CT scanner-specific organ dose coefficients generated by Monte Carlo calculation for the ICRP adult male and female reference computational phantoms

Abstract

Objective: Provide an update to the widely-utilized collection of organ dose coefficients (hereafter referred to as normalized doses) for CT published 30 years ago in NRPB-SR250. Approach: In order to reflect changes in technology, and also ICRP recommendations concerning use of the computational phantoms Adult Male (AM) and Adult Female (AF), 102 series of new Monte Carlo simulations have been performed covering the range of operating conditions for 12 contemporary models of CT scanner from 4 manufacturers. Normalized doses (relative to free air on axis) have been determined for 39 organs, and for every 8 mm or 4.84 mm slab of AM and AF, respectively. Main results: Analyses of results confirm the significant influence (by up to a few tens of percent), on values of normalized organ (or contributions to effective dose (E103,phan)), for whole body exposure arising from selection of tube voltage and beam shaping filter. Use of partial (when available) rather than a full fan beam reduced both organ and effective dose by up to 7%. Normalized doses to AF were smaller than corresponding figures for AM by up to 30% for organs and by 10% for E103,phan. Additional simulations for whole body exposure have also demonstrated that: practical simplifications in the main modelling (point source, single slice thickness, neglect of patient couch and immobility of phantom arms) have sufficiently small (<5%) effect on E103,phan; mis-centring of the phantom away from the axis of rotation by 5 mm (in any direction) leads to changes in normalized organ dose and E103,phan by up to 20% and 6%, respectively; and angular tube current modulation can result in reductions by up to 35% and <15% in normalized organ dose and E103,phan, respectively, for 100% cosine variation. Significance: These analyses help advance understanding of the influence of operational scanner settings on organ dose coefficients for contemporary CT, in support of improved patient protection. The results will allow the future development of a new dose estimation tool.

Development of a generalized method to allow the estimation of doses to the ICRP reference adults from CT, on the basis of normalized organ and CTDI dose data determined by Monte Carlo calculation for a range of contemporary scanners

“…Updated Monte Carlo simulations for a range of 'contemporary' CT scanners (Jansen et al 2022) have been used to develop a generalized method to allow the estimation of doses to the ICRP adult reference computational phantoms (ICRP 2009) for any CT scanner. Series of analyzes have been performed in relation to 102 Monte Carlo generated organ dose data sets representing 12 models of CT scanner, from 4 manufacturers, operating under a range of conditions in order to establish linear models of dose.…”

“…CT scanners can broadly be characterized by values of CTDI measured at central and peripheral locations in both the 32 cm and 16 cm diameter standard CT dosimetry phantoms, when normalized to the CTDI free-inair. Accordingly, normalized values of organ dose and CTDI from Monte Carlo calculations for a range of CT scanners (Jansen et al 2022) have been analyzed in order to derive a linear model for use with measurements of n CTDI for other scanners that were not included in the simulations. Relationships have also been established between the calculated values of n CTDI to ensure that similar measurements for other scanners are within the range of values inherent in the organ dose model, with reliance thus on interpolation rather than extrapolation, which might lead to increased uncertainties in dose estimates.…”