Evaluating Size-Specific Dose Estimate (SSDE) as an Estimate of Organ Doses Derived from Monte Carlo Simulations of CT Exams
In 2011, the American Association of Physicist in Medicine (AAPM) devised the Size-Specific Dose Estimate (SSDE) quantity in Report 204. SSDE is a dose metric that adjusts the commonly-reported CTDIvol metric to account for patient size. SSDE was originally developed with fixed tube current (FTC) exams of the abdomen and was later extended to the chest. SSDE represents an average dose to the center of the scan volume. As such, it gives some information concerning the radiation dose received from Computed Tomography (CT) exam but does not provide a direct estimate of organ dose by definition. AAPM Report 204 notes that the difference between the actual patient dose and SSDE may differ by 10-20%. Currently, the International Electrotechnical Commission (IEC) has introduced a measure that would allow future CT scanners to report SSDE, meaning the SSDE will become widely available dose metric.
Currently, adaptive dose reduction strategies such as attenuation-based tube current modulation (TCM) are in routine clinical use. Studies comparing SSDE to organ dose thus far have used patient models or TCM descriptions that are not seen clinically. Often times, these studies are also limited to routine chest and abdomen/pelvis exams. Moreover, recent developments in dose reduction technologies and, even federal recommendations, have produced other protocols that are commonly utilized, such as, low-dose CT lung cancer screening (LDCT-LCS) and organ-based tube current modulation (OBTCM) chest exams. The purpose of this dissertation was therefore to address the aforementioned shortcomings by evaluating an estimates of organ dose from routine and non-routine exams across a range of patient sizes to SSDE.
This dissertation evaluated SSDE in light of organ doses from four routine protocols: (1) routine FTC head exams, (2) routine TCM chest exams, and (3) routine TCM abdomen/pelvis exams. Additionally, SSDE in relation to fetal dose from routine FTC and TCM abdomen/pelvis exams was also evaluated. Furthermore, this dissertation also evaluated SSDE in light of two non-routine protocols: (1) LDCT-LCS chest exams and (2) OBTCM chest exams. In contrast to previous studies, this investigation employed patient models generated from image data. Additionally, this study also employed “whole body” voxelized phantom models that are based on image data. Where appropriate (i.e., for scans employing TCM), tube current information was either extracted directly from raw projection data or estimated based on the methodology of one manufacturer. The voxelized patient models and tube current information were used in detailed Monte Carlo (MC) simulations, currently deemed the “gold standard” of CT dosimetry, for organ dose estimation. Organ doses from MC simulation were normalized by CTDIvol (CTDIvol,16 for head and CTDIvol,32 for body exams) and were compared with the SSDE f-factors from AAPM Report 293 for head exams and 204 for body exams. Specifically, this study investigated brain parenchyma doses from FTC routine head exams; lung and glandular breast tissue doses from TCM routine chest exams; and liver, spleen, and kidney doses from TCM routine abdomen/pelvis exams in relation to the SSDE f-factors. In addition, fetal doses from both TCM and FTC routine abdomen/pelvis exams were also investigated in relation to the SSDE f-factors. For non-routine exams, lung and breast doses both from LDCT-LCS and OBTCM chest exams were investigated in relation to the SSDE f-factors. Specifically, for each routine and non-routine protocol, a one-sided tolerance interval was utilized to estimate the upper tolerance limit needed to cover 95% of the population of cases (p = 0.95) with a confidence level (α) of 5% (α = 0.05). For each evaluation, the point of comparison in terms of the tolerance window is the 20% upper limit noted in AAPM Report 204.
For routine FTC head exams, this dissertation found that the upper tolerance limit for the difference between normalized brain parenchyma dose and the SSDE f-factors needed to cover 95% of the population with 95% confidence was observed to be 12.5%. This dissertation observed that, for normalized lung and breast dose from routine TCM chest exams, the upper tolerance limit for the difference between lung and breast dose the SSDE f-factors needed to cover 95% of the population with 95% confidence was observed to be 35.6% and 68.3%, respectively. For TCM abdomen/pelvis exams, this study found that that the upper tolerance limit for the difference between normalized liver, spleen, and kidney dose and the SSDE f-factor needed to cover 95% of the population with 95% confidence was observed to be 30.7%, 33.2%, and 33.0%, respectively. This investigation found that the upper tolerance limit for the difference between TCM and FTC fetal dose and the SSDE f-factor needed to cover 95% of the population with 95% confidence was observed to be 35.7% and 24.8%, respectively. For LDCT-LCS chest exams, this study observed that the upper tolerance limit for the difference between normalized lung and breast dose and the SSDE f-factor needed to cover 95% of the population with 95% confidence was observed to be 40.0% and 70.1%, respectively. For OBTCM chest exams, this study observed that the upper tolerance limit for the difference between normalized lung and breast dose and the SSDE f-factor needed to cover 95% of the population with 95% confidence was observed to be 50.5% and 64.0%, respectively.
The upper threshold limit of 20% between SSDE and organ dose was found to be insufficient to cover 95% of the population with 95% confidence for all of the organs and protocols investigated in this dissertation, with the exception of brain parenchyma dose from routine FTC head exams. Results of this dissertation suggest that a wider upper limit may be more appropriate if SSDE is to be used as an estimate for organ doses. For the routine body exams, a wider threshold difference of ~30-36% will be wide enough to cover 95% of the organs with 95% confidence investigated in this chapter, excluding the breasts. This tolerance difference may also be sufficient to cover 95% fetal dose with 95% confidence from abdomen/pelvis exams of pregnant patients pending adequate sample size. SSDE is likely to serve as a conservative estimate for breast dose from routine TCM chest and lung and breast dose from LDCT-LCS and OBTCM non-routine protocols. Another dose model that takes TCM into consideration may be needed.