A comparison of breast and lung doses from chest CT scans using organ‐based tube current modulation (OBTCM) vs. Automatic tube current modulation (ATCM)

Abstract Purpose The purpose of this work was to estimate and compare breast and lung doses of chest CT scans using organ‐based tube current modulation (OBTCM) to those from conventional, attenuation‐based automatic tube current modulation (ATCM) across a range of patient sizes. Methods Thirty‐four patients (17 females, 17 males) who underwent clinically indicated CT chest/abdomen/pelvis (CAP) examinations employing OBTCM were collected from two multi‐detector row CT scanners. Patient size metric was assessed as water equivalent diameter (Dw) taken at the center of the scan volume. Breast and lung tissues were segmented from patient image data to create voxelized models for use in a Monte Carlo transport code. The OBTCM schemes for the chest portion were extracted from the raw projection data. ATCM schemes were estimated using a recently developed method. Breast and lung doses for each TCM scenario were estimated for each patient model. CTDIvol‐normalized breast (nDbreast) and lung (nDlung) doses were subsequently calculated. The differences between OBTCM and ATCM normalized organ dose estimates were tested using linear regression models that included CT scanner and Dw as covariates. Results Mean dose reduction from OBTCM in nDbreast was significant after adjusting for the scanner models and patient size (P = 0.047). When pooled with females and male patient, mean dose reduction from OBTCM in nDlung was observed to be trending after adjusting for the scanner model and patient size (P = 0.085). Conclusions One specific manufacturer’s OBTCM was analyzed. OBTCM was observed to significantly decrease normalized breast relative to a modeled version of that same manufacturer’s ATCM scheme. However, significant dose savings were not observed in lung dose over all. Results from this study support the use of OBTCM chest protocols for females only.


