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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 10  |  Issue : 2  |  Page : 108-116

In-house-developed phantoms for organ dose measurements using bovine tissues: A comparison study with CT-Expo simulation software


1 Department of Radiation Biology, Radiotherapy and Radiodiagnosis, College of Medicine, University of Lagos, Lagos, Nigeria
2 Department of Radiology, Medical Physics Unit, Federal Medical Centre, Asaba, Nigeria
3 Department of Physics, University of Ibadan, Ibadan, Oyo, Nigeria

Date of Web Publication9-Sep-2019

Correspondence Address:
Mr. Akintayo Daniel Omojola
Department of Radiology, Medical Physics Unit, Federal Medical Centre, Asaba
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jrcr.jrcr_13_19

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  Abstract 


Background: Estimating organ dose from computed tomography (CT) procedures is still ongoing. The aim is to reduce induced cancer risk associated with over dose. Aim and Objectives: The aim of this study was to estimate organ dose using CT-Expo software, to compare obtained values with thermoluminescent dosimeter (TLD) measurements from validated in-house phantoms, and to compare the CT-Expo results with other related studies. Materials and Methods: Four CT diagnostic centers denoted as A, B, C, and D were randomly selected for this study. A CT-Expo software (version 2.5 Germany) was used. A preliminary study was carried out to determine organ dose from the in-house phantoms using bovine tissues. The CT dose parameters used with the in-house phantoms were retrieved from the CT monitor and were used with CT-Expo worksheet to estimate organ dose as well. Results: The CT-Expo mean organ dose to the brain, eye lens, esophagus, and thyroid were 29.05 ± 10.78, 35.65 ± 15.1, 12.45 ± 10.13, and 4.25 ± 2.78 mGy, respectively; to the heart and lungs were 13.08 ± 9.84 and 11.5 ± 7.26 mGy, respectively; and to the liver, stomach, and kidney were 14.42 ± 9.07, 12.78 ± 7.97, and 11.73 ± 7.92 mGy, respectively. There was no statistically significant difference between the TLD measurements and CT-Expo (P = 0.361). The relative difference between CT-Expo and TLD measurements for brain, eye lens, heart, lungs, kidney, liver, and stomach were ≤21%. Investigated organ doses from the software were between 4.25 and 35.65 mGy. There was no difference in mean organ dose when compared to the studies in Thailand, Tanzania, Japan, USA (cadavers 1 and 2), and Nigeria. Conclusion: Large percentage differences were noticed in the thyroid and esophagus which was as a result of the software not recognizing them as organs in the head/neck but rather as organs in the chest; however, there was no difference in organ dose between the CT-Expo and TLD measurement from the in-house-validated phantom.

Keywords: Computed tomography, CT-Expo software, inhomogeneous phantom, thermoluminescent dosimeter, validated in-house phantom


How to cite this article:
Akpochafor MO, Omojola AD, Obed RI, Adeneye SO, Adewa OJ, Ekpo MAE. In-house-developed phantoms for organ dose measurements using bovine tissues: A comparison study with CT-Expo simulation software. J Radiat Cancer Res 2019;10:108-16

How to cite this URL:
Akpochafor MO, Omojola AD, Obed RI, Adeneye SO, Adewa OJ, Ekpo MAE. In-house-developed phantoms for organ dose measurements using bovine tissues: A comparison study with CT-Expo simulation software. J Radiat Cancer Res [serial online] 2019 [cited 2019 Nov 18];10:108-16. Available from: http://www.journalrcr.org/text.asp?2019/10/2/108/266116




  Introduction Top


Radiological examination utilizing Xrays remain the most commonly used ionizing radiation in the field of medicine, which is responsible as the most substantial artificial source of radiation exposure to the world population,[1],[2] which is seen to have contributed to patient dose in recent years.[3] Furthermore, according to the National Council on Radiation Protection and Measurements Report No. 160, computed tomography (CT) scans were noted to contribute to half of the total patient medical exposure.[4] This has become a major issue because of the increasing awareness and greater observation of the effects of ionizing radiation. In diagnostic radiology, dose monitoring is carried out to ascertain if radiation exposures are within the reference limits and the established optimization range, which ensures adequate radiation protection of patients.[5],[6] Recently, various CT dose surveys have been undertaken in many diagnostic centers; these surveys were needful to observe significant variations in doses to patient between different centers for the same type of CT examination. Discrepancies in dose within centers justify dose assessment and give room for optimization in CT practice. Dose measurements are therefore required in every diagnostic center to ensure compliance with acceptable reference level as well as consideration to justification and appropriate optimization.[7],[8],[9],[10]

