulder radiograph 0.01 Thoracic spine radiograph 1.0 Other Abdomen angiography 20 Barium swallow 1.5 bone densitometry (DEXA) 0.001 Bone scan (nucleer) 6.3 Cardiac perfusion (sestamibi) 12.5 Cardiac resting ventriculography 7.8 Coronary angiography 5–15 Diagnostic cardiac catheterization 7 Lung ventilation/perfusion 2.0 Mammography 0.7 Myocardial perfusion imaging 15.6 Percutaneous coronary intervention 15 Upper GI series 6 Note. *Data were obtained from reference 40–43.
Table 3. Typical effective dose values for both CT and non-CT imaging examinations
Data Study type References Pediatric CT scans result in increased lifetime risk for cancer mortality Retrospective cohort study 46-47 Pediatric CT examinations cause an increased risk of leukemia Retrospective cohort study 48 Breast cancer risk is increased in subjects with scoliosis Retrospective cohort study 49-50 Head and neck CT examinations during childhood increases thyroid cancer risk Prospektif study 51 Pediatric CT examinations cause an increased risk of brain tumors Retrospective cohort study 48 Ir used for treatment of skin hemangioma during infancy results in intracranial tumors Retrospective cohort study 52 Ir used for treatment of tinea capitis in childhood results in brain tumors Retrospective cohort study 53 CT examinations during childhood increases brain tumor risk Retrospective cohort study 54
Table 4. The lifetime attributable risk of cancer due to medical radiation exposure in childhood
The coronary angiography and percutaneous coronary interventions are essential for the diagnosis and treatment of ischemic heart diseases. It was reported that low dose of IR used in cardiac imaging increases the risk of cancer in patients without a history of cancer.
Radiation therapy for benign diseases such as tinea capitis, enlarged tonsils, thymus, and thyroid gland was more common in the past. Although only moderate doses were used to treat various benign diseases, radiation-related tumors in or near the irradiated area have been reported. Treatment with radioiodine-131 is thought to have no side effects, but there is some evidence showing an increased risk for breast, kidney, and stomach cancers due to radioiodine-131 treatment.
Radiation therapy, solely or combined with other therapeutic approaches (surgery or chemotherapy), is used in treating many types of cancer. The goal of radiotherapy is to destroy tumor cells with a lethal dose of IR, while minimizing the exposure of healthy tissue. Majority of the patients are treated with a dose between 40–60 Gy of radiation, and this dose may be reduced depending on the distance between the radiation source and the target tissue. Like other treatments, radiotherapy also causes serious side effects. Scientists and radiologists take great efforts to direct the beams accurately on tumor cells and minimize the exposure of surrounding normal tissues. Targeted radionuclide therapy is one of the most rapidly developing approach of nuclear medicine. It is based on the use of molecular carriers of radionuclides with high affinity to antigens on the surface of tumor cells. Unlike conventional external beam therapy, targeted radionuclide therapy causes less damage to the normal tissues and allows targeted drug delivery to tumors. It is widely applied in the treatment of the most radiosensitive tumors, particularly leukemias and lymphomas. Unfortunately, even though this new treatment has resulted in longer survival, radiotherapy still causes a growing number of radiation-related secondary cancers.
Occupational exposure of medical staff to radiation is another aspect of this challenge. Although various protection methods have been applied to reduce occupational exposure to radiation, incidence of cancer in medical staff working in an imaging area is high on an average, in comparison to the general population.
Taken together, most cancers can be induced by radiation, and a linear dose-response has been noted for most cancers. According to the linear no-threshold model, established by Biologic Effects of Ionizing Radiation VII Committee (BEIR VII), to estimate the relationship between IR exposure and cancer, risk of cancer continues to linearly increase even at low doses of exposure, with no safe threshold; the lowest dose of radiation increases the risk, and there is no safe level of exposure.