| INTRODUCTION
Computed tomography (CT) was first introduced in the 1970s, and the technology has rapidly evolved making it an important and highly utilized diagnostic tool for clinicians. In 2007, an estimated 62 million CT exams were performed annually with an annual growth rate of 12.5% from 2008 to 2009. 1,2 The increased utilization of CT procedures, and particularly the frequency of exams per patient, has raised concern about potential carcinogenic health risk from the associated radiation, despite the fact that the potential cancer risk associated with a single CT scan is considered to be very low or non-existent. 1,3 Another important aspect when considering radiation-related carcinogenesis is the radiation dose to radiosensitive organs, such as the breast. The breast is one of the most radiosensitive organs and is incidentally irradiated during certain CT exams while not being the organ of interest, for example, when evaluating the lung for pulmonary embolism using a thoracic CT exam.
Several approaches have been explored to reduce radiation dose to the breast from CT procedures. For instance, radioprotective shields made with bismuth can attenuate the entrance exposure during the CT exam thereby reducing the breast dose. [4][5][6] However, it has been demonstrated that these protective devices can negatively impact image quality by increasing noise, create streak and beam hardening artifacts, and affect CT number accuracy. [7][8][9] CT technology advancements have widely replaced the use of bismuth breast shields by incorporating techniques such as automatic exposure control (AEC). One type of AEC which is used routinely in clinical practice is the attenuation-based, automatic tube current modulation (ATCM). ATCM adjusts the x-ray tube current along the angular and/or longitudinal direction to optimize the dose distribution based on the patient's overall attenuation and provide overall improved image quality at a reduced radiation dose. 10 A more recent evolution in CT technology provides additional radiation dose savings, particularly to radiosensitive organs, with organ-based tube current modulation (OBTCM). This technique reduces the x-ray tube current preferentially over projections containing the radiosensitive organ. For instance, the anterior (assuming the patient is positioned supine) x-ray tube current is reduced in a chest CT to decrease dose to the breast. One OBTCM approach reduces the x-ray tube current over the anterior 120°of the patient with the aim of redistributing the dose posteriorly to reduce dose to the anterior organs (including breast) while maintaining image quality. 11,12 Several investigators have quantified the anterior dose distribution from OBTCM relative to conventional ATCM using physical measurements with dosimeters in anthropomorphic phantoms. A study conducted by Lungren et al. used MOSFET detectors on an adult, female anthropomorphic thoracic phantom to measure the anterior dose distribution from OBTCM and reported a 17-47% decrease anteriorly with accompanying maximum 52% increase posteriorly relative to ATCM. 13 A similar study done by Matsubara et al. used radiophotoluminescence dosimeters to specifically investigate the breast dose with OBTCM and reported a 22% reduction. 14 The use of Monte Carlo (MC) simulation techniques have also been used to quantify breast dose reduction from OBTCM. MC studies conducted by Fu et al. and Franck et al. have noted 15% and 11% breast dose reduction, respectively, from angular tube current reduction strategies. 15,16 Additionally, a few recent MC studies have noted an increase of lung dose with the use of OBTCM relative to conventional ATCM. Franck et al., for example, noted an average 11% increase in lung dose with OBTCM relative to conventional ATCM in female, oncologic patients with MC simulations from ImpactMC. 15 Similarly, Lopez-Rendon et al., observed an average 7.8% increase in lung dose when comparing OBTCM to conventional ATCM in female and male cadavers. 17 Additionally, the MC studies conducted by Fu et al. using the female XCAT phantoms showed small decrease in lung dose when using a 180°of fluence reduction. 16,18 This can be confounding when considering both breast and lung tissues are equally radiosensitive per ICRP 103 tissue weighting recommendations. 19 Although the use of OBTCM may spare dose to anteriorly located radiosensitive organs, the potential increased dose to posteriorly located organs may result in no net benefit in terms of overall population risk. 18,20 While phantom studies consider the effects of OBTCM on dose distribution, they have two important limitations in terms of evaluating breast dose. The first limitation is that phantom models are often not representative of actual patient anatomy since positioning of the breast may place the radiosensitive tissue within the area of increased radiation due simply to tissue movement or deformation.
The study by Lungren et al. observed that for adult female patients, the average angle needed to contain all breast tissue was 155°. 13 Moreover, other investigators have demonstrated, for actual adult female patients, the breasts may not be positioned within the OBTCM angular range of reduced fluence when the patient is in the supine position. 15,21,22 The second limitation is that point dose measurements from dosimeters may not reflect the average dose to the organ of interest (e.g., the breast) due to the heterogeneity of the dose distribution within the patient (or more specifically the breast).
This can be especially true when that distribution is not uniform and particularly near the surface, 23 such as estimating dose to the breast from helical scans and especially when some form of tube current modulation (including angular) modulation is being used.
MC-based simulation studies also possess some limitations. The MC approach employed by Franck et al. only had direct access to the actual z-axis modulation of the tube current and therefore had to model the angular modulation of the tube current based on some approximations. 15 In addition, their within-patient comparison of OBTCM to ATCM involved using scans of oncology patients scanned 6 months apart. While this is a reasonable approximation, that approach did not account for any change in patient positioning, weight gain/loss or other issues between scans. Finally, their study did not consider the effects of patient size. Lopez-Rendon et al. was able to more directly compare OBTCM and conventional ATCM due to the use of cadavers but was limited in terms of the number of patients. 17 Both studies of Fu et al. utilized computational patients and theoretical expressions of OBTCM and ATCM that may not take into account limitations of clinical systems and thus used modulation functions that might not be realized in the clinical setting. 16,18 In addition, the use of only female models presents the same issue of not considering the clinical relevance of dose penalties or savings for males as mentioned above.
Therefore, the purpose of this work was to estimate and compare breast and lung doses of chest CT scans using OBTCM to estimates of ATCM for both female and male patients and across a range of patient sizes. To overcome the limitations of the previous work, a validated MC simulation approach was used to accurately model the CT scanner, the patient and actual organ-based TCM schemes that were patient-specific. In order to perform a direct within-patient comparison of lung and breast dose from OBTCM relative to conventional TCM, a previously developed and validated method was used to model the conventional ATCM scheme based on CAREDose4D 24 (Siemens Healthineers, Forchheim, Germany).
This study considered organ doses from OBTCM relative to ATCM for females, males, and pooled populations, with the pooled being used to determine the effects of having one OBTCM protocol used for both females and males.

| MATERIALS AND METHODS
This study employed MC simulation methods for CT radiation transport to compare lung and breast doses from clinical OBTCM scans to those of estimates of conventional ATCM. To obtain as realistic an estimate of organ dose as possible, actual tube current modulation data were extracted from clinical performed scans on individual patients (both male and female). Specifically, the OBTCM tube current information was extracted from the raw projection data collected from clinical patient scans obtained directly from the CT scanners. Additionally, the image data that resulted from the OBTCM scans were used to create voxelized patient models that were specific to the scan performed. Because the patients were only scanned once clinically, a direct comparison to ATCM was not possible, so the conventional ATCM tube current information was estimated using the attenuation information in the topogram as described by McMillan et al. 25 Both of these TCM schemes were then incorporated separately into MC simulations on a per-patient basis for a direct comparison between OBTCM and ATCM lung and breast dose. The details of this approach are provided below. Healthineers, Forchheim, Germany). For this study, only the chest portion of the CAP exams was used. The chest portion of all CAP examinations used OBTCM and were acquired with the patient in the supine position. Additionally, all images were reconstructed to 500 mm full field-of-view (FOV) in order to ensure that the patient anatomy was contained within the image data. This includes the glandular breast tissue for female patients. The patient size in terms of water equivalent diameter (D w ) was determined for the image data of each patient at the center of the volume. 26 Specifically, D w was estimated using an ROI that encompassed the patient anatomy in the central cross-sectional image.

