Effects of body habitus on internal radiation dose calculations using the 5-year-old anthropomorphic male models

Tianwu Xie, Niels Kuster, and Habib Zaidi, Physics in Medicine & Biology, Volume 62, Issue 15, pp. 6185–6206, online 13 July 2017, doi: 10.1088/1361-6560/aa75b4

Computational phantoms are commonly used in internal radiation dosimetry to assess the amount and distribution pattern of energy deposited in various parts of the human body from different internal radiation sources. Radiation dose assessments are commonly performed on predetermined reference computational phantoms, although the argument for individualized patient-specific radiation dosimetry exists. The aim of this study is to evaluate the influence of body habitus on internal dosimetry and to quantify the uncertainties in dose estimation correlated with the use of fixed reference models. The 5-year-old IT'IS male phantom was modified to match target anthropometric parameters, including body weight, body height, and sitting/height stature ratio (SSR) determined from reference databases, thus enabling the creation of 125 5-year-old habitus-dependent male phantoms with 10th, 25th, 50th, 75th, and 90th percentile body morphometries. We evaluated the absorbed fractions and the mean absorbed dose to the target region per unit cumulative activity in the source region (S-values) of F-18 in 46 source regions for the 125 anthropomorphic 5-year-old hybrid male phantoms generated by means of the Monte Carlo N-Particle eXtended (MCNPX) general purpose Monte Carlo transport code and calculated the absorbed dose and effective dose of five 18F-labelled radiotracers for children of various habitus. For most organs, the S-value of F-18 presents stronger statistical correlations with body weight, standing height, and sitting height than with BMI and SSR. The self-absorbed fraction and self-absorbed S-values of F-18 and the absorbed dose and effective dose of 18F-labelled radiotracers present with the strongest statistical correlations with body weight. For 18F-amino acids, 18F-brain receptor substances, 18F-FDG, 18F-L-DOPA, and 18F-FBPA, the mean absolute effective dose differences between phantoms of different habitus and fixed reference models are 11.4%, 11.3, 10.8, 13.3, and 11.4%, respectively. Total body weight, standing height, and sitting height have considerable effects on human internal dosimetry. Radiation dose calculations for individual subjects based on the most closely matched habitus-dependent computational phantom should be considered as an alternative to improve the accuracy of the estimates.

The scientific and technical impact of the study can be summarized as:

  • The detailed analysis of habitus-dependent dosimetric results for the 5-year-old child supports arguments for the application of individual patient dose assessment.
  • Total body weight, standing height, sitting height, BMI, and SSR influence the calculation of absorbed fractions and S-values of F-18 and absorbed dose and effective dose of 18F-labelled radiotracers.
  • For most organs, the absorbed fractions of F-18 increase as a function of total body weight and BMI, while the self-absorbed S-values of F-18 as well as absorbed and effective doses of 18F-labelled radiotracers decrease with body weight and BMI.
  • Dose comparisons between habitus-dependent phantoms and predefined phantoms corresponding to ICRP reference data reveal that the use of the most closely matched habitus-dependent phantom is superior to the use of a predefined reference phantom for estimation of internal dose in an individual patient. 
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