Size specific dose estimates in computed tomography examinations of thorax, abdomen and pelvis: Pediatric and adult patients

SIZE SPECIFIC DOSE ESTIMATES IN COMPUTED TOMOGRAPHY EXAMINATIONS OF THORAX, ABDOMEN AND PELVIS: PEDIATRIC AND ADULT PATIENTS İsmail Ozsoykal Dokuz Eylul University Health Sciences Institute Medical Physics Department, İnciralti-IZMIR ozsoykal@gmail.com Objective: Volume Computed Tomography Dose Index (CTDIvol) does not contain any dose information regarding the patient size in Computed Tomography (CT) exams. Size specific dose estimates (SSDE), calculated by the multiplication of CTDIvol and size dependent conversion factors (fDw), give more reliable results accounting for the patient size, but requires a complex series of calculations as well. This study aims to develop a software to automatically calculate SSDE values along the patient and compare mean SSDE value to the central SSDE value for each examination, owing to the consideration of a more practical method of SSDE determination. Material and Method: 55 consecutive pediatric CT exams of the combined chest, abdomen and pelvis (CAP) have been examined. MATLAB based algorithm has been developed to calculate size dependent conversion factors (fDw) by means of water equivalent diameter (Dw) of the patient cross section in every slice of image. Examination specific CTDIvol parameters have been taken from dose reports. SSDE values have been calculated for 6 hypothetical scan ranges: chest alone (1), abdomen alone (2), pelvis alone (3), chest and abdomen (4), abdomen and pelvis (5), and CAP (6). Two SSDE values have been recorded and compared: mean SSDE over each scan range (1) and SSDE in the middle of each scan range (2). Results: For scan ranges 1 to 6, the root mean square differences between both SSDE values have been found to be 0.4, 0.5, 0.3, 1.0, 1.5, 0.7 mGy corresponding to 3%, 4%, 2%, 7%, 10%, 5% percentage differences, respectively. Conclusion: For pediatric exams, instead of making mean SSDE calculation over the whole scan range, using the mean CTDIvol and the fDw information for the central slice of the scan range provides a reasonable mean SSDE approach. Keywords : Computed Tomography, Dose, Computed Tomography Dose Index, Size Specific Dose Estimates, Pediatry