The modern CT can function either in axial or in helical scanning modes.[11] Patient doses from CT procedures are relatively higher than doses from other imaging modalities based on how radiation beam enters the body.[12],[13],[14] From the data published in 2006, the use of CT has increased at a rate of 8%–15% per year for the last 7–10 years in the United States.[15] Nearly half of the total medical radiation exposure was from CT. According to the UNSCEAR 2010 Report, the usage and exposures of CT scanning account for 43% of the total collective effective dose due to diagnostic medical radiology, while the additional dose from radiotherapy CT usage was not include.[16],[17] In the current radiation therapy planning, radiation oncologists are required to define the target volume more precisely, not just in two dimensions but also in three dimensions. This is achieved from reconstructed CT images exported from the CT workstation to the treatment planning system (TPS) through a DICOM.[18] In routine radiotherapy, CT provides not only the clinical anatomical information for correct structure delineation but also the electron density map necessary for dose calculation.

Different software packages for dose calculation in CT have been developed. They include the following: WinDose,[3] CT Dose[19] imPACT,[20] and CT-Expo,[21] with the purpose to determine CT dose index (CTDI), dose length product (DLP), and effective doses using varying data codes. The most practical way to determine the radiation dose absorbed by the organs and tissues of the body during a CT examination is measuring by direct or indirect methods. In other to evaluate patient/phantom dose by direct method, dose measuring on patient or phantom is done using smallsized devices, such as thermoluminescent dosimeter (TLDs) and optically stimulated luminescent dosimeters (OSLDs) or an ionization chamber.[22],[23],[24],[25],[26],[27],[28],[29] Another way to assess organ doses as mentioned above is indirect method through measurement of CTDIs and published conventional factors are obtained from Monte–Carlo simulation and mathematical phantoms.[20],[30] Generally, the estimation of organ doses for CT procedures requires the user to supply dose and scan parameters for running simulation programs.[31] The aim of this study was to estimate organ dose using CT-Expo (SASCRAD, FritzReuterWeg, Buchholz, Germany); this was intended to be achieved using the same protocol from TLD measurements using a validated in-house head and body phantom, after which both results were compared. Similarly, the result from the CT-Expo was also compared to related studies.


  Materials and Methods Top


Four CT diagnostic centers with at least one CT scanner in each center were used. The type and manufacturing year of the CT scanners denoted as A–D were between 2012 and 2016 [Table 1]. CT scan parameters with individual center were used [Table 2]. The initial stage was the design of the in-house inhomogeneous phantom with Plexiglas similar to a standard (commercially available phantom) one with a diameter of 16 cm for the head and 32 cm for the body, respectively, and with depth of 14 cm [Figure 1] and [Figure 2]. Both phantom consisted of five inserts each (four on the peripheral and one at the center) and with a water inlet placed at the peripheral. Validation was done by filling the in-house phantom with water and carefully inserting TLDs (three chips per insert). In a similar manner, TLDs were carefully placed in the inserts of the commercially available phantom (acrylic) using the same CT protocols. Another validation was done by scanning the in-house phantom filled with water and a standard phantom to determine CTDIvol.[32]
Table 1: Machine specification

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Table 2: Summary of computed tomography parameters used for the CT -Expo software

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Figure 1: Validated in-house head inhomogeneous phantom

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Figure 2: Validated in-house body inhomogeneous phantom

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The type of TLDs used was lithium fluoride doped with magnesium and titanium. The TLDs were used to measure energy deposited by X-rays and were subsequently retrieved and analyzed using a TLD reader (RADOS-RE2000) to give point estimates of organ dose.

In the same vein, CT organ doses for head/neck (brain, eye lens, thyroid, and esophagus), chest (heart and lung), and abdomen (liver, stomach, and kidney) were determined. The organs (bovine tissues) were crushed and put into individual inserts alongside the TLDs chip for image acquisition. Hounsfield Unit (CT number) was determined for the above organ.[33] A minimum of three chips were used per insert to estimate dose to determined organ dose (mGy).

