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Establishment of national diagnostic reference levels for radiotherapy computed tomography simulation procedures in Slovenia
European Journal of Radiology 127 (2020) 108979
Contents lists available at ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Establishment of national diagnostic reference levels for radiotherapy computed tomography simulation procedures in Slovenia
T
Nika Zalokara, Valerija Žager Marciuša,b, Nejc Mekiša,* a b
University of Ljubljana, Faculty of Health Sciences, Medical Imaging and Radiotherapy Department, Zdravstvena pot 5, 1000, Ljubljana, Slovenia Institute of Oncology Ljubljana, Teleradiotherapy Department, Zaloška cesta 2, 1000 Ljubljana, Slovenia
ARTICLE INFO
ABSTRACT
Keywords: Computed tomography DRL Radiotherapy simulation CT
Purpose: To propose national diagnostic reference levels (DRLs) for radiotherapy (RT) computed tomography (CT) localization purposes, compare both CT units used in the largest RT department in the country and to compare gathered results with other published DRLs in order to discover any need of optimization. Methods: In total, 1631 patient data (time spend of 4 months) regarding sex, examination type, total dose-length product (DLP) and CTDIvol was collated on two CT units. Those simulation procedures account for more than 80 % of all simulation procedures performed nationwide. Then, total DLP and CTDIvol was calculated and mean, median and 3rd quartile for both units together were presented to determine national DRLs for simulation procedures. The same data was later compared between both units to discover any potential need for optimization. Results: 3rd quartile values of DLP for abdomen, breast, chest, head, head and neck, pelvis and spine were 1116.2, 606.6, 832.4, 1942.4, 969.2, 677.1 and 1042.4 mGy∙cm, respectively. 3rd quartile CTDIvol values for the same sequence of procedures were 18.7, 13.3, 19.2, 76.9, 22.6, 17.9 and 22.2 mGy, respectively. Among the two units, the mentioned dose values were on average significantly higher on one CT unit than on the other unit. Conclusions: When comparing collected dose values with other studies, RT CT DRLs showed that radiation doses from our institution were similar or even lower. Some variations were found between both CT units in certain protocols, so exposure parameters should be reviewed and optimized.
1. Introduction Computed tomography (CT) plays a crucial role in the radiotherapy treatment process [1] as it is used for radiotherapy (RT) CT simulation. RT CT simulation determines the position and the extent of the disease as accurately as possible. Only one scan is generally required. The imaging position during the CT scan represents the position during the radiation treatment, that is why the preciseness and the reproducibility of the patient position is fundamental [2]. The key role of CT simulation is also the precise contouring of the target volumes and normal tissues, so called organs at risk (OAR) [1,3], which contributes to more accurate and precise RT treatment process [3]. This requires the provision of good quality images [4]. CT images are also used for electron density calculations and for generating digitally reconstructed radiographs (DRRs) [1]. The CT scan range includes the tumour volume and normal tissue volumes superiorly and inferiorly. Radiosensitive organs are also included in the scan. Since CT is a relatively high dose imaging technique
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[5] with a subsequent high risk of carcinogenesis [4] and radiosensitive organs in the primary field of irradiation, the optimization of dose is essential. There is not much information available on radiation exposures during CT radiotherapy simulations due to patient exposures not being a matter of concern so far. Despite that CT localization doses are considered to be irrelevant in comparison to therapeutic doses, they are not insignificant [3,4,6]. A cancer patient may need to go through many CT procedures in the process from diagnosis to treatment, so the exposure to radiation accumulates [1]. If patients recover from the primary cancer, they may still have a long life expectancy, so the additional imaging doses should be as low as reasonably achievable [1]. The CT dose should be optimized to minimize risk to patients [4]. The International Commission on Radiological Protection (ICRP) has implemented Diagnostic Reference Levels (DRLs) for radiological imaging techniques [7–9]. DRLs do not represent a dividing line between good and poor medical practice, they are neither dose limits [9]. Its purpose is to prevent excessive radiation dose that does not contribute additional clinical information [4]. DRLs are used as a tool to
Corresponding author at: Zdravstvena pot 5, 1000, Ljubljana, Slovenia. E-mail address: [email protected] (N. Mekiš).
https://doi.org/10.1016/j.ejrad.2020.108979 Received 26 December 2019; Received in revised form 25 February 2020; Accepted 25 March 2020 0720-048X/ © 2020 Elsevier B.V. All rights reserved.
European Journal of Radiology 127 (2020) 108979
N. Zalokar, et al.
discover the need for exposure optimization for standardized patients. DRLs do not apply to radiotherapy [3,8,9], but they should be considered for imaging for treatment planning, treatment rehearsal, and patient set-up verification in radiotherapy [3,8]. DRLs can be set based on patient-based dosimetry or phantom-based dosimetry [9]. DRL values are usually set at the 75th percentile of the distribution of the medians of distributions of the DRL quantity, which is obtained from surveys or other means [8,10]. DRLs can be set on the local, regional, national or international level [8]. For proper optimization of CT protocols in facilities, the ICRP recommends the comparison of national DRLs to international [8]. DRL values for CT procedures already exist, however, different image quality requirements and different scanning volumes between diagnostic and RT procedures prevents the use of these DRLs for RT CT simulation [6]. Some studies have already published DRLs for RT CT simulation procedures [1,3,4,6]. The purpose of the study was to establish national DRLs for radiotherapy localization CT procedures and compare them with European DRLs and DRLs established in other research to discover potential protocols in need of optimization.
