Patient Specific Organ and Effective Dose Estimates in Adult Oncology Computed Tomography

Abstract

Purpose

Patient specific organ and effective dose provides essential information for CT protocol optimization. However, such information is not readily available in the scan records. This study is to develop a method to obtain accurate examination- and patient-specific organ and effective dose estimates employing available scan data and patient body size information for a large cohort of patients.

Methods

The data was randomly collected under HIPAA compliance for over a thousand patients who received a CT scan in a two-year period. Physical characteristics of the patients and CT technique were processed as inputs for the dose estimator. Organ and effective doses were estimated using the information and computational human phantoms that matched to patients based on sex and effective diameter. Size-based ratios were applied to correct for patient-phantom body size differences.

Results

On average a patient received 60 mGy to the lens of eye per brain scan, 10 mGy to the thyroid per chest scan, 18 mGy to the liver per abdomen and pelvis scan, or 19 mGy to the liver per body scan. A factor of two difference in dose estimates was observed between patients of various body habitus.

Conclusions

Examination- and patient-specific organ and effective doses were estimated for 1200 adult oncology CT patients. The dose conversion factors calculated facilitate rapid organ and effective dose estimation in clinics. Compared to non-specific dose estimation methods, patient doses estimated with data specific to the patient and exam can be different by a factor of two.

Introduction

Computed Tomography (CT) assists in the localization and diagnosis of primary and metastatic malignances, as well as the planning of the radiological treatment [1]. CT has become one of the most popular noninvasive diagnosis measures that many physicians all over the world rely on, but it also results in radiation dose to patients that needs to be both justified and optimized [2–4]. CT scans are estimated to account for half of the medical diagnostic radiation exposure to the population in the United States (US) [5]. The principle of justification in medical imaging posits that the benefit of the exam be weighed against the biological risk incurred by radiation exposure. In this sense, assessment of image quality and estimation of radiation dose are important considerations in the medical decision-making process. According to the concept of optimization, the radiation dose shall be no greater than that needed to achieve the imaging objective. Guidelines for justification and optimization of imaging dose in the radiological community are established in the US, but some are subject to outdated data and need to be updated [3, 6–8]. In light of the dynamic nature of CT technology, in which advances in detectors and data processing are continuously implemented, radiation dose data of patients who received CT scans utilizing the latest protocols and technologies is needed to provide solid ground for dose optimization, protocol assessment, and protocol development. Ensuring dose estimates reflect the actual CT imaging conventions allows clinicians to better inform not only the patient but also themselves during the clinical decision-making process.

Diagnostic reference levels (DRL) are the recommended dose levels specified by scan type, established by national, regional or local radiological protection institutions, and are used to identify the CT practices with unusually high doses and assist with the dosimetric optimization of CT protocols [3, 9–11]. The dose quantities employed in establishing CT DRLs typically include volumetric CT dose index (CTDI_vol), dose length product (DLP), and more recently the size-specific dose estimate (SSDE) [3, 10–12]. DRLs are typically set as the 75^th percentiles of the distributions of the dose quantities based on national, regional or local dose surveys [3, 10, 11]. Countries all over the world have established DRLs for CT dose optimization [10, 11, 13–17]. The establishment and application of DRLs has been shown to facilitate protocol optimization and reduce unnecessary radiation dose in CT scans while maintaining image quality [17, 18].

Optimization of existing protocols and assessment of new protocols ponder the tradeoffs between the benefit of timely diagnosis with the risk of radiation-induced effects such as secondary cancer [19–22]. Organ dose and effective dose are required to estimate the risk of radiation-induced stochastic effects [22], but they are not readily available in scan records or images. Complex modern CT protocols further complicate the estimation of organ doses to patients. For oncologic patients, multiple diagnostic CT scans may be prescribed before, during, and after treatments [23, 24]. Moreover, loosely defined CT scan range, which varies according to preference and local convention, can have significant impact on radiation dose. The scan range of a protocol often varies among institutions and among physicians and technologists, where an extended range can include more radiosensitive organs at the ends of the scan [25]. The dose to small radiosensitive organs such as the thyroid, male gonads, or salivary glands can increase up to five times in a matter of a few centimeters of scan range extension [25]. Modern scan techniques also affect radiation dose and are not necessarily accounted for in all dose estimation methods, as is the case with tube current modulation (TCM) technique. TCM changes the X-ray quantity based on the attenuation of the body section, and it can change the dose to superficial organs by about 60% comparing to the dose from scans without this technique at all [26–28]. AAPM report No. 96 provides conversion factors (k factors) for effective dose estimation but not for organ dose estimation, and the effect of TCM on dose is not considered [29]. Thus, sophisticated organ dose estimation tools, which can account for small variations in scan range and utilization of modern scanner features, should be used to perform organ dose estimation for the complex and various modern CT scan protocols.

