With the recent completion of AAPM Report 293, a trilogy of reports by the American Association of Physicists in Medicine (AAPM) on the calculation of CT Size-Specific Dose Estimates (SSDE) now includes head CT.  SSDE provides a more accurate estimate of patient dose to better inform end-users when they are developing and reviewing size-based CT protocols, especially for pediatrics.  Initially, SSDE was developed to more accurately estimate radiation dose for chest and abdominopelvic CT. To calculate SSDE, a conversion factor (CF), ranging between 2.75-to-0.75 (newborns-to-adults), was multiplied by the examination’s CTDI_vol. The CF was derived based on a measurement of the patient’s effective diameter (Report 204), and later by the water equivalent diameter (D_w) of the patient (Report 220). D_w accounts for both the physical thickness of the body part and its attenuation properties. The accuracy of SSDE, especially in areas of low attenuation (lungs) or high attenuation (bone), was improved by accounting for D_w.