2.A | Patient models
In order to use patient data in MC simulations, voxelized models of each patient's anatomy were created from the image data. Voxels within each image series were modeled as either lung, fat, water, muscle, bone or air then subdivided into one of 17 density levels depending on their CT number. 27 The lung and glandular breast tissues were semi-automatically contoured and identified in the female models. Lung tissue was also segmented in male models, but glandular breast tissue was not in male patients. 28 Figure 1 contains images of a female patient with lung and breast tissues segmented.

2.B | CT scanning protocol
The CT chest protocols used to acquire the raw data from the two scanners are shown in Table 1. For 10 cases acquired on the Flash scanner, a tube voltage of 100 kVp was applied due to CARE kV (Siemens Healthineers, Forchheim, Germany) being utilized for these scans. For all 34 cases, scans were acquired using the OBTCM technique offered by the manufacturer (XCARE, Siemens Healthineers, Forchheim, Germany). This OBTCM algorithm employs an angle of 120°for fluence reduction that cannot be changed by the user. To provide comparisons with an ATCM acquisition on the same patients, the methods developed by McMillan et al. 25 were applied to produce predicted tube current modulation profiles for each patient using the same scanner parameters shown in Table 1. Table 1 are the software versions for the Force and Flash. The details of generating predicted ATCM schemes are based on these scanning parameters are briefly described below in Section 2. C.  25 :

2.C | Modeling tube current modulation schemes
where μ water, kVp is the linear attenuation coefficient of water for a given beam energy. For this investigation, μ water, kVp was set to 0.2 cm -1 for a 120 kVp beam. The maximum attenuation at each where QRM is the quality reference effective tube current-product set by the user on the CT scanner, t is the gantry rotation time, A ref is the protocol-specific reference attenuation hard coded into the ATCM algorithm, and b is a strength parameter set by the user to control the rate by which the tube current increases or decreases. In this study, the QRM was set to 140 based on the radiologist's preference for the desired image quality of an average sized patient. The strength parameter (b) was set to "Average", which corresponds to 0.33 for attenuation greater than A ref and 0.5 for attenuation less than A ref . 25,29 The angular current tube was calculated at each table position, i, for a helical scan using [Eq. (3)] where hROT is the half rotation of the tube given by one half the collimation multiplied by the helical pitch, A(i -hROT) is the patient attenuation at the table position a half rotation prior to the current table position, A min is minimum patient attenuation over the previous half rotation, A max is the maximum patient attenuation over the previous half rotation, q is an optimization parameter between 0.5 and 1.0, and μ(i) is a gantry rotation time-dependent parameter that limits the amount of modulation allowed at a given table position. 25 The complete estimated ATCM schemes were calculated by multi- together.
Predicted ATCM schemes were generated for the table positions corresponding to the chest portion of the CAP topogram. In order to perform a direct per-patient comparison with the OBTCM schemes extracted from the raw projection data, the ATCM schemes were estimated in accordance with the same imaging protocols specified in Section 2. B. Figure 2 depicts an extracted OBTCM scheme and predicted ATCM scheme of overlaid on a patient topogram. Figure 3 shows three-dimensional renders of the OBTCM and ATCM tube current profile.

2.D | MC simulations and dose calculations
A modified version of the MC software package MCNPX (Monte Carlo N-Particle eXtended version 2.7.a) was utilized for all the simulations in this study. 30,31 The modification allowed for the modeling of MDCT scanner geometry and beam spectrum. [32][33][34][35] Specifically, in this investigation, the appropriate beam energy spectrum data were generated using the equivalent source method developed by Turner et al. 36

2.E | CTDI vol values
For each patient, CTDI vol for the chest portion of the CAP scan using OBTCM was taken from the patient protocol page. In order to estimate CTDI vol for the predicted ATCM simulated scans, the collimation and bowtie-specific CTDI vol per mAs values given in Table 1 were multiplied by the average tube current-time product across the entire simulated scan length.  | 101 lung doses (nD breast and nD lung ). Comparisons between OBTCM and ATCM for normalized doses were done on a per-patient basis by calculating within-patient difference (%) relative to ATCM. Comparisons were done for: 1) nD breast and nD lung for females, 2) nD lung for males, and 3) nD lung for both females and males (pooled). Comparisons were conducted with CTDI vol -normalized doses in order to account for the radiation output from two scanners and the different protocols used in this investigation. 39 Negative differences were interpreted as dose savings relative to ATCM, while positive differences were interpreted as dose penalties relative to ATCM.  Table 3 of that study. 25 A tolerance limit interval covering 90% of the population with 95% confidence level was then calculated with non-central t-distribution per organ for females, males, and the pooled population of females and males.