Effective dose has been recommended for comparing doses to different populations, doses by different protocols or doses given at different institutions [29, 30]. Effective dose can be accurately calculated from the weighted sum of tissue equivalent doses of both sexes [30], or roughly estimated from a CT tube output parameter such as DLP [29]. Accurate organ doses (and subsequently tissue equivalent doses) are the foundation of accurate effective dose estimates. On the other hand, quick effective dose estimates can be obtained by converting from a CT output indicator such as DLP with a dose conversion coefficient as a fast and efficient approximation method in the clinical setting [31–34]. The convenience of such coefficients is compromised by the influence of patient body size variations and patient-specific scan parameter variations. Researchers have found that k factors decrease as body mass index (BMI) increases for adult populations [35]. BMI-based or size-based k factors were generated to provide more reasonably accurate dose estimates in clinics for cohorts of patients of various sizes [35, 36].

The objectives of this study are to develop a method to obtain accurate examination- and patient-specific organ and effective dose estimates employing available scan data and patient body size information as well as Monte Carlo based approaches for over a thousand adult oncology patients who have received CT scans at our institution. More accurate organ and effective doses can assist in clinical protocol optimization, clinical decision making, and future epidemiology studies. Patient-specific results are compared to other methods and national DRLs, while conversion factors are generated to characterize the patient population and facilitate fast dose estimation in our clinic.

Materials and Methods

Patient population

This HIPAA-compliant retrospective study had obtained institutional review board approval with a waiver of patient informed consent for the use of CT scans of adult oncology patients performed during a contiguous 2-year period. All DICOM images were anonymized prior to use. A total of 1200 adult CT cases were selected for diagnostic protocols covering four body regions: head (brain), chest, abdomen-pelvis (AP), and chest-abdomen-pelvis (CAP). Specifically, for each body region and each sex, 150 cases were randomly selected.

The patient body anterior-posterior size and lateral size were reported by the vendor-provided application DoseWatch™ (General Electric Inc., Milwaukee, USA), and then effective diameter was calculated using the methods described in AAPM TG 204 [6]. The body sizes reported by DoseWatch were verified by manually measuring the anterior-posterior and lateral sizes on the CT images for 20 randomly selected patients. The dose estimator, VirtualDose™ CT, compared the effective diameters of the patients to these of anthropomorphic computational phantoms by subtracting one diameter from the other and calculating the absolute differences. By matching each patient with a phantom of the least size difference, the software then categorized the patients into five sub-groups (normal-weight to obese level-III), characterized by phantoms that had been made to fit in the World Health Organization BMI category classification as shown in Supplemental Table 1 [37, 38].

CT protocols

All scans were performed on GE Discovery CT750 HD scanners (General Electric Inc., Milwaukee, USA), as shown in Table 1. The tube voltage for all scans was 120 kVp. The tube current varied from case to case depending on the use of TCM, the size and density of scanned region, and the noise index prescribed for the diagnostic task. The exposure time per gantry rotation ranged from 0.5 second to 1 second. The pitch was 0.984 for body scans and 1 for head scans. The nominal total collimation was 20 mm for head scans and 40 mm for body scans. Body bow-tie filter was used in body scans and medium bow-tie filter was used in head scans. A 2 cm overscan (half of a rotation) was assumed and added to both ends of helical body scans in dose estimation.

Table 1.

CT technique summary

Examination	Tube potential (kVp)	Tube Current* (mA)	Revolution time (s)	Bow tie filter	Pitch	Collimation (mm)	Scan Range
Head	120	300	1	Medium Filter	1.000	20	Hard palate-base of occipital bone to vertex
Chest	120	TCM (120–380)	0.5	Body Filter	0.984	40	Supraclavicular through adrenal glands (level of L2 vertebrae)
Abdomen and pelvis with contrast	120	TCM (220–380)	0.7	Body Filter	0.984	40	Diaphragm to pubic symphysis
Chest, abdomen and pelvis with contrast	120	TCM (220–380)	0.7	Body Filter	0.984	40	Supraclavicular to pubic symphysis

^*TCM is Tube Current Modulation.

Routine CT protocols for four types of regions were investigated: head, chest, AP, and CAP. The scan ranges were defined by anatomical landmarks as shown in Table 1. The most commonly used protocols were selected. For head scans, the selected protocols were head with contrast, head without contrast, and head with or without contrast. For chest scans they were chest with contrast and chest without contrast. For AP scans it was AP with contrast. For CAP scans it was CAP with contrast.

Scan technique and basic dosimetric information (CTDIvol, DLP, and SSDE) of adult CT scans were gathered through DoseWatch™ (General Electric Inc., Milwaukee, USA). Then the desired scans (routine clinical scan of the four investigated body regions, excluding impromptu extra scan and non-clinical scan, with scan parameters and patient size information) were filtered out of tens of thousands of scans with in-house database queries in Microsoft Access (Microsoft Inc., Redmond, Washington). A list of male patients and a list of female patients were generated for each of the four regions. The third step was randomized selection of 150 male patients and 150 female patients for each body region from the corresponding list in Matlab (version 2016b, Natick, Massachusetts, USA). Finally, since most of the cases used TCM technique, archived and anonymized DICOM images of these CT scans were gathered through the HERMES GOLD™ (Hermes Medical Solutions Inc., Stockholm, Sweden) system as the source of slice-by-slice scan parameters for dose estimation.

