SU‐E‐I‐08: Do CTDI Measurements Approximate Peak Skin Dose for Wide‐Beam Volumetric Scans?

Purpose: To investigate the relationship between peak skin dose and CTDI measurements for wide‐beam volumetric (stationary table) CT scanning. Methods: Air‐kerma (AK) measurements were performed in head and body CTDI phantoms using conventional CTDI, extended CTDI (CTDIe), and AAPM TG‐111 protocols. These measurements were compared to skin dose estimates obtained using a small volume (0.1 ml) ion chamber (IC) mounted on the anterior surface of the phantoms. Measurements were made on a volumetric CT scanner (Toshiba Aquilion ONE) for collimations of 40, 80, 120 and 160 mm and at kVp settings of 80, 100, and 120. Anterior and lateral surface dose measurements were also made on anthropomorphic phantoms at the same fixed techniques. Results: In the CTDI body phantom, peripheral CTDIe nearly equals surface dose while CTDIevol underestimates surface dose by 10–30% over the collimation range. In the CTDI head phantom, either the peripheral or the volume CTDIe values fall within approximately 5% of the surface dose for collimations greater than 80 mm. At smaller collimation, CTDIe values overestimate surface dose in the head phantom. TG‐111 f(0) measurements in the anterior peripheral location slightly overestimate anterior surface dose measurement results for both head and body CTDI phantoms. Surface measurements on the anthropomorphic phantoms exhibit variations between anterior and lateral locations, with lateral doses being smaller. Conclusion: Values derived from CTDIe measurements are useful indicators of peak skin dose for cylindrical phantoms at beam widths in excess of 80 mm. In patients, geometrical correction factors similar to the Size Specific Dose Estimate (SSDE) methodology might be used to account for actual body habitus. Toshiba America Medical Systems is supporting an associated research project on the scanner used for this work. Dr Di Zhang is an employee of Toshiba America Medical Systems.