Size-specific dose estimates for pediatric head CT protocols based on the AAPM report TG-293

Purpose: To establish the Size Specific Dose Estimate (SSDE) typical values for pediatric head CT examinations based on the AAPM report TG-293; to provide a new stratification based on the water-equivalent diameter (D_w), given that SSDE is related to the head size.

Methods: 296 Head CT scans of pediatric patients collected using a dose monitoring software were retrospectively analysed. Typical values were derived stratifying data by age in three methods: the first proposed by the European Guidelines on Diagnostic Reference Levels for Pediatric Imaging (RP185), the second by the National Istisan Report 20/22 and a local one related to the clinical protocols (LStrata). For each scan, a self-developed Matlab routine calculated the water-equivalent diameter (D_w) and related SSDE values with the conversion factors f^H16and f^B16provided by the AAPM reports TG-293 and TG-204, respectively. Eventually, a D_wstratification was introduced starting from a measure of the lateral dimension of the head.

Results: SSDE based on TG-204 overestimatesthe dose up to 12%. Four D_wgroups were identified thanks to the good correlation between the head lateral dimension andD_w: D_w < 14 cm, 14 ≤ D_w < 16 cm, 16 ≤ D_w< 17 cm, D_w≥ 17 cm. The D_w-stratified dosimetric indices presentgreater variability than those grouped by age because of the large variability of the size of the infant's head.

Conclusions: The variability of the SSDE metric underlines that age-optimized protocols are not when size is considered.