Patient Size-Dependent Dosimetry Methodology Applied to 18F-FDG Using New ICRP Mesh Phantoms

Abstract

Despite the known influence of anatomic variability on internal dosimetry, dosimetry for 18F-FDG and other diagnostic radiopharmaceuticals is routinely derived using reference phantoms, which embody population-averaged morphometry for a given age and sex. Moreover, phantom format affects dosimetry estimates to a varying extent. Here, we applied newly developed mesh format reference phantoms and a patient-dependent phantom library to assess the impact of height, weight, and body contour variation on dosimetry of 18F-FDG. We compared the mesh reference phantom dosimetry estimates with corresponding estimates from common software to identify differences related to phantom format or software implementation. Our study serves as an example of how more precise patient size-dependent dosimetry methodology could be performed. Methods: Absorbed dose coefficients were computed for the adult mesh reference phantoms and for a derivative patient-dependent phantom series by Monte Carlo simulation using the Particle and Heavy Ion Transport Code System (PHITS) within the software called PARaDIM (PHITS-Based Application for Radionuclide Dosimetry in Meshes). The dose coefficients were compared with reference absorbed dose coefficients obtained from International Commission on Radiological Protection publication 128 or were generated using software including OLINDA 2.1, OLINDA 1.1, and IDAC-Dose 2.1. Results: Differences in dosimetry arising from anatomic variations were shown to be significant, with detriment-weighted dose coefficients for the percentile-specific phantoms varying by up to 640% relative to the corresponding reference phantom effective dose coefficients, irrespective of phantom format. Similar variations were seen in the individual organ absorbed dose coefficients for the percentile-specific phantoms relative to the reference phantoms. The effective dose coefficient for the mesh reference adult was 0.017 mSv/MBq, which was 5% higher than estimated by a corresponding voxel phantom and 10% lower than estimated by the stylized phantom format. Conclusion: We observed notable variability in 18F-FDG dosimetry across morphometrically different patients, supporting the use of patient-dependent phantoms for more accurate dosimetric estimations relative to standard reference dosimetry. The methodology employed may help in optimizing imaging protocols and research studies, in particular when longer-lived isotopes are used.