region, but it acquires 5–10 cm above and under the irradiation target region. All images were inspected by the reporting radiotherapist and all of the images were stated appropriate for simulation (localization) purposes. The collated DLP and CTDIvol were also compared between the two CT units to determine if any of the procedures are in need of optimization. The data were analysed using IBM SPSS STATISTICS 26.0 software. First, the frequency table was performed. Then the descriptive statistical analysis was performed in which the average, standard deviation, median and 3rd quartile for each procedure were calculated for CTDIvol and total DLP. Shapiro-Wilk test was used to determine the normal data distribution. Mann-Whitney U test was used to calculate the difference between investigated CT units regarding DLP and CTDIvol for each procedure. A significance of p < 0.05 was used for all the tests. 3. Results In total there was 804 female patients and 827 male patients included in the study. The distribution of the procedures is presented on the pie chart (Fig. 1). Fig. 1 presents that the most frequent CT simulation examination is pelvic (n = 443) followed by the breast (n = 298), chest (n = 289), head and neck (n = 278), head (n = 162), abdomen (n = 128) and spine (n = 33). The results for the total DLP for both CT units are presented in the Table 3 and CTDIvol in Table 4. We wanted to compare the DLP and CTDIvol data between CT units for each examination separately to determine if any protocol on a specific CT unit needs to undergo optimization, so DLP data is presented in Table 5 and CTDIvol data in Table 6. Based on the statistical test we have found that there are statistically significant differences in DLP value in breast protocol (p < 0.001), head protocol (p < 0.001) and head and neck protocol (p < 0.001). The head protocol had lower DLP on a Philips unit whereas breast, head and neck protocol had lower DLP on a Siemens unit. The statistical analysis has shown statistically significant differences in breast protocol (p < 0.001), head protocol (p < 0.001), head and neck protocol (p = 0.001) and pelvis protocol (p < 0.001). The results for the CTDIvol values were lower on Philips unit for the head, breast and pelvis protocol whereas the head and neck exam protocol had lower CTDIvol on Siemens unit.
2. Methods A retrospective study with secondary data analysis was performed. Two CT simulators (Siemens SOMATOM Definition AS and Philips Brilliance Big Bore) that are in use for radiotherapy simulations were included in the study. On these two CT units more than 80 % of all radiotherapy localizations are performed nationwide. In total 1631 patient data was collated in a time spend of four months (from April 1st 2019 until July 31st 2019). The data about the patient sex, date of the procedure, total doselength product (DLP), CTDIvol, information on the phantom used for dose display calibration and examination protocol including exposure parameters were collated. Prior to the start of the study approval from the hospital research ethics committee was obtained. Any potentially identifiable data collected during the study was fully anonymised. The data was collected only for the adult patients age 18 or older. The CT procedures that were observed and analysed in our study are: abdomen, breast, chest, head, head and neck (h&n), pelvis and spine. The exposure parameters for each examination used on both CT units are described in Tables 1 and 2. The scanning length per examination differed between different examination protocols. For the abdomen the scan length is from the bottom of the ischiatic bone up to 3 cm above the diaphragm. Scan length for the breast protocol is from the middle of the neck to the middle of the kidneys. Chest protocol scan length is from the middle of the neck all the way to the end of the rib cage. Scan length for the head and neck examination is from the top of the head to the carina. Head protocol scan length is from the top of the head to the vertebrae C7. Scan length for the pelvis protocol is from the L3-L4 region to 2 cm under the ischiatic bone. Spine protocol scan length depends on the
4. Discussion According to the linear no-threshold model, all imaging procedures that utilize ionizing radiation, including CT simulations in radiotherapy, should be optimised to minimize risk to patients [4]. Diagnostic Reference Levels are a beneficial tool in regard to dose optimisation and radiation protection of patients in medical imaging. They
Table 1 Scanning parameters of the simulation protocols on Siemens CT unit. Siemens
Acquisition type
VOLTAGE (kV)
Reference (mAs)
Collimation (N × mm)
AEC
Rotation Time (sec)
Pitch
Slice thickness (mm)
