Simplified approach to estimation of organ absorbed doses for patients undergoing abdomen and pelvis CT examination

The volumetric computed tomography (CT) dose index (CTDIvol) is the measure of output displayed on CT consoles relating to dose within a standard phantom. This gives a false impression of doses levels within the tissues of smaller patients in Southeast Asia. A size-specific dose estimate (SSDE) can be calculated from the CTDIvol to provide an assessment of doses at specific positions within a scan using size-specific conversion factors. SSDE is derived using the water equivalent diameter (D (w)) of the patient, but calculation of D (w) requires sophisticated computer software. This study aimed to evaluate relationships between D (W) and effective diameter (D (Eff)), which can be measured more readily, in order to estimate SSDE at various positions within a routine clinical abdomen and pelvis CT examination for Thai patients. An in-house ImageJ algorithm was developed to measure D (w), effective diameter (D (Eff)), and SSDE on CT slices located at the heart, liver, kidneys, colon, and bladder, on 181 CT examinations of abdomen and pelvis. Relationships between D (Eff) and D (w) were determined, and values of organ absorbed dose using D (Eff) were estimated. This approach was validated using a second cohort of 54 patients scanned on a different CT scanner. The results revealed that ratios between D (Eff) and D (w) at the heart level were 1.11-1.13 and those for the others were about 1.00. Additionally, the SSDE/CTDIvol ratio was estimated for each organ in terms of exponential functions using the relationships between D (w) and D (Eff) for individual organs. In summary, this study proposed a simple method for estimation of organ absorbed doses for Southeast Asian patients undergoing abdomen and pelvis CT examinations where sophisticated computer software is not available.