Similarly, a CT-Expo (SASCRAD, Fritz-Reuter-Weg, Buchholz, Germany) software was used to estimate organ dose (mGy) to the head/neck (brain, eye lens, thyroid, and esophagus), chest (heart and lung), and abdomen (liver, stomach, and kidney) [Figure 3] and [Figure 4].
Figure 3: CT-EXPO mathematical phantom

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Figure 4: An overview CT-EXPO worksheet

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The main purpose of this study was to verify if the mean organ dose between the TLD measurements and the CT-Expo software was statistically the same and to compare CT-Expo organ dose/relative difference (RD) result with international studies. The mathematical expression for RD is given by:



where |ΔD| = Dose difference and ΣD = Dose summation.

Statistical tool

Data analysis was done using SPSS Inc. Released 2008. SPSS Statistics for Windows, Version 17.0. (Chicago, USA). Descriptive statistics was used to determine the mean organ dose and relative percentage difference; one sample t-test, independent sample t-test, Pearson correlation, and one-way ANOVA were used to analyze the estimated organ dose. P-value < 0.05 was considered statistically significant.


  Results Top


Validation of the in-housed-designed phantom was done against a standard phantom experimentally using TLDs and CTDI/DLP measurement from the monitor of the CT. It was recommended by the American Association of Physicist in Medicine (AAPM), American College of Radiology (ACR) and Radiation Protection in Radiology safety code 35 document that CT radiation dose must remain within ±20% of the nominal value or from baseline value.[34],[35],[36] Initial performance test was carried out, in which the standard phantom was within the manufacturer's specification of ±20%. The mean CTDIvol for the head and body of the locally designed phantom was 47.50 and 12.05 mGy, respectively (P = 0.057), while the mean CTDIvol for head and body of the standard phantom was 57.93 and 14.39 mGy, respectively (P = 0.063). The RD between the local and standard phantom for both the head and body was 19.7 and 17.7%, respectively, which was within the acceptance range of ± 20% as reported by the ACR 2017 Document.

An independent sample t-test showed that there was no difference in mean between the standard and local head phantom (P = 0.127). Similarly, there was no difference between the standard and local body phantom (P = 0.149) [Table 3].
Table 3: Thermoluminescent dosimeter reading for standard and local head and body phantom

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Organ dose to the brain for center A–D using a one-way ANOVA showed that there was generally statistically significant difference in dose (P = 0.004). Multiple comparison (using post hoc) test show that organ dose to the brain for center A versus B, C, and D was statistically different (P = 0.019, P = 0.004, P = 0.012). In the same vein, organ dose to the brain for center B versus C and D was not statistically different (P = 0.589 and P = 0.988). Furthermore, brain organ dose for center C versus D was as well not statistically different (P = 0.762).

A look at thyroid for center A–D showed that there was generally no statistically significant difference in dose (P = 0.891). Multiple comparison (using post hoc) test showed that organ dose to the thyroid for center A versus B, C, and D was not statistically different (P = 0.863; P = 0.985; P = 0.990), similarly, center B versus C and D were not statistically different (P = 0.972 and P = 0.962). In addition, thyroid organ dose for center C versus D was as well not statistically different (P = 1.000).

In addition, one-way ANOVA test for the esophagus for center A–D showed that there was generally no statistically significant difference in dose (P = 0.274). Multiple comparison (using post hoc) test showed that organ dose to the esophagus for center A versus B, C, and D was not statistically different (P = 0.469, P = 0.939, P = 0.946), similarly, esophagus dose to the esophagus for center B versus C and D was not statistically different as well (P = 0.778 and P = 0.243). Further, esophagus organ dose for center C versus D was as well not statistically different (P = 0.693) [Table 4].
Table 4: Organ dose measurement for four computed tomography units to the head and neck region

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Organ dose to the lungs was generally not the same (P = 0.011). Multiple comparison (using post hoc) test showed that organ dose to the lungs for center A versus B, C, and D was statistically different (P = 0.039, P = 0.025, P = 0.013). Mean dose comparison for center B and C and center C and D was P = 0.987 and 0.963, showing no statistically significant difference, respectively.