Reconstruction kernel
RT RT RT RT RT RT RT
Helical Helical Helical Helical Helical Helical Helical
120 120 120 120 120 120 120
370 165 140 110 180 210 330
20 × 0.6 16 × 1.2 16 × 1.2 16 × 1.2 16 × 1.2 16 × 1.2 16 × 1.2
on on on on on on on
1 1 0.5 0.5 0.5 0.5 1
0.55 0.8 1 1.2 0.6 0.8 0.8
3 2 3 3 3 3 3
H30s B31s B31f B31f B30f B30f B30s
HEAD H&N BREAST THORAX ABDOMEN PELVIS SPINE
2
European Journal of Radiology 127 (2020) 108979
N. Zalokar, et al.
Table 2 Scanning parameters of the simulation protocols on Philips CT unit. Philips
ACQUISITION TYPE
VOLTAGE (kV)
Reference (mAs)
Collimation (N × mm)
AEC
Rotation Time (sec)
Pitch
Slice thickness (mm)
Reconstruction kernel
RT RT RT RT RT RT RT
Helical Helical Helical Helical Helical Helical Helical
120 140 120 120 120 120 120
350 300 250 300 300 200 250
16 × 1.5 16 × 0.75 16 × 1.5 16 × 1.5 16 × 1.5 16 × 1.5 16 × 1.5
off on on on on on on
0.75 1 0.75 0.75 0.75 0.75 1
0.93 0.94 0.81 0.81 0.81 0.68 0.93
3 2 3 3 3 3 3
Brain standard (UB) Smooth (A) Sharp (C) Standard (B) Standard (B) Smooth (A) Smooth (A)
HEAD H&N BREAST THORAX ABDOMEN PELVIS SPINE
∙ cm for pelvis, for DLP and CTDIvol, respectively. Our study proposed a DRL of 1116.2 mGy ∙ cm and 18.7 mGy for abdomen, which is 39.5 % higher and 25.2 % lower for DLP and CTDIvol, respectively. Variations in DLP could suggest that the RT CT scan length is longer than that of the diagnostic CT scan, which is due to treatment planning needs. The localisation CT scan range contains the tumour volume, with a volume of normal tissue superiorly and inferiorly. The scanned volume should be at least 5 cm superiorly and inferiorly to the treatment volume [4]. CTDIvol defines the intensity of radiation used to perform a particular CT examination. The CTDIvol is settled for a given CT unit and a set of acquisition parameters, so it does not depend on patient size or scan length. DLP is the product of CTDIvol and the scan length, so it can be used as an indicator of patient dose from a CT scan [4]. Since scan length is linked to DLP, decreasing scan length could decrease the radiation dose. A study comparing diagnostic (DI) and RT CT imaging in the thorax found that radiation dose in RT was four times higher than DI CTs. The variation in this study could suggest that the RT scan length is longer than that of the DI protocols [11]. In our study the RT CT protocol for chest had more than two times higher radiation dose than the diagnostic chest CT examination. We also found a two times higher radiation dose in head RT CT. For the pelvis protocol, the DLP was only 23 % higher. The CTDIvol was not given by the Dose Datamed 2 report for the pelvis examination [11]. Dose levels are generally higher in CT simulations than those used in diagnostics [1]. O’Connor et al. [6] wanted to determine DRLs for the breast cancer CT protocol in radiation therapy. They have also compared CT localisation radiation dose for breast with the chest diagnostic CT radiation dose. They discovered two times higher radiation dose (DLP and CTDIvol) for breast RT CT compared to the diagnostic chest CT. The study claims longer scanning margins may account in part for higher DRLs. Their study proposed DRLs for the breast CT localization of 732 mGy cm and 26 mGy for DLP and CTDIvol, respectively. Our study proposed a DRL for the breast RT CT 606.6 mGy cm and 13.3 mGy for DLP and CTDIvol, respectively. Our DLP and CTDIvol values are lower in comparison to the study by O’Connor et al., which is the only found and investigated study that published proposed DRLs for breast RT CT scan, and all inspected images were stated as appropriate for simulation purposes. However, since statistically significant differences in CTDIvol values have been found between both CT simulators, imaging protocols of both CT units should be reviewed and optimized.
Fig. 1. Pie chart of procedure distribution on both CT units.
can be used to detect excessive radiation doses that do not contribute any additional diagnostic information [8]. In the institution where the study was carried out currently covers more than 80 % of all radiotherapy localizations that are performed nationwide so data for a representative selection of patients was obtained. A Nordic survey of patient doses in diagnostic radiology [12] suggested that national DRLs should include data from 5 to 25 % of the country’s departments. Since the institution where the study was carried out covers more than 80 % of all radiotherapy localizations in the country National DRLs can be suggested based on this study’s data. This is the first time CT simulation DRLs in radiotherapy are established. CT scanners used in diagnostic radiology and in radiotherapy have few differences. CT scanners in RT use the large scanner bore size due to different fixations systems and pads that are used in radiotherapy [2]. A larger CT-scanner bore size will most likely contribute to either a loss of image quality or an increased mAs setting (with all other scanning parameters kept the same) to achieve the same image quality as obtained with smaller diameter bore size scanner [2]. A study by GarciaRamirez et al. [13] reported 10−20 mGy higher doses with large bore CT scanner compared with normal diagnostic CT. The use of diagnostic DRLs in RT is not sympathized due to the different scanning volumes, protocols and image quality requirements between diagnostic and RT CT simulation [6,14]. However, RT CT simulation reference levels are lacking, so diagnostic CT examinations might approximate CT protocols in this study. Dose Datamed 2 [11] project estimated European population doses from CT procedures based on data collection. Published European DRLs for abdomen were 800 mGy ∙ cm and 25 mGy, 400 mGy ∙ cm and 10 mGy for chest, 1000 mGy ∙ cm and 60 mGy for head and for 550 mGy Table 3 Mean, median and 3rd quartile values for DLP.
