TY - JOUR T1 - Effective Dose from CT scanning in Eyes-to-Thighs PET/CT JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 1867 LP - 1867 VL - 57 IS - supplement 2 AU - Matthew Palmer AU - Da Zhang AU - J Anthony Parker Y1 - 2016/05/01 UR - http://jnm.snmjournals.org/content/57/supplement_2/1867.abstract N2 - 1867Objectives Oncologic 18-FDG PET studies are most commonly acquired over an axial extent spanning the mid skull to just below the pelvis. Even when low dose protocols are used for the anatomical localization and attenuation correction (ACAL) CT scan, the radiation dose imparted by CT constitutes a large fraction of the total effective dose. Estimating the effective dose from an eyes-to-thighs CT study acquired with a modern multi-slice CT scanner is complicated due to a number of factors: lack of agreed upon dose scaling factors (k-factors); dose reports based solely on the 32 cm phantom (not normally used as references for head and neck) and the availability of only the average CTDIvol to characterize the dose when the x-ray output intensity, due to tube current modulation (TCM), varies over a wide dynamic range. We therefore sought a new method to estimate effective dose valid for eyes-to-thighs CT.Methods We modified a popular CT dosimetry software tool (ImPACT CT Dosimetry Calculator V1.04) to allow us to import a tube-current modulation curve. This tool has the inherent ability to apply tissue weighting factors from either ICRP 60 or ICRP 103 and imports Monte Carlo data for a number of commercial scanners including the Definition AS which is incorporated into our Siemens Biograph mCT-64 PET/CT. Because the total effective dose can be considered as the sum of effective doses due to individual scans, the modified calculator applies a weight to the irradiation profile for each 5mm-wide axial slice, in 5mm increments. We wrote additional software to extract the tube current from the DICOM header of each slice in an axial sequence. The output of the calculator is the effective dose given the TCM profile, protocol parameters and the choice of tissue weights. During eyes-to-thighs traversal of the x-ray tube, radiation sensitivity varies and can be expressed as an effective dose differential (dD/dz). The sensitivity is low over the eyes (0.02 mSv/cm per 100 mAs) and peaks over the chest (0.4 mSv/cm per 100 mAs), for 140 kV and using ICRP 103 weighting factors. To evaluate the modified calculator, we obtained data from five patients scanned in the clinic chosen to represent “normal” height and weight (175 +/- 0.3 cm, 69 +/- 3 kg). We also scanned the CIRS Atom adult male phantom (173 cm, 73 kg) using the same ACAL CT protocol (140 kV and variable mA with quality reference of 50 mAs).Results For the patients, the ACAL CT protocol resulted in a calculated effective dose of 6.95 mSv using ICRP 103 tissue weights. That compared to 7.24 mSv when a fixed tube current equal to the average was assumed (equivalent to no TCM-correction). Substituting the ICRP 60 tissue weights yielded TCM-corrected effective dose estimate of 7.48 mSv. For the phantom, the effective dose calculation (ICRP 103) yielded 5.11 mSv with TCM-correction and 5.34 mSv without. For both the patients and phantom, tube current varies during table traversal with relative maxima in the vicinity of the shoulders and pelvic girdle and relative minima over the head, neck and chest. To a large extent, this appears to counter the effect of variation in tissue radiation sensitivity. The effect of applying the correction was muted - indeed only half the magnitude of the difference between estimates using alternate sets of tissue weighting factors. For both the patients and phantom scans, the ratio of TCM-corrected effective dose (ICRP 103) to CTDIvol was found to be 1.5 mSv/mGy.Conclusions For eyes to thighs CT scans, variation in beam intensity due to tube current modulation tends to normalize the variations in tissue radiation sensitivity. Errors introduced by ignoring the effects of tube current modulation are small. Effective dose can be estimated by multiplying the average CTDIvol by a factor of 1.5 mSv/mGy. Since CTDIvol (average) is reported by the scanner both prospectively based on protocol and retrospectively in the dose report, this allows for a rapid and practical estimate of effective dose due to the CT in the PET/CT setting. ER -