Dose percentiles for Diagnostic Reference Level

CTDI_vol, DLP, and SSDE were gathered from the records of DoseWatch™. The SSDE for head scans were ignored due to (1) incorrectly estimated head sizes by the vendor software in about 10% of the investigated cases, (2) the fact that adult head does not vary as much as body, and (3) fixed adult routine head scan technique. The 75^th percentiles of the three quantities were summarized for each protocol covered in this study. They were compared to the national DRLs derived from over one million CT records of the Dose Index Registry (DIR) [12].

Estimation of organ and effective doses with body region specific corrections

Protocol parameters, including slice-by-slice tube current were extracted from the DICOM headers of images with a previously developed tool (named “DICOMDataImportExportTool”, written in C# programming language) and then were sent to the API (Application Programming Interface) of the Monte Carlo based organ dose and effective dose estimator, VirtualDose™ CT and stored in a database ready for retrieval in the following steps [38]. Further, VirtualDose™ CT was modified to automatically match a human phantom to each patient by comparing effective diameter of the patient’s scanned body region (calculated from DoseWatch™ data) with that of the phantoms which were previously measured in the software 3D-Doctor (Lexington, Massachusetts, USA) following AAPM TG 204 methodology [6]. The phantom with the least diameter difference from the patient was selected. Normal weight phantoms were used for all head scans. After phantom selection, the parameters of the CT scan protocol were retrieved from the database and automatically used to estimate the organ doses and effective doses. The effect of the remaining size difference between the phantom and the patient on dose was addressed by applying a table of SSDE ratio for phantom-patient size correction presented according to the following description.

To account for size variations among patients and between body regions, ratios of body-region-specific patient SSDE to phantom SSDE were calculated and used during dose estimation. Specifically, as shown in the equation, for chest scans the ratios of chest SSDE were applied. For AP scans, the ratios of abdomen SSDE were applied to abdomen region, and the ratios of pelvis SSDE were applied to pelvis region. For CAP scans, the ratios of chest SSDE, the ratios of abdomen SSDE, and the ratios of pelvis SSDE were applied to chest, abdomen, and pelvis regions respectively. These ratios were summarized in Supplemental Table 1. As a result, the dose to any organ from a body scan was the sum of the dose from chest scan (if any), abdomen scan (if any) and pelvis scan (if any) after applying SSDE ratios respectively.

$$Organ Dose=\sum _{i=1}^N{D}_i\times \frac{SSD{E}_{i,patient}}{SSD{E}_{i,phantom}}$$

Where Organ Dose is dose in mGy to any organ defined in the phantom. N is the total number of scanned region: one region (chest) for chest scans, two regions (abdomen and pelvis) for AP scans, and three regions (chest, abdomen and pelvis) for CAP scans. The ratio was calculated for each region individually. D_i is dose in mGy to the organ from the primary and/or scatter photons of the scanned region i. SSDE_i,patient is the SSDE of the patient for the scanned region i. SSDE_i,phantom is the SSDE of the phantom for the scanned region i, generated by multiplying CTDI_vol for the region and the AAPM $f_{size}^{32X}$ factor for the size of the body region [6, 39].

With the method described previously in this section, organ doses were estimated and were compared to doses estimated with size-specific methods in the literature [40, 41]. Effective doses were calculated by VirtualDose™ CT, which uses the ICRP 103 methodology. For comparison purposes, effective doses were also calculated by converting DLPs using DLP-to-Effective-Dose k factors from AAPM report No.96 [29].

Size-based dose conversion factors

In this study, adult patients were categorized into five groups by the BMI category: normal weight, overweight, obese class I, obese class II, and obese class III. The k factors (DLP-normalized effective dose) for each category were calculated for all the body regions. The k_org factors for converting DLP to organ doses for male and female patients of various sizes were also calculated to enable rapid clinical dose estimations.

Results

Patient demographics

A total of 1200 adult patient CT scan data were used for dose calculations in this study with 50% males and 50% females. The percentages of patients in the normal weight category were 38% (chest), 45% (AP), and 45% (CAP). The percentages of patients in the overweight category were 13% (chest), 36% (AP), and 27% (CAP). The percentages of patients in obese class I and above categories were 49% (chest), 19% (AP), and 28% (CAP).

Dose percentiles for Diagnostic Reference Level

Table 2 shows the 75^th percentiles, minimum and maximum of our study CTDIvol, SSDE and DLP. The table also shows the US national 75^th percentiles of same parameters (national DRLs) from the literature [12]. The 75^th percentiles of the present study were very close to the national DRLs [12]. Most of the 75^th percentiles were lower than those of the national values, and the rest of the 75^th percentiles were no more than 7% higher than the corresponding national values.

