Radioprotective efficacy of plastic polymer against the toxicogenomic effects of radiopharmaceutical ¹⁸F-FDG on human lymphocytes

Radioactive fluorodeoxyglucose (2-[¹⁸F]-fluor-2-deoxy-D-glucose, or ¹⁸F-FDG) is a radiopharmaceutical widely used in PET/CT (positron emission tomography/computed tomography) exams. ¹⁸F-FDG is captured by glucose transporters, abundant in neoplastic cells due to their high metabolism, but not consumed by cells and therefore remain in the cytoplasm [19]. Radioactive fluorine (¹⁸F) is a positron emitter with a half-life of approximately 110 min. The positron travels a short distance in tissues or water before consuming its kinetic energy and combining with an electron. The interaction, named positron annihilation, results in the simultaneous emission of two photons with both having a high specific energy (511 keV) [2] that increases the risk to occupationally exposed individuals [33].

High doses of ionizing radiation (IR) can have deleterious consequences in humans, like the development of cardiovascular disease and cataracts [6, 28, 41], in addition to producing reactive oxygen species and inducing DNA damage [27, 34, 45]. The genotoxic effects of IR can cause genomic instability and mutations that lead to the development of cancer in exposed individuals [27, 37, 43]. Professional radiation protection/shielding is an important method of protection against IR exposure. Protective devices are used in hot laboratory areas during the preparation and handling of radiopharmaceuticals. They are also used when injecting patients with radiopharmaceuticals through syringes or vial shields. These shields are typically made of lead, tungsten, or lead-coated steel. Studies have shown the unprotected radiation dose to be 10 to 20 times higher than the exposure under lead shielding [1, 26, 30, 46]. The choice of leaded or unleaded aprons, apron thickness, and durability according to the manufacturer’s warranty are all important factors to consider.

The biggest drawback of protective equipment despite the radiological protection is the heavy weight (especially of those containing lead), which can cause back pain, discomfort, and muscle fatigue, and thus reducing ergonomics. The development of new, more ergonomic shields that provide radiological protection similar to lead aprons could contribute to a solution. Polyvinyl chloride (PVC) consists of long carbon chains, where every carbon atom has a chlorine atom attached to it. PVC is one of the most widely used plastic polymers in the world. Its widespread use in industry is due to its low cost and versatility, with applications ranging from thermoplastics and thermosets, to elastomeric shapes [40].

Considering the importance of constant radiological biomonitoring in individuals occupationally exposed to IR, the cytokinesis-block micronucleus (CBMN) assay in human peripheral blood lymphocytes has become one of the most commonly used tests to measure numerical and structural chromosomal alterations in human cells in vitro and in vivo [22, 36]. This assay is a reliable test for assessing radiation-induced chromosome damage and is a valuable biomarker in many biomonitoring studies among individuals that are occupationally or environmentally exposed to IR [13, 16, 34, 37]. Thus, the objective of this study was to evaluate the radiological protection efficacy of a plastic polymer, in the presence or absence of a lead shield, against the toxicogenomic effects of ionizing radiation emitted by the radiopharmaceutical ¹⁸F-FDG in human lymphocytes, using the CBMN assay.

As observed in the shielding test (Table 1) of the IR emitted by the ¹⁸F-FDG, the lead shield was not effective in reducing the number of counts registered by the radiation monitor. There was an increase in counts for all activities at all distances, except for A = 0.68 mCi, which recorded 4000 counts per minute without shielding, and 3800 with the lead shield, at 60 cm. This increase in counts was due to the photoelectric effect, which is predominant for energies less than 600 keV and chemical elements with a high atomic number, such as lead (Z = 82) [5, 39]. When the IR reaches the lead shield, the transfer of energy from the ionization electron to the material medium produces a proportional secondary ionization that expresses the incident radiation energy at the end of the process. Due to this increase in counts recorded by the radiation monitor behind the shield, it is expected that the individual exposed to the ¹⁸F-FDG, for example, will not use any type of lead protection equipment when handling radiopharmaceuticals, increasing exposure to IR. The polymer + lead shield fluctuated in the increase and decrease of the counts. The polymer and lead + polymer shields reduced the counts for all the radioactive activities and distances analyzed, highlighting the novelty of this study. Previous experiments showed a similar efficacy of 0.5 mm and 1 mm polymer thickness (data not presented).

These findings support the simulation performed by Fonsêca et al. [15]. Using the Monte Carlo N-Particle method, they demonstrated that the use of a 0.5-mm thick lead apron increased the dose absorbed by the individual exposed to IR of the ¹⁸F-FDG by up to 26%, instead of a reduction, depending on the distance between the radiation source and the individual. This increase occurred due to the generation of secondary and scattered particles from the interaction of the incident photons (511 keV) with the lead apron, which would not exist without the apron. Consequently, these new ionizations led to an increased flow of photons and electrons that increased the dose absorbed by the individual.

The CBMN assay results (Table 2) indicated a statistically significant increase in the frequency of MN in binucleated cells (p < 0.0001), compared to the negative control, where the lymphocyte cultures were exposed to the ¹⁸F-FDG radiopharmaceutical without shielding. Shielding significantly reduced the MN frequencies (lead p = 0.0001, polymer p = 0.0133, and lead + polymer p < 0.0001) compared to the unshielded group. Similarly, for the NPB and NBUD frequencies, a higher induction of DNA damage was observed in cultures exposed to IR (p < 0.0001 and p = 0.0026, respectively) compared to the negative control. The polymer shield differed from the negative control (NPB p = 0.0171 and NBUD p = 0.0442), despite reducing the incidence of damage (NPB p = 0.0001 and NBUD p = 0.0108). The lead shield (NPB p = 0.6877 and NBUD p = 0.4033) and the lead + polymer shield (NPB p = 0.1773 and NBUD p > 0.9999) significantly reduced IR-induced damage to the level observed in the negative control (Fig. 3).

