Radiation protection from external exposure to radionuclides: A Monte Carlo data handbook

Occupational radiation exposure during blood sample collection post I-131 therapeutic administration in thyroid cancer patients

The blood sampling required for the verification of dose delivered during radioactive iodine (131I) therapy is a source of radiation exposure for healthcare staff. This study aims to estimate staff exposures, using Monte Carlo modelling, as well as experimental measurements. The study further aimed to validate the models with staff exposure measurements and examine the impact of protective measures on the procedure. The clinical set-up of blood sampling post131I patient administration, within a dedicated131I ablation therapy suite, was modelled using EGSnrc Monte Carlo simulations (MCSs). The dose scoring regions representing deep dose (Hp(10)) and skin dose (Hp(0.07)) were estimated and validated with an experimental approach and clinical monitoring of staff members using electronic personal dosimeters. MCSs and experimental values did not show significant differences between the two approaches (p> 0.05), with simulated values having lower uncertainties. It is demonstrated that the model is capable of being tailored to clinical scenarios at any centre. The simulations were corroborated with dosimetry data of blood sampling from 14 patients post administering 3.7 GBq131I, amounting to a total of 54 measurements from 14 staff members. With the employed protective shielding, none of the staff were exposed to a dose rate approaching the annual dose constraint of 0.3 mSv.yr-1. The experimental and MCS data show good agreement with the clinical data, demonstrating the advantages of MCS approaches for providing useful information for planning and carrying out risk assessments before implementing a new dosimetry practice.

Evaluation of the contamination droplet model used for the assessment of skin dose during the manipulation of radiopharmaceuticals

The manipulation of radiopharmaceuticals in nuclear medicine can result in the droplet contamination of operators resulting in the accumulation of a significant skin dose. Current methods to estimate this skin dose often utilise a 50μl cylindrical droplet model, which can lead to unrealistically high estimated skin doses for some radiopharmaceuticals. By conducting experiments to measure the volume of real droplets arising from simulating the manipulation of radiopharmaceuticals, this work found that 50μl is an overestimation of a realistic contamination droplet. For almost all radiopharmaceuticals considered in this work, incorporating a smaller droplet volume into skin dose simulations resulted in higher estimates of skin dose rate per unit of activity, which, when combined with appropriate activity concentrations and droplet volumes, resulted in lower skin doses for contamination droplet incidents. The results presented in this work challenge the 50μl contamination droplet volume and highlight the importance of having an accurate model when estimating the skin dose for contamination scenarios.

Assessment of occupational exposure from shielded and unshielded syringes for clinically relevant positron emission tomography (PET) isotopes-a Monte Carlo approach using EGSnrc

¹⁸F has been the most widely used radionuclide in positron emission tomography (PET) facilities over the last few decades. However, increased interest in novel PET tracers, theranostics and immuno-PET has led to significant growth in clinically used positron-emitting radionuclides. The decay schemes of each of these radioisotopes are markedly different from¹⁸F, with different endpoint energies for the emitted positrons and, in some cases, additional high energy gamma radiation. This has implications for the occupational exposure of personnel involved in the manipulation and dispensing of PET radiopharmaceuticals. The EGSnrc Monte Carlo simulation software was used to estimate the doses to extremities in contact with unshielded and shielded syringes containing⁶⁴Cu,¹⁸F,¹¹C,¹³N,¹⁵O,⁶⁸Ga and⁸⁹Zr, respectively. Dose rates at various distances from the syringe were also modelled, with dose rates reported in terms of eye (Hp(3)), skin equivalent (Hp(0.07)) and deep (Hp(10)) doses. The composition and geometry of the simulated syringe shields were based on a selection of commercially available PET shields. Experimental dose rate measurements were performed for validation purposes where possible. Contact skin dose rates for all isotopes, except for⁶⁴Cu, were found to be higher than¹⁸F for the unshielded syringe. The addition of a shield resulted in approximately equal contact skin dose rates for nearly all isotopes, for each shield type, with the exception of⁸⁹Zr which was notably higher. Dose rate constants (µGy/MBq.hr) for a range of PET isotopes and shields are presented and their significance discussed.

Assessment of radionuclide impurities in [18F]fluoromethylcholine ([18F]FMCH)

PURPOSE

[¹⁸F]Fluoromethylcholine ([¹⁸F]FMCH) is a radiopharmaceutical used in positron emission tomography (PET) imaging for the study of prostate, breast, and brain tumors. It is usually synthesized in cyclotron facilities where ¹⁸F is produced by proton irradiation of [¹⁸O]H₂O through ¹⁸O(p,n)¹⁸F reaction. Due to the activation of target materials, the bombardment causes unwanted radionuclidic impurities in [¹⁸O]H₂O, that need to be removed during the radiopharmaceutical synthesis. Thus, the aim of this study is to quantify the radionuclide impurities in the ¹⁸F production process and in the synthesized [¹⁸F]FMCH, demonstrating the radionuclidic purity of this radiopharmaceutical.

METHODS

Long-lived radionuclide impurities were experimentally assessed using high-resolution gamma and liquid scintillation spectrometries, while short-lived impurities were monitored analyzing the decay curve of the irradiated [¹⁸O]H₂O with an activity calibrator. As spectrometric radionuclide library, a Geant4 Monte Carlo simulation of the ¹⁸F-target assembly was previously performed.

RESULTS

³H, ^52,54Mn, ^56,57,58Co, ^95m,96Tc, ¹⁰⁹Cd, and ¹⁸⁴Re were found in the irradiated [¹⁸O]H₂O, but no radionuclide was found in the non-irradiated [¹⁸O]H₂O neither in the final [¹⁸F]FMCH solution with an activity concentration greater than the minimum detectable activity concentration. A total impurity activity <6.2 kBq was measured in the irradiated [¹⁸O]H₂O, whereas a [¹⁸F]FMCH radionuclide purity >99.9999998% was estimated. Finally, the decay curve of the irradiated [¹⁸O]H₂O revealed a very low maximum of ¹³N activity (<0.03% of ¹⁸F) even immediately after the end of bombardment.

CONCLUSIONS

This study demonstrated the radionuclidic purity of [¹⁸F]FMCH according to the EU Pharmacopeia.

Enhancement of radiation exposure risk from β-emitter radionuclides due to Internal Bremsstrahlung effect: A Monte Carlo study of 90Y case

Employment of β-decaying radionuclides, used in many fields (industrial, clinical, research) requires a correct assessment of the operators' radiological exposure. Usually, in the dosimetric evaluation, the contribution coming from Internal Bremsstrahlung (IB) accompanying the β-decay is not kept into account; nevertheless, this negligibility does not always appear justified, at least for high-energy β-emitters. By means of Monte Carlo (MC) simulations, we showed how the contribution from IB photons is noteworthy for the evaluation of the overall radiation absorbed dose in the case of ⁹⁰Y source. We evaluated an increase of the absorbed doses, respectively for a point source and the considered receptacles, up to + 34% and + 60% or + 15% and + 28%, depending on the adopted model of IB spectrum. These results demonstrate the relevance of IB phenomenon in radiation protection estimations and suggest extending future theoretical and experimental studies to other β-decaying radionuclides.