Abdomen (n = 128) Breast (n = 298) Chest (n = 289) Head (n = 162) Head and neck (n = 278) Pelvis (n = 443) Spine (n = 33)
Mean (mGy cm)
Median (mGy cm)
St. dev. (mGy cm)
3rd Quartile (mGy cm) National RT CT DRLs
European DRLs for diagnostic CT (mGy cm) [11]
864.9 514.3 682.2 1527.9 741.3 615.0 872.6
816.8 464.0 471.9 1284.6 683.8 433.3 723.5
457.6 307.9 522.6 587.0 362.5 2142.9 724.6
1116.2 606.6 832.4 1942.4 969.2 677.1 1042.4
800 NA 400 1000 NA 500 NA
3
European Journal of Radiology 127 (2020) 108979
N. Zalokar, et al.
Table 4 Mean, median and 3rd quartile values for CTDIvol.
Abdomen (n = 128) Breast (n = 298) Chest (n = 289) Head (n = 162) Head and Neck (n = 278) Pelvis (n = 443) Spine (n = 33)
Mean (mGy)
Median (mGy)
St. dev. (mGy)
3rd Quartile (mGy) National RT CT DRLs
European DRLs for diagnostic CT (mGy) [11]
18.2 11.2 19.2 55.0 16.9 14.5 16.6
17.5 10.0 12.0 41.9 15.8 13.8 14.8
9.7 4.4 17.4 25.2 6.7 5.5 8.8
18.7 13.3 19.2 76.9 22.6 17.9 22.2
25 NA 10 60 NA NA NA
The study by Clerkin et al. [4] proposed a National Irish DRL of 882 mGy cm and 21 mGy for the head and neck RT localisation CT. Our study’s results comply with the results of this study with the DLP of 969.2 mGy cm and CTDIvol of 22.6 mGy. The study by Toroi et al. [1] investigated patient exposure levels in CT simulations. Only CTDIvol values were given. The given third quartile values were 36 mGy, 24 mGy, 37 mGy and 86 mGy for the prostate, the breast, the head and neck and the whole brain RT CT simulations, respectively. All CTDIvol values in our study were lower compared to that study. However, wide variations in CTDIvol have been found between the CT units. Such variations are unjustified given the same clinical requirements of the images, so imaging protocols should be harmonized. When comparing the general scan lengths of our study to diagnostic CT units’ from the European guidelines on CT [15] we can see a major difference. For the abdomen the scan length in our paper was from the bottom of the ischiatic bone up to 3 cm above the diaphragm while in diagnostic CT the scan length is from the dome of the liver to the aortic bifurcation. For the chest protocol scan length was from the middle of the neck all the way to the end of the rib cage and in the diagnostic CT from the apex of the lungs to the base of the lungs. Head protocol scan length in our study was from the top of the head to the vertebrae C7, in diagnostic CT it is from foramen magnum to the skull vertex. Scan length for the pelvis protocol was from the L3-L4 region to 2 cm under the ischiatic bone. In diagnostic CT it is from iliac crest to pelvic floor. This corresponds to the results of the higher DLP levels. So far radiation doses cancer patients receive during imaging have not been a major concern due to the high received treatment doses. Additional and continuous optimisation is typically not performed for these CT systems. However, cancer patients may need to go through many CT imaging procedures and the radiation dose accumulates.
Table 5 Comparison of mean, median and 3rd quartile values for DLP between CT units. examination
data
Siemens (mGy cm)
Philips (mGy cm)
Abdomen
Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile
838.5 731.6 490.7 1093.4 464.2 403.5 353.1 531.9 705.9 437.9 587.8 877.0 2050.8 2038.2 404.9 2299.3 554.7 551.1 186.0 631.1 661.6 437.3 2598.0 663.1
921.0 942.3 377.4 1143.3 555.7 517.7 258.6 666.2 623.3 575.8 246.7 779.7 1109.5 1173.2 307.1 1242.7 789.4 761.8 381.2 1016.8 519.1 411.1 336.4 708.2
Breast
Chest
Head
Head and neck
Pelvis
Table 6 Comparison of mean, median and 3rd quartile values for CTDIvol between CT units. examination
data
Siemens (mGy)
Philips (mGy)
Abdomen
Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile
19.4 15.4 11.2 21.9 13.7 12.8 4.3 16.2 21.7 12.0 14.9 25.4 80.1 72.3 12.7 85.2 14.6 14.2 2.3 15.0 15.6 14.4 4.8 18.3
15.7 17.7 4.2 17.7 9.1 8.3 3.8 10.1 13.1 11.8 4.6 15.6 34.8 37.1 9.7 37.1 17.5 17.8 7.3 23.4 12.1 9.8 6.0 17.7
Breast
Chest
Head
Head and neck
Pelvis
5. Conclusions The first national RT CT simulation DRLs were proposed and provide a platform for dose comparison and dose optimisation. Due to the lack of literature about RT CT DRLs, a comparison with diagnostic DRLs showed much higher radiation doses in RT CT than those used for diagnostics. However, imaging purposes and consequently scanning volumes, protocols and image quality requirements of both are different. A comparison to already published RT CT DRLs showed that radiation doses from our institution were similar or even lower. Some variation was found between both CT units in certain protocols, so exposure parameters should be reviewed and optimized. CRediT authorship contribution statement Nika Zalokar: Methodology, Formal analysis, Investigation, Resources, Writing - original draft, Writing - review & editing. Valerija Žager Marciuš: Conceptualization, Methodology, Investigation, Resources, Writing - original draft, Writing - review & editing. Nejc Mekiš: Conceptualization, Methodology, Validation, Formal analysis, Writing original draft, Writing - review & editing, Visualization, Supervision. 4