The increase in the frequency of MN, NPB, and NBUD in lymphocytes reflects the chromosomal instability induced by IR. MN can be formed during anaphase when an acentric chromosome, or chromatid fragments from DNA repair failures are not incorporated into the cell’s main nucleus. NPB instead originates from dicentric chromosomes, which can occur due to the inadequate repair of breaks in DNA or terminal fusions of telomeres. NBUD represents the process of eliminating amplified DNA, complex DNA repair mechanisms, possible chromosomes of aneuploid cells, and may also result from NPB disruption [9‐11, 16, 22, 36]. The direct interaction of IR with DNA induces single and double-strand breaks, changes in nitrogen bases, and DNA cross-linking [27, 45]. The genotoxic effects of IR may cause genomic instability and mutations that lead to cancer induction in exposed individuals [27]. Several studies have reported increased levels of chromosomal aberrations in lymphocytes from workers occupationally exposed to IR, compared to those from unexposed workers [3, 8, 21, 25, 35, 42, 47]. Additionally, some epidemiological studies have revealed that individuals who are occupationally exposed to IR may have an increased risk of developing leukemia and other cancers [4, 18, 20, 38, 44].

The CBPI analysis (Table 2) is a useful tool to understand the cell cycle kinetics of lymphocyte cultures, especially after exposure to IR [32, 36]. Evaluation of this parameter in the present study indicated that direct exposure to IR had a cytotoxic effect on lymphocytes, compared to the negative control (p < 0.0001). The polymer (p < 0.0001) and lead (p = 0.0031) shields were not effective in protecting lymphocytes from the cytotoxic effects of radiation, compared to the negative control. Only the lead + polymer shield was effective in protecting the cells, since the CBPI of this group did not present a statistically significant difference compared to the negative control (p = 0.2903). The results of the CBMN assay are consistent with the CBPI results, suggesting that the lead + polymer shield was equally effective in protecting the DNA and in maintaining the integrity of the lymphocytes exposed to ¹⁸F-FDG.

IR generates an explosion of free radicals capable of inducing DNA damage in exposed cells, because of the ionization of water molecules and the direct ionization of target molecules [24, 37]. These free radicals are produced in microseconds. However, their effects persist long after their production, owing to the cascade of events triggered at the molecular and cellular level. This ultimately leads to increased oxidative stress, lipid peroxidation, and genome instability [17, 37]. All these mechanisms can be explained by the cytotoxic and genotoxic effects observed and reaffirms the uniqueness of this study.

Considering the analyzed population (n = 3 individuals, 6000 cells for CBMN assay, and 3000 cells for CBPI, in each group), it is essential to consider that results with more blood donors could provide even more relevant observations on interindividual and temporal differences. It is also noteworthy that a more significant number of samples would result in a longer exposure time of researchers to ¹⁸F-FDG, even with the adoption of all security protocols. Moreover, the harmful effect of IR is not limited to lymphocytes. The radiosensitivity of healthy cells, tissues, and organs depend on several factors, including the ability to repair the damage, hypoxia, cell cycle position, and the effective absorbed dose of radiation. Then, further investigations with different cell types must be carried out.

Thus, compliant with the basic guidelines of CNEN (NN 3.01 and 3.05) which aim to optimize radiological protection to provide occupationally exposed individuals with better health care, we found that the use of a lead + polymer shield is more effective than lead aprons. The association of lead + polymer reduces the number of counts and mitigates the biological effects of IR, increasing the protection of individuals working in PET/CT procedures with ¹⁸F-FDG. Similar to the PVC polymer, Prokhorenko et al. [31] demonstrated that polystyrene composite (19.1%) combined with tungsten (66.9%) and aluminum (14%) was also effective in the production of radioprotective equipment of low weight and high durability, supporting our results.

The results from this study show that the addition of a lightweight, low-cost polymer layer to a lead apron is highly recommended and provides enhanced radiological and biological protection to individuals who are occupationally exposed to radiation, particularly during ¹⁸F-FDG handling. In cases where lead aprons are avoided due to their low ergonomics, we recommended the use of polymer aprons by qualified service personnel at least. Other polymer-based safety equipment may be developed to reduce the biological effects of IR, like glasses and other types of eye protection, and polymer-coated syringe/vial holders and coatings for radiopharmaceuticals. Furthermore, a minimum distance of 25 cm between healthcare professionals and radiopharmaceuticals is recommended during transportation from hot laboratories to patients’ rooms. Workers occupationally exposed to ionizing radiation who are better protected will be able to carry out routine activities safely, allowing them to work longer with less radiation exposure, reducing staff turnover costs in addition to minimizing their risk of exposure to the harmful effects of IR.

Ionizing radiation is mutagenic and known to induce cell damage, including the formation of micronuclei, nucleoplasmic bridges, and nuclear buds. The results presented in this study suggest that the coating of conventional radiation shielding equipment, like lead aprons, with a polymer layer could increase the radiological protection of occupationally exposed individuals, particularly during ¹⁸F-FDG PET/CT examinations. The toxicogenomic biomonitoring of workers exposed to ionizing radiation, the application of radiation protection programs, and physical dosimetry procedures are all crucially important in minimizing occupational radiation exposure. Moreover, the development of new protective equipment is key in terms of biotechnology for the protection of individuals and the environment. Better protective equipment will increase the safety of health workers exposed to radiation, and also that of researchers, technicians, and students exposed to radiation in universities, research centers, and industries.