Monte Carlo-based methodology for the calculation of calibration coefficient for in vivo monitoring systems according to the activity distribution on the human body

Calibration of whole-body counters (WBCs) with standard homogeneous phantoms introduces considerable uncertainties in internal dosimetry because it doesn't account for the complexity, spatial heterogeneity, and time-dependent radionuclide biodistribution in the human body. The aim is to validate a novel WBC calibration methodology to lower the system calibration uncertainties. The methodology includes Monte Carlo simulations to systematically analyze the counting efficiency of a WBC composed of an 8”x4” Na(Tl) detector and a partial lead shield bed. The ICRP-110 Adult Female reference phantom was positioned at the WBC bed modeled in MCNP. Monoenergetic photon sources in the energy range of 100 to 2500 keV were distributed within each of 135 tissues in the model. The efficiency curve as a function of the photon energy was obtained for each tissue. The calibration coefficient of the system for a given gamma emission of an incorporated radionuclide is given by the sum of the calibration coefficients (CC) of each tissue in the model, each one weighted by the ratio of the activity of the tissue in question and the total activity in the body at the time evaluated. The validation of the methodology involved comparing the CC obtained using the new approach with direct simulations with biodistribution data from 18 F-FDG and 131 I. Results showed that the energy with maximum efficiency depends on tissue anatomy, varying from ∼100 keV for superficial tissues to ∼1000 keV for deep structures such as the spine. Analytical functions that fit the energy-efficiency data for each tissue in the model were obtained. Validations demonstrated good agreement (less than 1.2% differences) with the direct biodistribution simulations. It was concluded that the new methodology was suitable for calculating calibration coefficients for different photon-emitting radionuclides with virtually any biodistribution in the body, using tissue-specific, energy-dependent efficiency functions.