Organ dose to the heart generally among the four centers was statistically not different (P = 0.499). In the same vein, comparison among centers showed there was no difference in dose values to the heart [Table 5].
Table 5: Organ dose measurement to the chest for four computed tomography units to the chest region

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There was large difference in organ dose among the centers to the kidney (P > 0.001). Multiple comparison test show that there were different doses to the kidney among the centers. Conversely, center B versus C and D organ dose was statistically the same. Dose to the liver showed an overall difference as well (P = 0.008). Similarities were only seen in organ dose for center D versus B and C, respectively. Organ dose to the stomach showed that there was statistically significant difference in dose (P = 0.005). Multiple comparison test showed that center A versus B, C, and D and center B versus C were statistically the same.

One sample t-test revealed that there was statistically significant difference in organ dose for brain, eye lens, and liver (P = 0.013, 0.018, and 0.049, respectively). In the same vein, there was no difference in organ dose for esophagus, thyroid, lung, heart kidney, and stomach (P = 0.091, 0.055, 0.077, 0.051, 0.050, and 0.060, respectively) [Table 6].
Table 6: Organ dose measurement to the chest for four computed tomography units to the abdomen

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CT-Expo measurement ranged from 4.25 to 35.65 mGy [Figure 5]. Comparison of the organ dose from TLD measurement using the local phantom and CT-Expo mathematical software revealed that there was no statistically significant difference in dose (P = 0.361) [Table 7]. This study was compared with imPACT, TLD, CT-Expo, and cadaver studies. The maximum RD was with thyroid in cadaver 2 (141%)[Table 8].
Figure 5: CT-Expo organ dose measurements for centre A-D

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Table 7: Comparison of mean CT-Expo and thermoluminescent dosimeter organ dose and relative difference

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Table 8: Comparison of CT-Expo mean organ dose between this work and international studies

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  Discussion Top


Technical parameters

There was statistically significant difference in kVp (P = 0.025) and rotation time (P = 0.002), among the centers respectively, using a one-way ANOVA. Studies have shown that with increase in kVp, image noise would decrease however at higher dose to the patient.[37],[38] A study by Beeres et al., who considered whether rotation time affected image quality. His finds showed that there was no statistically significant difference in faster and slower rotation time from chest CTs.[39] A critical parameter like mAs has been studied to affect overall patient dose.[40] Conversely, mAs in this study was statistically the same (P = 0.441).

Comparison of this study's with same or imPACT software only

Comparison of this study with Ekpo et al.,[41] who used two CT facilities, showed that there was statistically significant difference using similar software for CT one (P = 0.024) and no difference in organ dose for CT two (P = 0.130) for brain, eye lens, thyroid, lungs, liver, and stomach. Differences could be as a result of the kind protocol used which included mAs, kVp, slice thickness, and pitch. The maximum RD was 147.1 and 112.3% for liver and eye lens, respectively. There was no statistically significant difference in organ dose when this study was compared to Chipiga et al.,[42] who similarly estimated organ dose using pediatric protocol (for age five) with CT-Expo software parameter (P = 0.077).

The mean organ dose was compared with a study by Puekpuang et al.[43] in Thailand, who used the imPACT software; the findings showed that there were no statistically significant difference for the lung, esophagus, heart, stomach, liver, and kidney (P = 0.542). The maximum RD in dose was noticed in esophagus (57.9%). Furthermore, the mean organ dose was statistically the same when this study was compared with Ngaile and Msaki et al.,[44] who used the imPACT dosimetric software (P = 0.081). Maximum RD was seen in stomach with 101%. Similarly, there was mean dose difference between this study and Akpochafor et al.,[45] who used the imPACT organ dose software (P = 0.044). A change in trend was also noticed in a study by Kawaguchi et al.[46] in Japan, who like wise used imPACT software with tube current modulation which is a useful dose reducing technique with Aquilion RXL and 64. Comparison of their study with ours showed significant mean dose difference (P = 0.047 and P > 0.001, respectively), with maximum RD of 138.1 and 168.7% in thyroid, respectively. More difference was noticed with Aquilion 64 compared to RXL CT model. This points out to the fact that CT configuration is one of the factors that affect dose output. Similarly, comparison with a study by Nishizawa et al.[47] showed that there was no statistically significant difference in dose (P = 0.204). Although the highest RD in dose was 148.8% in the stomach, TLD measurements by Cakmak et al.[48] showed that there was difference in dose values compared with this study's CT-Expo measurement (P = 0.038) for brain, eye lens, thyroid, lungs, heart, kidney, and stomach. The highest RD was seen to be 175.2% for thyroid.