Table 2.

CTDIvol, DLP, and SSDE of each type of CT examination

Examination	No.	CTDIvol (mGy)			SSDE (mGy) †		DLP (mGy-cm)
		75th, Min-Max	75th from DIR*	75th, Min-Max		75th from DIR*	75th, Min-Max	75th from DIR*
Head	300	60, 56–60	57	-		-	1078, 958–2156	1011
Chest	300	12, 5–15	15	14, 7–18		16	475, 150–658	545
Abdomen and pelvis with contrast	300	17, 12–26	19	20, 16–25		19	1007, 555–1439	995
Chest, abdomen and pelvis with contrast	300	16, 10–21	19	19, 14–22		19	1263, 684–1708	1193

^*The Dose Index Registry (DIR) data was summarized by Kanal et al. (2017) from 1.3 million records. CTDIvol is volumetric Computed Tomography Dose Index. SSDE is Size Specific Dose Estimate. DLP is Dose Length Product.

^†The symbol “-” means Not Applicable.

Patient-specific organ doses and effective doses

Table 3 summarizes the estimated organ doses and effective doses. The data of male, female and all patients were presented as mean ± standard deviation (SD), and the range from minimum to maximum in parentheses. After considering the ICRP 103 tissue weighting factors [30] and the amount of radiation dose the organs received from the scans, we presented the top 5 of 29 organ doses for head scans and the top 10 of 29 organ doses for body scans when sorted in descending order. Between patients, the ratio of the maximum to minimum of the top organ doses were 2.5 for head scans, 3.4 for chest scans, 2.2 for AP scans, and 2.1 for CAP scan on average.

Table 3.

