Optimising cylinder model dimensions for VARSKIN to simulate a droplet of radionuclide skin contamination using Geant4 Monte Carlo code

Abstract

Aim VARSKIN provides a convenient way of calculating skin dose from predefined geometries but the models are limited to concentric shapes such as discs, cylinders and point sources. The aim of this article is to use the Geant4 Monte Carlo code to independently compare the cylindrical geometries available in VARSKIN to more realistic droplet models obtained from photography. It may then be possible to recommend an appropriate cylinder model that can be used to represent a droplet within acceptable accuracy. Method Geant4 Monte Carlo code was used to model various droplets of radioactive liquid on the skin based on photographs. The dose rates were then calculated to the sensitive basal layer 70 µm beneath the surface for three droplet volumes (10, 30 and 50 µl) and 26 radionuclides. The dose rates from the cylinder models were then compared against the dose rates from the ‘true’ droplet models. Results The table gives the optimum cylinder dimensions that best approximate a true droplet shape for each volume. The mean bias and 95% confidence interval (CI) from the true droplet model are also quoted. 10 μl 30 μl 50 μl Radius, falser 1.26 mm 2.02 mm 2.51 mm Height, falseh 2.01 mm 2.35 mm 2.53 mm Mean bias: −0.5% ± 0.7% (P = 0.436) 0.3% ± 0.7% (P = 0.647) −0.5% ± 0.6% (P = 0.381) 95% CI: −7.4% → 6.3% −6.8% → 7.4% −6.3% → 5.3% Conclusion The evidence from the Monte Carlo data suggests that different droplet volumes require different cylinder aspect ratios to approximate the true droplet shape. Using the cylinder dimensions in the table in software packages such as VARSKIN, dose rates from radioactive skin contamination are expected to be within ± 7.4% of a ‘true’ droplet model at 95% CI.

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

“…When comparing the dose rate and associated dose for the 10 µl droplet model measured in this work with results from Amato et al (2018), Delacroix et al (2002) and James et al (2023), although doses were found to be lower with the smaller droplet models, significant differences were found between the two 10 µl droplet models. For example, the skin dose for the 10 µl droplet model used in this work was 282 mSv for 18 F-FDG compared with 116 mSv measured by James et al (2023).…”

“…The impact on the skin dose for a reduced droplet volume of 10 µl, using the droplet dimensions measured in this work, was evaluated with Monte Carlo simulations. Figure 4 compares the dimensions of the droplet model used with the 10 µl droplet model proposed by James et al (2023) and the 50 µl droplet model adopted by Amato et al (2018) and Delacroix et al (2002).…”

“…The measured skin dose rates are compared with those reported by James et al (2023) and with Amato et al (2018) and Delacroix et al (2002) for a 50 µl droplet, where available, in figure 5. The dose rates measured using GAMOS for all eight radionuclides were greater with the 10 µl droplet proposed in this work than the taller 10 µl droplet and the 50 µl droplet.…”

“…The results are similar, with the 68 Ga droplet having the greatest difference in skin dose rate. Figure 6 compares the skin doses for a 10 min exposure to the measured 10 µl droplet, the 10 µl droplet proposed by James et al (2023) and the 50 µl droplets as modelled by Amato et al (2018) and Delacroix et al (2002). The greatest skin dose for all droplet models resulted from the 90 Y-microspheres (SIR-Spheres ® ) contamination droplet due to the combination of the high measured dose rate and high activity concentration of the radiopharmaceutical.…”

“…A recent publication from James et al (2023) used photographs of real droplets formed on surgical gloves to determine realistic dimensions for 10, 30 and 50 µl cylindrical droplet models. The proposed droplet models were simulated using the Geant4 Monte Carlo code to provide dose rates for a range of radionuclides.…”