Comparison of this study with imPACT/CT-Expo and thermoluminescent dosimeter/optically stimulated luminescent dosimeter from other studies

The maximum RD from this study between CT-Expo and TLD measurement was 21% excluding esophagus and thyroid. The maximum RD from Groves et al.,[49] who used computer simulation (imPACT) and TLD, was 66.7%. Our study was approximately seen to be one-third of Groves's result. Similarly, RD between Monte–Carlo-simulated software (imPACT) and measured organ doses by Fujii et al.[50] was between 2.5%–11.0% and 1.5%–10.5% for organs in chest and abdominopelvis region, respectively; their minimum ranges were quite lower compared to the RD observed in our study between CT-Expo and measured organ dose using TLDs, which were between 13.06%–17.46% and 1.98%–17.91% for chest and the abdomen, respectively. This study was also compared to a study by Yusuf et al.,[51] who used OSLDs with a Rando phantom with various insert. Discrepancies between measured and calculated dose using OSLD and imPACT dosimetric software were within 10%. Apart from large discrepancy observed for esophagus and thyroid in this study, the maximum dose difference was seen to be 21%. In addition, the maximum RD for CT-Expo/TLD in our study was 159.8% for thyroid, and Cakmak dose difference using imPACT/TLD was 211% for the testis. A study by Chipiga et al., who used CT-Expo software with dosimetric anthropomorphic phantom for age one and five with TLDs, showed that RD was 166 and 178%. Organ dose discrepancies may occur from different point of measurement within the investigated organ. It is wordy to note that dose discrepancy tends to reduce if phantom or mathematical software measurements are done using similar CT scanner and protocol and if more points of measurement are taken within the organ and vice versa.

Comparison of CT-Expo with thermoluminescent dosimeter measurement from other studies

There was no statistically significant difference between TLD measurements from the locally designed phantom and CT-Expo software (P = 0.361). Large differences were noticed for esophagus and thyroid which were 91.81 and 159.8%, respectively; these differences were attributed to lack of recognition by the CT-Expo software for esophagus and thyroid as organs in the head/neck region. In a similar manner, Dabin et al.[52] used a pediatric phantom for organ dose verification; their findings showed that RD between measured and simulated dose was ±20%, this result was close to the maximum RD observed in this study, which was 21%. In a related study carried out by Hsieh et al.[53], who used TLDs to take measurement from an anthropomorphic Rando Phantom with a Philips Brillance iCT and 64 Multi-Slice Computed Tomography (MSCT). Obtained result showed that there was no difference between his study and those obtained in this study (CTExpo software) for esophagus, thyroid, lung, and liver (P = 0.099 and 0.534), respectively. Maximum organ dose in this study for chest/abdomen was 17.9 mGy, which was lower than a postmortem (cadaver) study by Griglock et al.,[54] who used OSLDs; their maximum dose for chest/abdomen-pelvis was >32 mGy. Cadaver 1 and 2 mean organ dose from Griglock et al. showed no difference with our study (P = 0.214 and P = 0.065), but there was difference in mean dose in cadaver 3 (P > 0.001).


  Conclusion Top


Organ dose measurements from readymade or anthropomorphic phantom with TLDs have been established. This study has estimated organ doses with locally designed phantoms using TLDs and has used mathematical software to compare results. The use of organ dose software will help centers who cannot purchase readymade or anthropomorphic phantoms to optimize their protocol before scanning their patients. CT configuration and position of TLDs within tissues were major factors that contributed to dose discrepancies in this study.

Acknowledgment

We would like to thank the management of the following diagnostics centers for their assistance in the collection of data: Lagos University Teaching Hospital, Arrive Alive Diagnostics Center, and Foremost Diagnostics Center.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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