Organ and Effective Doses of Head Scans and Body Scans

Organs	Mean ± Standard Deviation (Min—Max)
	Male	Female	All
Head Scan
Lens of eye	60.1 ± 13.0 (54.6—114.8)	59.7 ± 6.7 (55.2—118.6)	59.9 ± 10.3 (54.6—118.6)
Brain	51.9 ± 11.3 (46.2—99.3)	46.6 ± 5.2 (43.1—92.7)	49.3 ± 9.2 (43.1—99.3)
Salivary glands	41.9 ± 9.1 (38.0—79.9)	49.6 ± 5.6 (45.9—98.5)	45.8 ± 8.4 (38.0—98.5)
Extrathoracic region	37.2 ± 8.1 (33.7—70.8)	44.6 ± 5.0 (41.2—88.6)	40.9 ± 7.7 (33.7—88.6)
Red bone marrow	5.7 ± 1.2 (5.2—10.8)	6.6 ± 0.7 (6.1—13.1)	6.1 ± 1.1 (5.2—13.1)
Effective Dose	2.8 ± 0.6 (2.5—5.3)	2.7 ± 0.3 (2.5—5.3)	2.7 ± 0.5 (2.5—5.3)
Effective Dose_A96*	2.3 ± 0.5 (2.0—4.5)	2.1 ± 0.2 (2.0—4.0)	2.2 ± 0.4 (2.0—4.5)
Chest Scan
Breasts	7.8 ± 2.3 (5.0—12.6)	6.8 ± 1.9 (4.2—11.4)	7.3 ± 2.2 (4.2—12.6)
Colon	5.5 ± 1.5 (3.2—9.6)	4.9 ± 1.3 (2.4—11.2)	5.2 ± 1.4 (2.4—11.2)
Esophagus	6.3 ± 1.3 (3.6—9.3)	6.6 ± 1.8 (4.1—11.4)	6.4 ± 1.6 (3.6—11.4)
Gonads	0.3 ± 0.1 (0.2—0.5)	0.5 ± 0.1 (0.3—1.1)	0.4 ± 0.2 (0.2—1.1)
Liver	10.1 ± 2.1 (6.3—16.1)	8.7 ± 2.1 (4.9—17.1)	9.4 ± 2.2 (4.9—17.1)
Lungs	9.4 ± 1.9 (6.3—13.8)	8.8 ± 2.3 (5.5—15.5)	9.1 ± 2.1 (5.5—15.5)
Red bone marrow	3.7 ± 0.6 (2.4—5.1)	3.4 ± 0.8 (2.1—5.9)	3.6 ± 0.8 (2.1—5.9)
Stomach	8.0 ± 1.6 (5.4—12.8)	7.1 ± 1.6 (3.8—14.4)	7.5 ± 1.7 (3.8—14.4)
Thyroid	9.5 ± 2.1 (5.7—16.1)	10.7 ± 3.2 (6.2—20.4)	10.1 ± 2.8 (5.7—20.4)
Urinary bladder	0.6 ± 0.1 (0.3—0.9)	0.5 ± 0.1 (0.3—1)	0.5 ± 0.1 (0.3—1)
Effective Dose	6.3 ± 1.0 (4.2—8.4)	5.4 ± 1.2 (3.2—9.8)	5.9 ± 1.2 (3.2—9.8)
Effective Dose_A96*	5.9 ± 1.6 (2.8—9.2)	4.6 ± 1.9 (2.1—9.2)	5.2 ± 1.9 (2.1—9.2)
Abdomen and Pelvis Scan
Breasts	6.8 ± 3.5 (2.7—15.8)	4.2 ± 1.8 (2.5—8.1)	5.5 ± 3.1 (2.5—15.8)
Colon	15.5 ± 1.7 (12.5—19.5)	16.4 ± 1.6 (12.8—20.7)	16.0 ± 1.7 (12.5—20.7)
Esophagus	1.0 ± 0.1 (0.8—1.3)	1.3 ± 0.2 (1.1—1.9)	1.1 ± 0.2 (0.8—1.9)
Gonads	3.4 ± 0.7 (2.3—5.9)	13.0 ± 1.9 (8.8—17.3)	8.2 ± 5.0 (2.3—17.3)
Liver	17.9 ± 2.4 (14.1—23.4)	17.0 ± 1.9 (13.2—23.4)	17.5 ± 2.2 (13.2—23.4)
Lungs	6.2 ± 0.8 (4.9—7.9)	5.9 ± 0.7 (4.8—8.3)	6.0 ± 0.7 (4.8—8.3)
Red bone marrow	5.5 ± 0.5 (4.5—6.8)	5.9 ± 0.5 (4.9—7.6)	5.7 ± 0.6 (4.5—7.6)
Stomach	13.5 ± 1.6 (10.4—17.9)	12.8 ± 1.4 (9.7—17.1)	13.1 ± 1.5 (9.7—17.9)
Thyroid	0.7 ± 0.1 (0.5—1.1)	0.8 ± 0.1 (0.6—1.1)	0.7 ± 0.1 (0.5—1.1)
Urinary bladder	11.1 ± 1.0 (9.1—14.8)	13.6 ± 1.9 (9.4—18.2)	12.3 ± 1.9 (9.1—18.2)
Effective Dose	8.9 ± 0.9 (7.1—10.9)	8.6 ± 0.8 (7.2—11.7)	8.7 ± 0.9 (7.1—11.7)
Effective Dose_A96*	13.1 ± 3.0 (8.6—21.6)	12.0 ± 3.0 (8.3—19.8)	12.6 ± 3.1 (8.3—21.6)
Chest, Abdomen and Pelvis Scan
Breasts	15.8 ± 3.0 (8.2—20.0)	12.7 ± 2.4 (8.6—19.1)	14.3 ± 3.1 (8.2—20.0)
Colon	15.5 ± 1.5 (11.1—19.6)	17.2 ± 2.0 (11.3—22.7)	16.3 ± 2.0 (11.1—22.7)
Esophagus	7.6 ± 1.7 (5.1—11.4)	8.8 ± 1.9 (6.7—14)	8.2 ± 1.9 (5.1—14)
Gonads	3.7 ± 0.8 (2.5—5.9)	13.6 ± 1.9 (8—17.8)	8.6 ± 5.2 (2.5—17.8)
Liver	19.1 ± 2.4 (14.3—24.5)	18.8 ± 2.2 (14.5—24.8)	19.0 ± 2.3 (14.3—24.8)
Lungs	15.1 ± 2.6 (10.3—21.7)	14.9 ± 2.4 (12.1—22.3)	15.0 ± 2.5 (10.3—22.3)
Red bone marrow	8.4 ± 1 (6.5—10.5)	8.8 ± 0.9 (7.4—11.2)	8.6 ± 1 (6.5—11.2)
Stomach	15.8 ± 2.0 (11.0—20.9)	15.7 ± 1.7 (10.7—20.6)	15.8 ± 1.9 (10.7—20.9)
Thyroid	10.7 ± 2.4 (6.6—21.3)	13 ± 3.3 (9.1—22)	11.8 ± 3.1 (6.6—22)
Urinary bladder	10.9 ± 1.0 (8.3—14.1)	14.2 ± 1.8 (8.7—18.6)	12.5 ± 2.2 (8.3—18.6)
Effective Dose	13.0 ± 1.4 (9.9—15.9)	12.6 ± 1.3 (10.6—16.8)	12.8 ± 1.3 (9.9—16.8)
Effective Dose_A96*	17.1 ± 3.8 (11.3—25.6)	14.7 ± 3.7 (10.3—24.4)	15.9 ± 3.9 (10.3—25.6)

^*Effective dose derived by using the DLP in our study and the k factors in AAPM report No.96.

The average effective doses from the dose estimator and the ones from using k factors of AAPM No. 96 report were also shown in Table 3. Between patients the ratios of maximal effective dose to minimal effective dose were 2.1 (Head), 3.1 (Chest), 1.6 (AP), and 1.7 (CAP).

