Influence of Body Posture on Internal Organ Dosimetry: Radiocesium Exposure Modeling Using Novel Posture-dependent Mesh Computational Phantoms.

Current practice in reference internal dosimetry assumes a fixed upright standing posture is maintained throughout the dose-integration period. Recently, the mesh-type ICRP adult reference computational phantoms were transformed into different body postures (e.g., sitting, squatting) for use in occupational dose reconstruction applications. Here, for the first time, we apply this phantom series to the study of organ dose estimates following radionuclide intake. We consider the specific cases of 137Cs and 134Cs ingestion (accidental/occupational intake) with attention to variability in absorbed dose as a function of posture. The ICRP Publication 137 systemic biokinetic model for soluble cesium ingestion was used to compute organ-level time-integrated activity coefficients for reference adults, over a 50-y dose-integration period, for 134Cs and 137Cs (and its radioactive progeny 137mBa). Mean posture time-allocations (h d-1 for standing, sitting, and lying) were taken from published survey data. In accord with modern dosimetry formalisms (e.g., MIRD, ICRP), a posture weighting factor was introduced that accounts for the fraction of time spent within each independent posture. Absorbed dose coefficients were computed using PHITS Monte Carlo simulations. ICRP 103 tissue weighting factors were applied along with the posture weighting factors to obtain committed effective dose per unit intake (Sv Bq-1). For 137Cs ingestion, most organ absorbed dose coefficients were negligibly to marginally higher (< ~3%) for sitting or crouched (lying fetal/semi-fetal) postures maintained over the dose commitment period, relative to the upright standing posture. The committed effective dose coefficients were 1.3 × 10-8 Sv Bq-1 137Cs for standing, sitting, or crouched postures; thus, the posture-weighted committed effective dose was not significantly different than the committed effective dose for a maintained upright standing posture. For 134Cs ingestion, most organ absorbed dose coefficients for the sitting and crouched postures were significantly larger than the standing posture, but the differences were still considered minor (< ~8% for most organs). The committed effective dose coefficients were 1.2 × 10-8 Sv Bq-1 134Cs for the standing posture and 1.3 × 10-8 Sv Bq-1 134Cs for the sitting or crouched posture. The posture-weighted committed effective dose was 1.3 × 10-8 Sv Bq-1 134Cs. Body posture has minor influence on organ-level absorbed dose coefficients and committed effective dose for ingestion of soluble 137Cs or 134Cs.