European Journal of Radiology 127 (2020) 108979
N. Zalokar, et al.
Declaration of Competing Interest [8]
No potential conflicts of interest were disclosed. References
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Contents lists available at ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Establishment of national diagnostic reference levels for radiotherapy computed tomography simulation procedures in Slovenia
T
Nika Zalokara, Valerija Žager Marciuša,b, Nejc Mekiša,* a b
University of Ljubljana, Faculty of Health Sciences, Medical Imaging and Radiotherapy Department, Zdravstvena pot 5, 1000, Ljubljana, Slovenia Institute of Oncology Ljubljana, Teleradiotherapy Department, Zaloška cesta 2, 1000 Ljubljana, Slovenia
ARTICLE INFO
ABSTRACT
Keywords: Computed tomography DRL Radiotherapy simulation CT
Purpose: To propose national diagnostic reference levels (DRLs) for radiotherapy (RT) computed tomography (CT) localization purposes, compare both CT units used in the largest RT department in the country and to compare gathered results with other published DRLs in order to discover any need of optimization. Methods: In total, 1631 patient data (time spend of 4 months) regarding sex, examination type, total dose-length product (DLP) and CTDIvol was collated on two CT units. Those simulation procedures account for more than 80 % of all simulation procedures performed nationwide. Then, total DLP and CTDIvol was calculated and mean, median and 3rd quartile for both units together were presented to determine national DRLs for simulation procedures. The same data was later compared between both units to discover any potential need for optimization. Results: 3rd quartile values of DLP for abdomen, breast, chest, head, head and neck, pelvis and spine were 1116.2, 606.6, 832.4, 1942.4, 969.2, 677.1 and 1042.4 mGy∙cm, respectively. 3rd quartile CTDIvol values for the same sequence of procedures were 18.7, 13.3, 19.2, 76.9, 22.6, 17.9 and 22.2 mGy, respectively. Among the two units, the mentioned dose values were on average significantly higher on one CT unit than on the other unit. Conclusions: When comparing collected dose values with other studies, RT CT DRLs showed that radiation doses from our institution were similar or even lower. Some variations were found between both CT units in certain protocols, so exposure parameters should be reviewed and optimized.
1. Introduction Computed tomography (CT) plays a crucial role in the radiotherapy treatment process [1] as it is used for radiotherapy (RT) CT simulation. RT CT simulation determines the position and the extent of the disease as accurately as possible. Only one scan is generally required. The imaging position during the CT scan represents the position during the radiation treatment, that is why the preciseness and the reproducibility of the patient position is fundamental [2]. The key role of CT simulation is also the precise contouring of the target volumes and normal tissues, so called organs at risk (OAR) [1,3], which contributes to more accurate and precise RT treatment process [3]. This requires the provision of good quality images [4]. CT images are also used for electron density calculations and for generating digitally reconstructed radiographs (DRRs) [1]. The CT scan range includes the tumour volume and normal tissue volumes superiorly and inferiorly. Radiosensitive organs are also included in the scan. Since CT is a relatively high dose imaging technique
⁎
[5] with a subsequent high risk of carcinogenesis [4] and radiosensitive organs in the primary field of irradiation, the optimization of dose is essential. There is not much information available on radiation exposures during CT radiotherapy simulations due to patient exposures not being a matter of concern so far. Despite that CT localization doses are considered to be irrelevant in comparison to therapeutic doses, they are not insignificant [3,4,6]. A cancer patient may need to go through many CT procedures in the process from diagnosis to treatment, so the exposure to radiation accumulates [1]. If patients recover from the primary cancer, they may still have a long life expectancy, so the additional imaging doses should be as low as reasonably achievable [1]. The CT dose should be optimized to minimize risk to patients [4]. The International Commission on Radiological Protection (ICRP) has implemented Diagnostic Reference Levels (DRLs) for radiological imaging techniques [7–9]. DRLs do not represent a dividing line between good and poor medical practice, they are neither dose limits [9]. Its purpose is to prevent excessive radiation dose that does not contribute additional clinical information [4]. DRLs are used as a tool to
Corresponding author at: Zdravstvena pot 5, 1000, Ljubljana, Slovenia. E-mail address: [email protected] (N. Mekiš).
https://doi.org/10.1016/j.ejrad.2020.108979 Received 26 December 2019; Received in revised form 25 February 2020; Accepted 25 March 2020 0720-048X/ © 2020 Elsevier B.V. All rights reserved.
European Journal of Radiology 127 (2020) 108979
N. Zalokar, et al.
discover the need for exposure optimization for standardized patients. DRLs do not apply to radiotherapy [3,8,9], but they should be considered for imaging for treatment planning, treatment rehearsal, and patient set-up verification in radiotherapy [3,8]. DRLs can be set based on patient-based dosimetry or phantom-based dosimetry [9]. DRL values are usually set at the 75th percentile of the distribution of the medians of distributions of the DRL quantity, which is obtained from surveys or other means [8,10]. DRLs can be set on the local, regional, national or international level [8]. For proper optimization of CT protocols in facilities, the ICRP recommends the comparison of national DRLs to international [8]. DRL values for CT procedures already exist, however, different image quality requirements and different scanning volumes between diagnostic and RT procedures prevents the use of these DRLs for RT CT simulation [6]. Some studies have already published DRLs for RT CT simulation procedures [1,3,4,6]. The purpose of the study was to establish national DRLs for radiotherapy localization CT procedures and compare them with European DRLs and DRLs established in other research to discover potential protocols in need of optimization.