Table 4 showed the derived k factors based on the effective doses and DLP values of this study. The k factors decreased as the BMI category increased for body scans. In the same table the DLP-to-organ-dose factors (k_org factors) for 10 organs (or 5 organs for head scans) that received relatively high dose were also included for both male and female patients of various body sizes. Although these factors will not be as accurate as using VirtualDose™ CT and TCM information in estimating patient organ doses, they provide an alternative method for quick estimation of organ dose from DLP in clinics.

Table 4.

The k_org factors (DLP-normalized organ doses) and the k factors (DLP-normalized effective dose)^*

Scanned Region	Organs							korg (mGy/mGy-cm) or k (mSv/mGy-cm)
		Normal weight			Overweight				Obese-I		Obese-II		Obese-III
		Male	Female		Male		Female		Male	Female	Male	Female	Male		Female
Head	Lens of eye	0.056	0.060		-		-		-	-	-	-	-		-
	Brain	0.048	0.047		-		-		-	-	-	-	-		-
	Salivary glands	0.039	0.050		-		-		-	-	-	-	-		-
	Extrathoracic region	0.034	0.045		-		-		-	-	-	-	-		-
	Red bone marrow	0.005	0.007		-		-		-	-	-	-	-		-
	Effective dose	0.0026			-				-		-		-
Chest	Thyroid	0.033	0.041		0.021		0.035		0.021	0.035	0.020	0.029	0.020		0.025
	Spleen	0.033	0.036		0.033		0.033		0.032	0.031	0.026	0.031	0.021		0.024
	Adrenals	0.031	0.034		0.031		0.031		0.031	0.028	0.025	0.027	0.022		0.022
	Liver	0.033	0.035		0.031		0.030		0.028	0.026	0.021	0.024	0.017		0.019
	Thymus	0.028	0.034		0.024		0.028		0.022	0.027	0.021	0.025	0.020		0.021
	Kidneys	0.032	0.033		0.031		0.029		0.028	0.025	0.022	0.024	0.017		0.017
	Lungs	0.028	0.034		0.023		0.028		0.023	0.026	0.021	0.025	0.020		0.022
	Heart	0.025	0.032		0.021		0.025		0.020	0.023	0.019	0.022	0.018		0.019
	Stomach	0.028	0.031		0.025		0.024		0.021	0.020	0.016	0.018	0.013		0.014
	Breasts	0.021	0.026		0.018		0.021		0.019	0.019	0.019	0.020	0.018		0.018
	Effective dose	0.021			0.017				0.016		0.014		0.012
Abdomen and Pelvis	Spleen	0.023	0.026		0.023		0.022		0.020	0.019	0.015	0.018	-		-
	Liver	0.023	0.025		0.022		0.020		0.017	0.016	0.013	0.014	-		-
	Adrenals	0.021	0.023		0.022		0.020		0.019	0.017	0.014	0.015	-		-
	Kidneys	0.023	0.023		0.022		0.020		0.018	0.016	0.013	0.014	-		-
	Colon	0.021	0.025		0.018		0.019		0.015	0.014	0.011	0.012	-		-
	Small intestine	0.021	0.026		0.017		0.018		0.013	0.013	0.009	0.010	-		-
	Pancreas	0.019	0.020		0.017		0.015		0.012	0.012	0.008	0.009	-		-
	Stomach	0.018	0.020		0.017		0.015		0.012	0.011	0.009	0.009	-		-
	Uterus/Prostate	0.018	0.020		0.014		0.014		0.011	0.011	0.010	0.009	-		-
	Urinary bladder	0.016	0.022		0.013		0.014		0.010	0.011	0.008	0.009	-		-
	Effective dose	0.012			0.010				0.009		0.007		-
Chest, Abdomen and Pelvis	Spleen	0.019	0.022		0.020		0.020		0.018	0.019	0.013	0.018	0.012		0.014
	Adrenals	0.018	0.021		0.020		0.018		0.019	0.017	0.013	0.016	0.012		0.012
	Liver	0.020	0.022		0.019		0.018		0.016	0.016	0.011	0.014	0.010		0.011
	Kidneys	0.019	0.020		0.019		0.017		0.016	0.015	0.011	0.014	0.010		0.010
	Colon	0.017	0.021		0.015		0.015		0.013	0.013	0.010	0.011	0.008		0.008
	Stomach	0.017	0.019		0.016		0.015		0.013	0.013	0.009	0.011	0.008		0.008
	Small intestine	0.017	0.022		0.014		0.014		0.011	0.012	0.009	0.010	0.007		0.007
	Lungs	0.015	0.017		0.014		0.014		0.013	0.014	0.011	0.013	0.011		0.012
	Breasts	0.014	0.014		0.015		0.013		0.014	0.012	0.012	0.012	0.011		0.011
	Pancreas	0.016	0.017		0.015		0.013		0.011	0.011	0.007	0.009	0.006		0.006
	Effective dose	0.014			0.012				0.011		0.009		0.008

^*The symbol “-” means Not Applicable.

