Patient exposures and consequent risks from nuclear medicine procedures

Assessing radiation doses and proposing DRLs for nuclear medicine procedures for pediatric and adult patients in Madinah, Saudi Arabia

BACKGROUND

Nuclear medicine diagnostic and treatment procedures represent significant sources of ionizing radiation exposure for both staff and patients. Consequently, assessing and optimizing radiation doses are crucial to minimize potential side effects.

AIM

This study seeks to evaluate the effective radiation doses associated with common diagnostic and treatment procedures, as well as propose diagnostic reference levels (DRLs), within two nuclear medicine centers in Madinah, Saudi Arabia.

METHODOLOGY

Data from 445 patients were gathered from two nuclear medicine centers in the Madinah region of Saudi Arabia. The data were categorized based on the type of nuclear medicine (NM) procedure, the chemical composition of the administered radiopharmaceutical, as well as patient age and weight. Effective radiation doses for prevalent NM procedures were computed, and suggested DRLs were formulated.

RESULTS

Effective radiation doses were analyzed for 16 adult and 2 pediatric NM procedures (divided into 8 groups). The effective radiation doses for adult diagnostic nuclear medicine procedures range from 0.05 mSv (Nanocoloid) to 29 mSv (67Ga-citrate). For pediatric procedures, the doses range from 0.80 mSv (5-year-old undergoing renal DTPA) to 1.6 mSv (1-year-old undergoing renal DMSA). Furthermore, DRL values were determined for both adult and pediatric NM procedures. The study's findings demonstrated a high degree of concordance between effective radiation doses and DRL values, aligning well with previously published research.

CONCLUSION

While the effective radiation doses outlined in this study were generally within acceptable limits and consistent with prior research findings, optimizing radiation doses remains imperative, particularly for pediatric NM procedures.

Replace the Linear No-threshold Model with a Risk-informed Targeted Approach to Radiation Protection

ABSTRACT

The linear no-threshold (LNT) model may be useful as a simple basis for developing radiation protection regulations and standards, but it bears little resemblance to scientific reality and is probably overly conservative at low doses and low dose rates. This paper is an appeal for a broader view of radiation protection that involves more than just optimization of radiation dose. It is suggested that the LNT model should be replaced with a risk-informed, targeted approach to limitation of overall risks, which include radiation and other types of risks and accidents/incidents. The focus should be on protection of the individual. Limitation of overall risk does not necessarily always equate to minimization of individual or collective doses, but in some cases it might. Instead, risk assessment (hazards analysis) should be performed for each facility/and or specific job or operation (straightforward for specialized work such as radiography), and this should guide how limited resources are used to protect workers and the public. A graded approach could be used to prioritize the most significant risks and identify exposure scenarios that are unlikely or non-existent. The dose limits would then represent an acceptable level of risk, below which no further reduction in dose would be needed. Less resources should be spent on ALARA and tracking small individual and collective doses. Present dose limits are thought to be conservative and should suffice in general. Two exceptions are possibly the need for a lower (lifetime) dose limit for lens of the eye for astronauts and raising the public limit to 5 mSv y -1 from 1 mSv y -1 . This would harmonize the public limit with the current limit for the embryo fetus of the declared pregnant worker. Eight case studies are presented that emphasize how diverse and complex radiation risks can be, and in some cases, chemical and industrial risks outweigh radiation risks. More focus is needed on prevention of accidents and incidents involving a variety of types of risks. A targeted approach is needed, and commitments should be complied with until they are changed or exemptions are granted. No criticism of regulators or nuclear industry personnel is intended here. Protection of workers and the public is everyone's goal. The question is how best to accomplish that.

In Patients We Trust: Reliability of Self-Reported Weight and Height in Nuclear Medicine Patients

The aim of the study was to assess the reliability of the self-reported weight and height of nuclear medicine patients in view of recommendations for weight-dependent tracer application for imaging and therapy. Methods: In total, 824 patients (334 men and 490 women) were asked to report their weight and height before imaging or therapy and their level of confidence. Subsequently, the weight and height of each patient were measured, and body mass index, body surface area, and lean body mass were calculated. Differences between reported and true values were compared for statistically significant differences. Results: The average patient age was 60 ± 14 y (range, 17-91 y). An over- or underestimation of weight by at least 10% was observed in 2% of patients, and height was overestimated by 1% by the patients. The BMI calculation was affected by incorrect self-reported values. Conclusion: Most self-reported weights and heights of nuclear medicine patients are accurate. However, since over- and underestimation of weight and height lead to incorrect body mass index, body surface area, and lean body mass values, patient weights should be measured at least for patients receiving a weight-adapted therapy or if quantification in PET/CT is needed.

Subcellular Targeting of Theranostic Radionuclides

The last decade has seen rapid growth in the use of theranostic radionuclides for the treatment and imaging of a wide range of cancers. Radionuclide therapy and imaging rely on a radiolabeled vector to specifically target cancer cells. Radionuclides that emit β particles have thus far dominated the field of targeted radionuclide therapy (TRT), mainly because the longer range (μm-mm track length) of these particles offsets the heterogeneous expression of the molecular target. Shorter range (nm-μm track length) α- and Auger electron (AE)-emitting radionuclides on the other hand provide high ionization densities at the site of decay which could overcome much of the toxicity associated with β-emitters. Given that there is a growing body of evidence that other sensitive sites besides the DNA, such as the cell membrane and mitochondria, could be critical targets in TRT, improved techniques in detecting the subcellular distribution of these radionuclides are necessary, especially since many β-emitting radionuclides also emit AE. The successful development of TRT agents capable of homing to targets with subcellular precision demands the parallel development of quantitative assays for evaluation of spatial distribution of radionuclides in the nm-μm range. In this review, the status of research directed at subcellular targeting of radionuclide theranostics and the methods for imaging and quantification of radionuclide localization at the nanoscale are described.