region, but it acquires 5–10 cm above and under the irradiation target region. All images were inspected by the reporting radiotherapist and all of the images were stated appropriate for simulation (localization) purposes. The collated DLP and CTDIvol were also compared between the two CT units to determine if any of the procedures are in need of optimization. The data were analysed using IBM SPSS STATISTICS 26.0 software. First, the frequency table was performed. Then the descriptive statistical analysis was performed in which the average, standard deviation, median and 3rd quartile for each procedure were calculated for CTDIvol and total DLP. Shapiro-Wilk test was used to determine the normal data distribution. Mann-Whitney U test was used to calculate the difference between investigated CT units regarding DLP and CTDIvol for each procedure. A significance of p < 0.05 was used for all the tests. 3. Results In total there was 804 female patients and 827 male patients included in the study. The distribution of the procedures is presented on the pie chart (Fig. 1). Fig. 1 presents that the most frequent CT simulation examination is pelvic (n = 443) followed by the breast (n = 298), chest (n = 289), head and neck (n = 278), head (n = 162), abdomen (n = 128) and spine (n = 33). The results for the total DLP for both CT units are presented in the Table 3 and CTDIvol in Table 4. We wanted to compare the DLP and CTDIvol data between CT units for each examination separately to determine if any protocol on a specific CT unit needs to undergo optimization, so DLP data is presented in Table 5 and CTDIvol data in Table 6. Based on the statistical test we have found that there are statistically significant differences in DLP value in breast protocol (p < 0.001), head protocol (p < 0.001) and head and neck protocol (p < 0.001). The head protocol had lower DLP on a Philips unit whereas breast, head and neck protocol had lower DLP on a Siemens unit. The statistical analysis has shown statistically significant differences in breast protocol (p < 0.001), head protocol (p < 0.001), head and neck protocol (p = 0.001) and pelvis protocol (p < 0.001). The results for the CTDIvol values were lower on Philips unit for the head, breast and pelvis protocol whereas the head and neck exam protocol had lower CTDIvol on Siemens unit.
2. Methods A retrospective study with secondary data analysis was performed. Two CT simulators (Siemens SOMATOM Definition AS and Philips Brilliance Big Bore) that are in use for radiotherapy simulations were included in the study. On these two CT units more than 80 % of all radiotherapy localizations are performed nationwide. In total 1631 patient data was collated in a time spend of four months (from April 1st 2019 until July 31st 2019). The data about the patient sex, date of the procedure, total doselength product (DLP), CTDIvol, information on the phantom used for dose display calibration and examination protocol including exposure parameters were collated. Prior to the start of the study approval from the hospital research ethics committee was obtained. Any potentially identifiable data collected during the study was fully anonymised. The data was collected only for the adult patients age 18 or older. The CT procedures that were observed and analysed in our study are: abdomen, breast, chest, head, head and neck (h&n), pelvis and spine. The exposure parameters for each examination used on both CT units are described in Tables 1 and 2. The scanning length per examination differed between different examination protocols. For the abdomen the scan length is from the bottom of the ischiatic bone up to 3 cm above the diaphragm. Scan length for the breast protocol is from the middle of the neck to the middle of the kidneys. Chest protocol scan length is from the middle of the neck all the way to the end of the rib cage. Scan length for the head and neck examination is from the top of the head to the carina. Head protocol scan length is from the top of the head to the vertebrae C7. Scan length for the pelvis protocol is from the L3-L4 region to 2 cm under the ischiatic bone. Spine protocol scan length depends on the
4. Discussion According to the linear no-threshold model, all imaging procedures that utilize ionizing radiation, including CT simulations in radiotherapy, should be optimised to minimize risk to patients [4]. Diagnostic Reference Levels are a beneficial tool in regard to dose optimisation and radiation protection of patients in medical imaging. They
Table 1 Scanning parameters of the simulation protocols on Siemens CT unit. Siemens
Acquisition type
VOLTAGE (kV)
Reference (mAs)
Collimation (N × mm)
AEC
Rotation Time (sec)
Pitch
Slice thickness (mm)
Reconstruction kernel
RT RT RT RT RT RT RT
Helical Helical Helical Helical Helical Helical Helical
120 120 120 120 120 120 120
370 165 140 110 180 210 330
20 × 0.6 16 × 1.2 16 × 1.2 16 × 1.2 16 × 1.2 16 × 1.2 16 × 1.2
on on on on on on on
1 1 0.5 0.5 0.5 0.5 1
0.55 0.8 1 1.2 0.6 0.8 0.8
3 2 3 3 3 3 3
H30s B31s B31f B31f B30f B30f B30s
HEAD H&N BREAST THORAX ABDOMEN PELVIS SPINE
2
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Table 2 Scanning parameters of the simulation protocols on Philips CT unit. Philips
ACQUISITION TYPE
VOLTAGE (kV)
Reference (mAs)
Collimation (N × mm)
AEC
Rotation Time (sec)
Pitch
Slice thickness (mm)
Reconstruction kernel
RT RT RT RT RT RT RT
Helical Helical Helical Helical Helical Helical Helical
120 140 120 120 120 120 120
350 300 250 300 300 200 250
16 × 1.5 16 × 0.75 16 × 1.5 16 × 1.5 16 × 1.5 16 × 1.5 16 × 1.5