Discussion

Understanding CT doses in a given clinic and comparing to national DRLs reveals opportunities for optimization with potential dose reduction while maintaining image quality. Conformance of clinical imaging CT doses to national and regional DRLs has been shown to effectively reduce CT radiation dose without compromising diagnostic outcome [17, 18]. This study summarized the 75^th percentiles of CTDI_vol, DLP, and SSDE for the protocols investigated. Across the four body regions, CTDI_vol was the highest for head scans as large numbers of photons are needed to penetrate the thick skull. DLP was also relatively high for head scans due to the high CTDI_vol, the extended scan length above patient head (up to 4 cm above the top of head), and multiple-phase scans with imaging contrast agent. For head with-or-without-contrast scans, one scan before and one scan after contrast agent administration were always performed, so the DLP was twice as much as other head scans. Overall our percentiles were consistent with and comparable to national DIR percentiles, showing successful management of CT radiation dose at our institution when compared with national optimization patterns.

The dose estimator VirtualDose™ CT used realistic computational human phantoms of five BMI categories (normal weight to obese class III) to represent patients. Effective diameter of the scanned region of each patient was used to find the closest-size human phantom. Even with the closest-size phantoms selected, the patient body sizes are usually different from their representative phantoms. Moreover, in each patient, the size of the chest, abdomen and pelvis can be different from those of the phantom, and the scanner output for each of these regions will be different, respectively. In this study, such size variation was addressed by calculating and applying the ratio of patient SSDE to phantom SSDE for each scanned body region (except head) of each patient. SSDE for head was ignored because about 10% of the reported adult head size by DoseWatch™ was erroneously comparable to adult body size. Moreover, adult head breadth and length in the US varies by about 12% (5^th or 95^th percentiles compared to 50^th percentiles) [42] so the head size does not vary as much as the body size. And finally, most clinical routine adult brain scans are performed with fixed mA in our institution, so scanner reported CTDIvol and SSDE (if correctly calculated) is very similar among adult patients. Subsequent investigation found apparatuses (e.g. pillow, contrast infusion catheters) surrounding patient head were mistakenly included in automated head size measurement by DoseWatch™, but this potential error was not observed in the manual verification of body size measurements.

The summarized SSDE ratios in Supplemental Table 1 showed that on average patient SSDE was slightly higher (3% – 14%) than phantom SSDE, suggesting the patient size was slighter smaller than the corresponding phantom. This was because underweight patients were represented by normal weight phantoms. On the other hand, patients of extreme large body sizes were represented by obese class III phantoms and doses were also corrected by their SSDE ratios. Thus, the use of SSDE ratios addressed the remaining effect of the size differences between phantoms and patients on doses. Moreover, the use of the ratios cancelled out most of the error caused by the difference between effective diameter (ED) and water-equivalent effective diameter (WED) for chest scans while for abdominal scans these two kinds of diameters agreed within 2% [39].

The AAPM Report No.96 provides k factors for converting DLP to effective dose for average size adults. Recent studies have shown the k factors decrease as patient body sizes increase and the k factors for obese patients can be half of these for average size patients [35, 43]. Our estimated k factors showed similar trend that agreed with and consolidated these findings in the literature [35, 36, 40, 41, 44]. Our DLP-normalized organ doses (k_org factors) decreased with patient size as well and the trend was consistent with literature [36, 40]. Student t-test between male and female organ doses for all investigated body regions showed significant (p<0.05) differences for most of the organs, so k_org factors for male and female were calculated and showed separately in Table 4. This table of k_org factors for ten organs received relatively high dose was to provide an alternative method to estimate organ dose with reasonable accuracy in clinical settings where time would be limited and TCM information would not be readily available. When more accurate doses are needed, the modified VirtualDose™ CT, DICOM images and DoseWatch™ records can be used following the method in this study to calculate organ and effective doses.

For our study protocols and patient population, the highest dose to an organ per scan was dose to the lens of eye for 60 mGy, which was due to the high mAs (300 mAs and pitch of 1 without TCM) used in head scans. For head scans using one scan before contrast infusion and another scan after, patient radiation dose is doubled, indicating the lens of eye can receive up to 120 mGy in one examination. Head CT scans should be carefully prescribed and optimized to control the dose, as recent studies showed a threshold of 500 mGy for the tissue reaction of lens opacities from fractionated/protracted exposures irrespective of the rate of dose delivery [45–47]. In chest scans, the thyroid received a relatively high dose of 10 mGy per scan due to the supraclavicular scan range that covered this superficial organ. In all three kinds of investigated body scans visceral organs such as liver received higher dose than other visceral organs because they were relatively close to the body surface and they were not as well shielded as the others by visceral fat that was modeled in the dose estimator [38, 48]. The average ratio between maximum and minimum of organ doses of different patients received similar scans was more than 2 for all investigated scans. This showed the use of a single dose value is insufficient and of large error when patients of various body habitus undertake CT scans of various techniques. Thus, patient-specific dose estimation methods should be used to provide a more accurate dose for individual patients.

