Estimation of size-specific dose estimates (SSDE) for paediatric and adults patients based on a single slice.

Volume averaged CT dose index (CTDI) is an important dose index utilized for CT dosimetry. Measurements of CTDI are performed in reference cylindrical phantoms of specified diameters. A size-specific dose estimate (SSDE) has been recommended for assessment of doses delivered to individual patients. Evaluation of the SSDE requires the size of the scanned region of the patient to be estimated in terms of water-equivalent diameter (D) to allow calculation of a dose value appropriate for the patient. Estimation of D, however, may be challenging and time consuming as it requires assessment of D for each slice within the scanned region. A study has been carried out to investigate the suitability of using D for a single slice at the middle of the scanned region to estimate a value of D to apply to all slices. 351 phantoms (158 paediatric and 193 adult) developed from reconstructed CT images of patients were employed. Six scan regions were studied: chest, abdomen, pelvis, chest and abdomen, abdomen and pelvis, and the whole trunk. Results show that the use of D can lead to over or underestimation of D by up to 13% for paediatric and adult patients. SSDE values based on D and D were assessed for each phantom, and a linear regression analysis was performed. Use of the analysis could provide a simple and practical approach to assessing SSDE for a given scan based on D with the root-mean-square errors estimated to be in the range of 1.2%-4.0% for paediatric and 1.2%-5.9% for adults.