off on on on on on on
0.75 1 0.75 0.75 0.75 0.75 1
0.93 0.94 0.81 0.81 0.81 0.68 0.93
3 2 3 3 3 3 3
Brain standard (UB) Smooth (A) Sharp (C) Standard (B) Standard (B) Smooth (A) Smooth (A)
HEAD H&N BREAST THORAX ABDOMEN PELVIS SPINE
∙ cm for pelvis, for DLP and CTDIvol, respectively. Our study proposed a DRL of 1116.2 mGy ∙ cm and 18.7 mGy for abdomen, which is 39.5 % higher and 25.2 % lower for DLP and CTDIvol, respectively. Variations in DLP could suggest that the RT CT scan length is longer than that of the diagnostic CT scan, which is due to treatment planning needs. The localisation CT scan range contains the tumour volume, with a volume of normal tissue superiorly and inferiorly. The scanned volume should be at least 5 cm superiorly and inferiorly to the treatment volume [4]. CTDIvol defines the intensity of radiation used to perform a particular CT examination. The CTDIvol is settled for a given CT unit and a set of acquisition parameters, so it does not depend on patient size or scan length. DLP is the product of CTDIvol and the scan length, so it can be used as an indicator of patient dose from a CT scan [4]. Since scan length is linked to DLP, decreasing scan length could decrease the radiation dose. A study comparing diagnostic (DI) and RT CT imaging in the thorax found that radiation dose in RT was four times higher than DI CTs. The variation in this study could suggest that the RT scan length is longer than that of the DI protocols [11]. In our study the RT CT protocol for chest had more than two times higher radiation dose than the diagnostic chest CT examination. We also found a two times higher radiation dose in head RT CT. For the pelvis protocol, the DLP was only 23 % higher. The CTDIvol was not given by the Dose Datamed 2 report for the pelvis examination [11]. Dose levels are generally higher in CT simulations than those used in diagnostics [1]. O’Connor et al. [6] wanted to determine DRLs for the breast cancer CT protocol in radiation therapy. They have also compared CT localisation radiation dose for breast with the chest diagnostic CT radiation dose. They discovered two times higher radiation dose (DLP and CTDIvol) for breast RT CT compared to the diagnostic chest CT. The study claims longer scanning margins may account in part for higher DRLs. Their study proposed DRLs for the breast CT localization of 732 mGy cm and 26 mGy for DLP and CTDIvol, respectively. Our study proposed a DRL for the breast RT CT 606.6 mGy cm and 13.3 mGy for DLP and CTDIvol, respectively. Our DLP and CTDIvol values are lower in comparison to the study by O’Connor et al., which is the only found and investigated study that published proposed DRLs for breast RT CT scan, and all inspected images were stated as appropriate for simulation purposes. However, since statistically significant differences in CTDIvol values have been found between both CT simulators, imaging protocols of both CT units should be reviewed and optimized.
Fig. 1. Pie chart of procedure distribution on both CT units.
can be used to detect excessive radiation doses that do not contribute any additional diagnostic information [8]. In the institution where the study was carried out currently covers more than 80 % of all radiotherapy localizations that are performed nationwide so data for a representative selection of patients was obtained. A Nordic survey of patient doses in diagnostic radiology [12] suggested that national DRLs should include data from 5 to 25 % of the country’s departments. Since the institution where the study was carried out covers more than 80 % of all radiotherapy localizations in the country National DRLs can be suggested based on this study’s data. This is the first time CT simulation DRLs in radiotherapy are established. CT scanners used in diagnostic radiology and in radiotherapy have few differences. CT scanners in RT use the large scanner bore size due to different fixations systems and pads that are used in radiotherapy [2]. A larger CT-scanner bore size will most likely contribute to either a loss of image quality or an increased mAs setting (with all other scanning parameters kept the same) to achieve the same image quality as obtained with smaller diameter bore size scanner [2]. A study by GarciaRamirez et al. [13] reported 10−20 mGy higher doses with large bore CT scanner compared with normal diagnostic CT. The use of diagnostic DRLs in RT is not sympathized due to the different scanning volumes, protocols and image quality requirements between diagnostic and RT CT simulation [6,14]. However, RT CT simulation reference levels are lacking, so diagnostic CT examinations might approximate CT protocols in this study. Dose Datamed 2 [11] project estimated European population doses from CT procedures based on data collection. Published European DRLs for abdomen were 800 mGy ∙ cm and 25 mGy, 400 mGy ∙ cm and 10 mGy for chest, 1000 mGy ∙ cm and 60 mGy for head and for 550 mGy Table 3 Mean, median and 3rd quartile values for DLP.
Abdomen (n = 128) Breast (n = 298) Chest (n = 289) Head (n = 162) Head and neck (n = 278) Pelvis (n = 443) Spine (n = 33)
Mean (mGy cm)
Median (mGy cm)
St. dev. (mGy cm)
3rd Quartile (mGy cm) National RT CT DRLs
European DRLs for diagnostic CT (mGy cm) [11]
864.9 514.3 682.2 1527.9 741.3 615.0 872.6
816.8 464.0 471.9 1284.6 683.8 433.3 723.5
457.6 307.9 522.6 587.0 362.5 2142.9 724.6
1116.2 606.6 832.4 1942.4 969.2 677.1 1042.4
800 NA 400 1000 NA 500 NA
3
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Table 4 Mean, median and 3rd quartile values for CTDIvol.
