An Assessment of Organ and Effective Dose of Patients who Undertake CT Examinations in two Teaching Hospitals of Mashhad&Isfahan

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1 An Assessment of Organ and Effective Dose of Patients who Undertake CT Examinations in two Teaching Hospitals of Mashhad&Isfahan *M.T.Bahreyni Toossi, **S.Mohandes Dastgherdi Medical Physics Dep., Faculty of Medicine, Mashhad University of Medical Sciences. Mashhad-Iran * ** Abstract. The relative public dose from CT examination depends on various factors and therefore its influence on the public dose from medical applications is varied widely in different societies. In Iran, little data are available in this field. In the present work, we have tried to estimate patient dose caused by routine CT examinations in four teaching hospitals. Organ and effective dose were estimated for 7 patients who underwent CT examination of brain, chest and abdomen pelvis. Four CT scanners were involved in this work. Application of CTDOSE software enabled us to compute for all patients doses to organs with the ICRP specified tissue weighting factors, Organs listed as remainder organs and some additional organs such as: lenses of eyes, heart and trunk. CT scans applied to all 7 patients who participated in this survey can be classified into three major examinations namely: brain, chest and abdomen pelvis. The results were analyzed to find out the CT machines and CT scans which can cause maximum and minimum organ and effective dose. Summary of our findings are as follows: patients who were scanned by G.E CT pace scanner in the abdomen pelvis region received maximum dose equal to.5 mgy to their U-bladder and minimum dose equal to.5 mgy to their thyroid. Maximum organ dose all in all was delivered to brain when brain scan was performed by G.E CT pace machine. Maximum effective dose (.97 msv) was produced when abdomen pelvis scans was carried out; minimum effective dose (. msv) was resulted from brain scan. Scan carried out by G.E CT pace scanner produced maximum effective dose (.5 msv), minimum effective dose (o.9 msv) was achieved when CT examination was performed by Siemens somatom.. Introduction In recent years application of a wide variety of CT examinations in diagnostic radiology has been increased sharply. Due to the nature of CT procedures it is considered as a relatively high dose technique, which could deliver much higher doses to patients in comparison to conventional radiology. The doses can often approach or exceed the levels at which the probability of cancer induction is known to be increased significantly [,]. In a 998 survey,the UK National Radiological Protection Board (NRPB) reported that % of the national collective dose from medical X-ray examinations was derived from CT-scans, although it represented only % in terms of the total number of such examinations. The NRPB has also reported that the contribution of CT examinations to national collective dose had risen up to one third in 995 and nearly to % in 998[,,,5,]. Many similar studies have been performed to estimate absorbed dose to organs and tissues of patients undergoing CT examinations in other countries. Iran has been no exception, and over past years the number of requests for CT examinations by medical professionals has gone up by many folds, but so far, non-or very little information are available as regards to the contribution of CT examinations in patient dose [,7]. In this research, for the first time in Iran, we have estimated organ and effective dose of 7 patients who undertook CT examinations in four hospitals of two major cities of Mashhad and Isfahan. When large number of patients and various organs are involved, indirect dosimetry is more appropriate. For this purpose different workers in different countries have compiled quite a few computer program. In this study and for our purpose CTDOSE software; compiled by John Le Heron; has been utilized. CTDOSE is a computer program, written to run on MSDOS computer to enable the calculation of organ doses and effective dose for CT examinations performed on a selection of common CT scanners. CTDOSE uses data files containing the results of Monte Carlo calculations performed at the National Radiological Protection Board (UK) [7,,].

2 . Materials and Methods CTDOSE requires a computer running MSDOS version. or later. It can also be run as an MSDOS task under Microsoft Windows.. Printed output from CTDOSE is text only and so any printer will do. The computer program is supplied as an executable file CTDOSE.EXE that should be placed in the same directory as the data files (named MCSET.DAT up to MCSET.DAT) and CTDI.DAT (CTDI.DAT contains a set of default values of CTDI per mas obtained during the survey of CT practice in the UK). The input of the software is divided into two parts. The first part is concerned with specifying the make and model of CT scanners. Scanning parameters are then entered to the second part of input. The rang of scanning parameters are as follows: -mas: values up to mas per slice are valid. -Slice width: values up to mm are valid. -Couch increment: values from to mm are acceptable. -CTDI/mAs: values up to mgy per mas are accepted. -Start position (mm): the base of the trunk is assigned a value of mm, and the top of the head 9mm.The software assumes that the sequence of slices is toward the head, values from -mm to 9mm are valid. -No of slices: it represents the number of slices in the region of interest []. Altogether 7 patients were selected from those who were referred during spring and summer of for their CT examinations to the radiology department of the following four hospitals (8 per hospital):imam Reza and Ghaem which are located in Mashhad, north east of Iran; Shariaty and AL-Zahra which are situated in Isfahan, in the center of Iran. Siemens somatom, GE CT 98, GE CT Max and GE CT Pace were the scanners used in Ghaem, Imam Reza, Shariaty and Al-Zahra respectively. The most common CT examinations carried out in these hospitals were: brain, chest, and abdomen-pelvis. According to the requirements dictated by the input file of CTDOSE software, scan parameters and the type of CT machine used for a particular examination had to be recorded. Therefore a questionnaire was designed to accommodate details of individual patient CT scan (table I). Table I. The questionnaire used in this research Name of hospital: Model of CT scanner: Start No of Couch Slice mas kvp Type of Name of Position Slices Increment Width Examination Patient 7 questionnaires were filled in for all examinations included in this study. Later this information were entered to the input file of the program. At this stage we were able to acquire from the output file organ dose in mgy for organs with ICRP- specified tissue weighting factors, the organs listed as the remainder organs in ICRP- and also dose to an additional organ and body regions. ICRP classifications for CTDOSE software are showed in table II [8]. Table II. ICRP classification for body organs/tissues applied to CTDOSE software ICRP Organs:,,,, Red Marrow,,,,,,,. Remainder Organs:,,,,,, Small Intestine,,,. Other organs:,,,.. Leg Reg. In addition to individual organ dose, effective dose was also computed for the relevant examination/patients. Organ and effective dose calculated for individual patient/examination were saved in separate files for further analysis.