In Supplemental Figure 1, our chest dose estimates were compared to the dose estimates generated by a size-based dose estimation method in the literature [41]. To obtain dose in mGy using their method, patient chest effective diameter was applied to their equations, and then CTDI_vol was multiplied to the resultant CTDI_vol-normalized organ dose. The largest difference of 38% was dose to the thymus. For most of the organs investigated, their dose estimates were higher than ours. This was because the effect of TCM was not considered in their method. In addition, their pitch was 1.375 while ours was 0.984. Given the same weighted CTDI, our pitch could lead to a 40% higher CTDI_vol and consequently 40% higher dose estimates using their method. Using their method spleen and liver dose estimates were lower than ours, and this was due to differences in scan range where our chest scans included the adrenals and at the same time covered the spleen and most of the liver. Their chest protocol was finished at the base of the lungs. On average our estimates were 11% lower than the ones by their method.

In Supplemental Figure 2 we compared our AP dose estimates to the dose estimates generated by another size-based dose estimation method in the literature [40]. Both patient effective diameter and CTDI_vol were used to generate dose estimates with their method. The largest difference of 47% was dose to the gonads and this was because our scans ended at pubic symphysis while theirs went on through ischium posterior. Even with 2cm overscan the male gonads were not fully covered in our scan range and led to our lower male gonads dose estimates and subsequently lower gender-averaged gonads dose. The large standard deviation for gonads dose was also due to averaging the low testis dose with the high ovaries dose. For most of the organs, their dose estimates were higher than ours, and this was again because no TCM was considered in their study and that our CTDI_vol was high due to our low pitch of 0.984 comparing to theirs of 1.4. Other factors that contributed to the differences were our region-specific SSDE correction, inherent differences between human phantoms, and their use of medium bow-tie filter versus our use of body bow-tie filter. On average our estimates were 18% lower than the ones by their method.

In this study, for dose estimation we tried to be as patient-specific as possible by matching patients to five pairs of realistic human phantoms in the organ dose estimator VirtualDose™ CT based on effective diameters, and then applying region specific SSDE corrections. Still, the ideal situation would be dose estimation with a computation phantom for each individual patient, which requires an exceedingly large amount of work for organ segmentation/registration and subsequent phantom creation. For TCM we only considered the longitudinal modulations, where the other part of TCM, i.e. angular modulation, could change the dose to organs at/near surface (such as lens of the eye or female breasts) by up to 38% in simulations or by up to 19% in experiments [39–43]. In addition, the effect of contrast on organ dose was not investigated, which could increase dose to the liver by 18% and dose to the kidneys by 27% [49]. The variation of patient positioning in clinical scans was not considered, but a 5cm off-centering of the patient in table height direction can lead to 13% breast dose change in chest CT scans [50]. Finally, for effective diameter calculation we used DoseWatch™ reported anterior-posterior and lateral dimensions, which was found to be up to 10% smaller than dimensions manually measured on the scout images. This discrepancy might lead to about 10% dose overestimation.

Conclusion

For 1,200 adult patients who previously received CT examinations, patient-specific organ and effective doses were estimated. Additionally, CTDI_vol, SSDE and DLP were summarized and k factors specific to our patient population were generated. The 75^th percentiles of CTDI_vol, SSDE and DLP were found to be lower than or up to 7% higher than the national DRLs. Lens of eye dose was 60 mGy per head scan without contrast and was 120 mGy per scan with-then-without contrast. For body scans, dose to organs that are close to body surface and not well shielded by abdominal/visceral fat (e.g. liver, thyroid) receive relatively high dose of about 10 mGy per chest scan, 18 mGy per AP scan, or 19 mGy per CAP scan. Compared to methods without TCM, the organ doses estimated in the present study showed 18% lower dose on average. Generated k factors were found to decrease with increase in body size, as previously demonstrated in the literature [35, 36]. As the dose estimation method in this study accounts for the effects of TCM and uses body region specific SSDE corrections for patient body size variations among patients and between body regions, it should be an appealing option for reasonably quick and accurate organ and effective dose estimations. The k factors and k_org factors for patients of various sizes generated in this study make possible rapid clinical dose estimations. By accounting for varying scan techniques and differences in patient habitus, this patient-specific dose estimation methodology results in estimates showing some patients can receive twice as much organ doses as others from scans of the same body region. In addition to the increased accuracy of patient dose resulting from incorporation of patient-specific factors, the process also characterizes dose for the clinic. Armed with more accurate dose information that is readily accessible, clinicians are better informed of risks, are more confident to communicate about risk, and can incorporate risk into medical decisions.

Supplementary Material

Supplemental figure 1~2

Supplemental Fig. 1—Chest CT scan organ dose comparison between this study and the method by Sahbaee et al. (2014).

Supplemental Fig. 2—Abdomen and pelvis CT scan organ dose comparison between this study and the method by Tian et al. (2013).