Abdomen (n = 128) Breast (n = 298) Chest (n = 289) Head (n = 162) Head and Neck (n = 278) Pelvis (n = 443) Spine (n = 33)
Mean (mGy)
Median (mGy)
St. dev. (mGy)
3rd Quartile (mGy) National RT CT DRLs
European DRLs for diagnostic CT (mGy) [11]
18.2 11.2 19.2 55.0 16.9 14.5 16.6
17.5 10.0 12.0 41.9 15.8 13.8 14.8
9.7 4.4 17.4 25.2 6.7 5.5 8.8
18.7 13.3 19.2 76.9 22.6 17.9 22.2
25 NA 10 60 NA NA NA
The study by Clerkin et al. [4] proposed a National Irish DRL of 882 mGy cm and 21 mGy for the head and neck RT localisation CT. Our study’s results comply with the results of this study with the DLP of 969.2 mGy cm and CTDIvol of 22.6 mGy. The study by Toroi et al. [1] investigated patient exposure levels in CT simulations. Only CTDIvol values were given. The given third quartile values were 36 mGy, 24 mGy, 37 mGy and 86 mGy for the prostate, the breast, the head and neck and the whole brain RT CT simulations, respectively. All CTDIvol values in our study were lower compared to that study. However, wide variations in CTDIvol have been found between the CT units. Such variations are unjustified given the same clinical requirements of the images, so imaging protocols should be harmonized. When comparing the general scan lengths of our study to diagnostic CT units’ from the European guidelines on CT [15] we can see a major difference. For the abdomen the scan length in our paper was from the bottom of the ischiatic bone up to 3 cm above the diaphragm while in diagnostic CT the scan length is from the dome of the liver to the aortic bifurcation. For the chest protocol scan length was from the middle of the neck all the way to the end of the rib cage and in the diagnostic CT from the apex of the lungs to the base of the lungs. Head protocol scan length in our study was from the top of the head to the vertebrae C7, in diagnostic CT it is from foramen magnum to the skull vertex. Scan length for the pelvis protocol was from the L3-L4 region to 2 cm under the ischiatic bone. In diagnostic CT it is from iliac crest to pelvic floor. This corresponds to the results of the higher DLP levels. So far radiation doses cancer patients receive during imaging have not been a major concern due to the high received treatment doses. Additional and continuous optimisation is typically not performed for these CT systems. However, cancer patients may need to go through many CT imaging procedures and the radiation dose accumulates.
Table 5 Comparison of mean, median and 3rd quartile values for DLP between CT units. examination
data
Siemens (mGy cm)
Philips (mGy cm)
Abdomen
Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile
838.5 731.6 490.7 1093.4 464.2 403.5 353.1 531.9 705.9 437.9 587.8 877.0 2050.8 2038.2 404.9 2299.3 554.7 551.1 186.0 631.1 661.6 437.3 2598.0 663.1
921.0 942.3 377.4 1143.3 555.7 517.7 258.6 666.2 623.3 575.8 246.7 779.7 1109.5 1173.2 307.1 1242.7 789.4 761.8 381.2 1016.8 519.1 411.1 336.4 708.2
Breast
Chest
Head
Head and neck
Pelvis
Table 6 Comparison of mean, median and 3rd quartile values for CTDIvol between CT units. examination
data
Siemens (mGy)
Philips (mGy)
Abdomen
Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile Average Median Standard deviation 3rd Quartile
19.4 15.4 11.2 21.9 13.7 12.8 4.3 16.2 21.7 12.0 14.9 25.4 80.1 72.3 12.7 85.2 14.6 14.2 2.3 15.0 15.6 14.4 4.8 18.3
15.7 17.7 4.2 17.7 9.1 8.3 3.8 10.1 13.1 11.8 4.6 15.6 34.8 37.1 9.7 37.1 17.5 17.8 7.3 23.4 12.1 9.8 6.0 17.7
Breast
Chest
Head
Head and neck
Pelvis
5. Conclusions The first national RT CT simulation DRLs were proposed and provide a platform for dose comparison and dose optimisation. Due to the lack of literature about RT CT DRLs, a comparison with diagnostic DRLs showed much higher radiation doses in RT CT than those used for diagnostics. However, imaging purposes and consequently scanning volumes, protocols and image quality requirements of both are different. A comparison to already published RT CT DRLs showed that radiation doses from our institution were similar or even lower. Some variation was found between both CT units in certain protocols, so exposure parameters should be reviewed and optimized. CRediT authorship contribution statement Nika Zalokar: Methodology, Formal analysis, Investigation, Resources, Writing - original draft, Writing - review & editing. Valerija Žager Marciuš: Conceptualization, Methodology, Investigation, Resources, Writing - original draft, Writing - review & editing. Nejc Mekiš: Conceptualization, Methodology, Validation, Formal analysis, Writing original draft, Writing - review & editing, Visualization, Supervision. 4
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Declaration of Competing Interest [8]
No potential conflicts of interest were disclosed. References
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