2.G | Statistical analysis
After identifying the cases within and outside of the tolerance limit, one-sample proportion tests were performed to determine whether the chance of occurring outside of the tolerance limit is random. 40 All statistical analyses were performed in Stata (v. 14.1, College Station, Texas).

3.A | 3. A Patient Size and CTDI vol values
The measured range of D w was 20.  Figure 5. The R 2 value between OBTCM and ATCM CTDI vol was observed to be 0.58.

3.B | Organ dose comparisons between OBTCM
and ATCM

3.B.1 | Breast and lung dose comparison for females
For females, the OBTCM difference relative to ATCM for nD breast (ΔnD breast ) was observed to range from −31% to 21%. The mean, median, and standard deviation (SD) of the difference from ATCM for females was −10%, −13%, and 16% for ΔnD breast , respectively.
An R 2 value of 0.28 was observed when correlating ΔnD breast from OBTCM with D w . Mean dose reduction of OBTCM in nD breast was significant after adjusting for the scanner models and D w (P = 0.047).
The nD lung for females demonstrated a difference relative to ATCM (ΔnD lung ) ranging from −18% to 26%. The mean, median, and standard deviation of the difference from ATCM for females was −2%,

3.B.2 | Lung dose comparison for males
For males, the OBTCM differences relative to ATCM for nD lung (ΔnD lung ) ranged from −21% to 36%. The mean, median, and standard deviation (SD) of the difference compared to ATCM for males was 9%, 13%, and 16% for ΔnD lung , respectively. Table 5 contains the lung dose comparison for males. The normalized dose in the Flash scanner model was observed to be significantly higher than the Force model with the mean of 16% and 0%, respectively (P = 0.038). The tolerance limit covering 90% of the population with 95% confidence was observed to be [1.29, 11.04%] in lung dose for males. Fourteen males (N outside = 82%) had larger differences than each group's tolerance limits (p = 0.0075).

3.B.3 | Pooled breast and lung dose comparison
When both populations were pooled (i.e., females and males), the mean, median, and standard deviation of the difference for ΔnD lung was 4%, 5%, and 15%, respectively. When correlating ΔnD lung with respect to D w , pooled across females and males, an R 2 value of 0.48 was observed. When pooled, mean dose reduction from OBTCM in nD lung was observed to be trending after adjusting for the scanner model and patient size (P = 0.085). A summary of ΔnD breast and ΔnD lung for all patients is given in Table 6. The tolerance limits covering 90% of the population with 95% confidence were observed to be [2.04%, 6.80%] in lung dose for the pooled group. Using the pooled tolerance limit, 28 out of 34 (N outside = 82%) had larger differences between OBCTM and ATCM than ATCM's reproducibility (P = 0.0002). A summary of ΔnD breast and ΔnD lung and tolerance limit coverage for all patients are given in Table 6 and Table 7, respectively.

| DISCUSSION
The objective of the introduction of OBTCM was to reduce the radiation dose to radiosensitive organs, particularly more anteriorly posi-   This study noted a significant mean dose reduction in nD breast from OBTCM after adjusting for the scanner models and patient size (P = 0.047). Furthermore, the results in Table 3 demonstrates an overall average reduction in the normalized breast dose (ΔnD breast = −10%) when using OBTCM compared to ATCM. For this study, male breast dose was not considered due to the absence of glandular tissue in the cross-sectional images of the male patients. Additionally, the cancer biology of male breast cancer appears to be distinct from that of female breast cancer, both of which being outside the scope of this study. 41 As can be seen in Table 3  T A B L E 6 Summary of female, male, and pooled mean, median, and standard deviation (SD) of differences (%) for ΔnD breast and ΔnD lung . Negative and positive differences were interpreted as dose savings and dose penalties relative to ATCM, respectively.   test. This study used estimated ATCM as opposed to an actual ATCM directly from the raw projection data to make comparisons with OBTCM. This estimated ATCM does introduce around 5% error in terms of dose estimates. 25 A comparison study of this nature would also ideally compare doses from ATCM values extracted from raw projection data. However, this was not an option because obtaining IRB approval for duplicate chest scans for this purpose alone was not considered feasible. This study did not investigate the effects of OBTCM on image quality relative to ATCM. OBTCM has been shown to increase image noise in relation to ATCM. 43,44 Lastly, this study did not consider the impact of patient centering as this study attempted to look at the best-case scenario, which entailed the patient being centered in the gantry. Patient mis-centering has been shown to yield consequences for the anterior-posterior dose profiles, thereby affecting patient dose. 45,46

| CONCLUSION
In this study, the OBTCM algorithm of one manufacturer (Siemens) was analyzed in relation to an estimated version of the ATCM algorithm of the same manufacturer. The OBTCM algorithm investigated in this study proports to reduce dose to anteriorly located organs such as the breast. 11 This study found, on average, a reduction of