Comparison of Size specific dose estimates (SSDE) and CTDIvol values in patients undergoing CT examinations of abdomen and pelvis

Abstract

Introduction: Computed Tomography Dose Index volume (CTDIvol) is a measure of radiation output of the CT scanner and does not represent patient dose. It often underestimates the radiation dose being given to small children and smaller adults and overestimates the dose to larger patients. The use of Size Specific Dose Index (SSDE) helps convert CTDIvol into more patient size specific radiation dose and is a better measure of estimated patient dose than CTDIvol. Methods: This cross-sectional study was conducted in the Department of Radiology and Imaging of Tribhuvan University Teaching Hospital (TUTH), Maharajgunj, Nepal. CT scans were performed on the Siemens Somaton Definition AS+ 128 slice scanner. During 96 CT examinations of the abdomen and pelvis collected over a period of 4 months, effective diameters were calculated from the AP and Lateral diameters at the mid-liver region. These were used to determine the conversion factors which were then used to convert the CTDIvol values to SSDE. Obtained SSDE values were compared with the displayed CTDIvol values. Results: The average CTDIvol was found to be 9.42 ± 3.26 mGy and the average SSDE was found to be 13.48 ± 3.53 mGy. Moderate positive correlation (r=+0.52, p<0.001) was found between CTDIvol and patient weight and low positive correlation (r=+0.17, p=0.08) was found between SSDE and patient weight. Similarly, moderate positive correlation (r=+0.5, p<0.001) was found between CTDIvol and patient BMI and low positive correlation (r=+0.14, p=0.15) was found between SSDE and patient BMI. Conclusion: In comparison with SSDE, CTDIvol seemed to underestimate the patient dose estimate by 30.11%. CTDIvol values showed dependency with patient weight and BMI, this dependency was significantly reduced when those values were converted into SSDE (i.e. r=+0.52, p<0.001 (CTDIvol and Patient weight) vs. r=+0.17, p=0.08 (SSDE and Patient weight) and r=+0.5, p<0.001 (CTDIvol and BMI) vs. r=+0.14, p=0.15 (SSDE and BMI). Thus, providing more concrete evidence to the fact that SSDE values are a more reliable patient dose estimate since it addresses the patient’s size.