3 . Results All 7 data files created for patients included in this research were divided into groups of equal numbers. Each group consisted files related to patients who undertook similar examination by one of the four CT scanners employed in this study. Average organ and effective dose were computed for pre-specified groups. Summary of all these calculations are presented in table III. Figures to show how the average dose to a single organ (as specified by ICRP-) is varied with the type of CT examination and also CT scanners of different model modify radiation dose to a particular organ arising from identical examinations. Dose ' mgy ' 8 FIG.. Organ doses by GE CT Max Dose ' mgy ' 8 8 FIG.. Organ doses by GE CT Pace

4 Dose 'mgy ' 8 FIG.. Organ dose by GE CT 98 5 Dose ' mgy ' FIG.. Organ Dose by Siemens Somatom

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6 Average effective dose of all patients were also examined for the effect of CT scanner model and the prescribed CT examination (fig 5). Fig. shows that over all average effective doses of patients depends on the scanner s make and model. Average effective dose of all patients due to undertaking of brain, chest and abdomen-pelvis examinations were determined. The results are given in figure 7. EDE ' msv ' 5 GE CT MAX GE CT PACE GE CT 98 SIEMENS SOMATOM The model of scanner FIG. 5. Average effective dose caused in different examinations by different scanners..5 EDE ' msv ' GE CT MAX GE CT PACE GE CT 98 SIEMENS SOMATOM The model of scanner FIG.. Average effective dose arising from three types of examinations by different CT scanners. EDE ' msv ' BRAIN CHEST ABDOMAN-PELVIS Type of examoination FIG. 7. Average effective dose received by all patients due to three specified CT examinations.

7 . Discussion and conclusion A careful review of our results summarized in table I and fig to 7 enabled us to specify: the organs which absorb maximum and minimum dose; type of CT examinations delivering the highest and lowest dose to a particular organ; and the CT scanner which its employment will give rise to the highest over all average dose to patients. Figures to show that regardless of the scanners make and model: average organ dose to gonads, stomach,,, red marrow, bone, U bladder, skin, kidneys, small intestine, uterus, muscle and G bladder are higher for abdomen-pelvis examination., esophagus, breasts, thymus and heart receive a higher average dose when chest CT-scan is carried out. Also brain scan is responsible for higher average dose to brain and eye lenses. In figure 5 average effective dose to patients produced by different CT machines can be compared. It is clearly evident that abdomen-pelvis examination will give rise to the highest effective dose irrespective of the scanner model. Overall average effective dose of patients examined by GE CT Pace scanner is equal to.5msv; it is also as low as.9 msv for Siemens somatom. On the other hand from figure 7 it is evident that average effective dose produced by all four CT-scanners investigated in this study, is distinctly higher when patient is undertaking abdomen-pelvis scans, and is at its lowest value for brain examination. 5. References.International Commission on Radiological Protection, Managing patient dose in computed tomography. Publication 87. Annals of the ICRP,,No., Pergamon Press, Oxford and New York ().. Shrimpton, P.C.,Wall, B.F., The increasing importance of X-Ray computed tomograpy as a source of medical exposure. Radiat. Prot. Dosim., 57: -5, (995).. Shrimpton, P.C., Edvyen, S., CT scanner dosimetry. BR. J. Radiol., 7: -, (998).. Shrimpton, P.C., Jones,D.G., Hillier, M.C., Wall, B.F., Le Heron, J.C., and Faulkner, K., Survey of CT practice in the UK.(part ): Dosimetric aspects. NRPB R-9: -, (99). 5. Shrimpton, P.C., Wall, B.F., Reference dose for pediatric CT. Radiat. Prot.Dosim., 9: 9-5, ().. Wall, B.F., Shrimpton, P.C., The historical development of reference doses in diagnostic radiology. Radiat. Prot. Dosim. 8: 5-, (998). 7. Jones, D.G., Shrimpton, P.C., Survey of CT practice in the U.K (part) : Normalized organ doses calculated using Monte Carlo techniques. NRPB R-5: -9, (99). 8. Recommendations of the International Commission on Radiological Protection. Publication. Analls of the ICRP,, No.-, Pergamon Press, Oxford and New York (99). 7