Influence of Speciation of Thorium on Toxic Effects to Green Algae Chlorella pyrenoidosa

Thorium (Th) is a natural radioactive element present in the environment and has the potential to be used as a nuclear fuel. Relatively little is known about the influence and toxicity of Th in the environment. In the present study, the toxicity of Th to the green algae Chlorella pyrenoidosa (C. pyrenoidosa) was evaluated by algal growth inhibition, biochemical assays and morphologic observations. In the cultural medium (OECD TG 201), Th(NO₃)₄ was transformed to amorphous precipitation of Th(OH)₄ due to hydrolysis. Th was toxic to C. pyrenoidosa, with a 96 h half maximum effective concentration (EC₅₀) of 10.4 μM. Scanning electron microscopy shows that Th-containing aggregates were attached onto the surface of the algal cells, and transmission electron microscopy indicates the internalization of nano-sized Th precipitates and ultrastructural alterations of the algal cells. The heteroagglomeration between Th(OH)₄ precipitation and alga cells and enhanced oxidative stress might play important roles in the toxicity of Th. To our knowledge, this is the first report of the toxicity of Th to algae with its chemical species in the exposure medium. This finding provides useful information on understanding the fate and toxicity of Th in the aquatic environment.

Absorbed dose evaluation of Auger electron-emitting radionuclides: impact of input decay spectra on dose point kernels and S-values

The aim of this study was to investigate the impact of decay data provided by the newly developed stochastic atomic relaxation model BrIccEmis on dose point kernels (DPKs - radial dose distribution around a unit point source) and S-values (absorbed dose per unit cumulated activity) of 14 Auger electron (AE) emitting radionuclides, namely 67Ga, 80mBr, 89Zr, 90Nb, 99mTc, 111In, 117mSn, 119Sb, 123I, 124I, 125I, 135La, 195mPt and 201Tl. Radiation spectra were based on the nuclear decay data from the medical internal radiation dose (MIRD) RADTABS program and the BrIccEmis code, assuming both an isolated-atom and condensed-phase approach. DPKs were simulated with the PENELOPE Monte Carlo (MC) code using event-by-event electron and photon transport. S-values for concentric spherical cells of various sizes were derived from these DPKs using appropriate geometric reduction factors. The number of Auger and Coster-Kronig (CK) electrons and x-ray photons released per nuclear decay (yield) from MIRD-RADTABS were consistently higher than those calculated using BrIccEmis. DPKs for the electron spectra from BrIccEmis were considerably different from MIRD-RADTABS in the first few hundred nanometres from a point source where most of the Auger electrons are stopped. S-values were, however, not significantly impacted as the differences in DPKs in the sub-micrometre dimension were quickly diminished in larger dimensions. Overestimation in the total AE energy output by MIRD-RADTABS leads to higher predicted energy deposition by AE emitting radionuclides, especially in the immediate vicinity of the decaying radionuclides. This should be taken into account when MIRD-RADTABS data are used to simulate biological damage at nanoscale dimensions.

Radiation-induced DNA damage and the relative biological effectiveness of 18F-FDG in wild-type mice

Clinically, the most commonly used positron emission tomography (PET) radiotracer is the glucose analog 2-[(18)F] fluoro-2-deoxy-D-glucose ((18)F-FDG), however little research has been conducted on the biological effects of (18)F-FDG injections. The induction and repair of DNA damage and the relative biological effectiveness (RBE) of radiation from (18)F-FDG relative to 662 keV γ-rays were investigated. The study also assessed whether low-dose radiation exposure from (18)F-FDG was capable of inducing an adaptive response. DNA damage to the bone marrow erythroblast population was measured using micronucleus formation and lymphocyte γH2A.X levels. To test the RBE of (18)F-FDG, mice were injected with a range of activities of (18)F-FDG (0-14.80 MBq) or irradiated with Cs-137 γ-rays (0-100 mGy). The adaptive response was investigated 24h after the (18)F-FDG injection by 1 Gy in vivo challenge doses for micronucleated reticulocyte (MN-RET) formation or 1, 2 and 4 Gy in vitro challenges doses for γH2A.X formation. A significant increase in MN-RET formation above controls occurred following injection activities of 3.70, 7.40 or 14.80 MBq (P < 0.001) which correspond to bone marrow doses of ~35, 75 and 150 mGy, respectively. Per unit dose, the Cs-137 radiation exposure induced significantly more damage than the (18)F-FDG injections (RBE = 0.79 ± 0.04). A 20% reduction in γH2A.X fluorescence was observed in mice injected with a prior adapting low dose of 14.80 MBq (18)F-FDG relative to controls (P < 0.019). A 0.74 MBq (18)F-FDG injection, which gives mice a dose approximately equal to a typical human PET scan, did not cause a significant increase in DNA damage nor did it generate an adaptive response. Typical (18)F-FDG injection activities used in small animal imaging (14.80 MBq) resulted in a decrease in DNA damage, as measured by γH2A.X formation, below spontaneous levels observed in control mice. The (18)F-FDG RBE was <1.0, indicating that the mixed radiation quality and/or low dose rate from PET scans is less damaging than equivalent doses of gamma radiation.