Methods Used to Estimate the Collective Dose in Denmark from Diagnostic Radiology

P-7-24

Methods used to estimate the collective dose in Denmark from diagnostic radiology

P. Grøn 1 1National Institute of Radiation Hygiene, Knapholm 7, DK-2730 Herlev, Denmark

ABSTRACT According to EU directive 97/43/Euratom all member states must estimate doses to the public from diagnostic radiology. In Denmark the National Institute of Radiation Hygiene (NIRH) is about to finish a project with the purpose of estimating the collective dose in Denmark from diagnostic radiology. In this paper methods, problems and preliminary results will be presented.

Patient doses were obtained from x-ray departments, dentist and chiropractors. Information about the frequencies of examination was collected from each of the Danish hospitals or counties. It was possible to collect information for nearly all of the hospitals. The measurements were done by means of dose area product meters in x-ray departments and by termoluminescens dosimetry at chiropractors and solid-sate detectors at dentists. Twenty hospitals, 3.200 patients and 23.000 radiographs were measured in this study. All data were stored in a database for quick retrieval.

The DAP measurements was done "automatically" controlled by PC based software. Later these recordings could be analysed by means of specially designed software and transferred to the database.

Data from the chiropractors were obtained by mail. NIRH sent each chiropractor TLD's and registration form. The chiropractor did the measurements him self and returned afterwards the TLD's and registration forms. On the registration form height, weight, age etc. of the patient was noted and so was information about applied high-tension, current-time product and projection.

Calculation of the effective dose from the DAP values and the surface entrance dose were done by Monte Carlo techniques. For each radiographs two pictures of the mathematical phantom were generated to ensure that the x-ray field where properly placed. The program ”diagnostic dose” developed by NIRH did the Monte Carlo calculations.

INTRODUCTION According to EU Directive 97/43/Euratom (1) all member states must estimate doses to the public from diagnostic radiology. In Denmark the National Institute of Radiation Hygiene (NIRH) is about to finish a project with the purpose of estimating the yearly collective dose in Denmark from diagnostic radiology. In this paper, methods, problems and preliminary results will be presented.

MATERIAL AND METHODS When estimating the collective dose to the public, one needs to know the number of various x- ray examinations performed annually and the mean effective dose for each type of examination. The collective effective dose (CED) can be calculated by means of the following equation:

K = ∗ (1) CED ∑ N i D i i=1 Where K is the number of different examinations.

Ni is the annual numbers of examinations of type i.

D i is the mean effective dose from examination i.

1 P-7-24

Thorax Lumbar spine 22% Knee 4% Hand & fingers 6% Hip Pelvis 2% 5% Mammography 3% Thoracic spine 2% 3% 2% 5% Anklejoint Cervical spine

Hospitals

The task is to collect information about Ni and Di. Each type of x-ray examinations in Denmark is given a four-digit number to identify the examination. Most of the Danish hospitals and/or counties being the owners of the hospitals keep computer records of the number of performed x-ray examinations. The hospitals or counties were asked to send the appropriate information to NIRH. We were able to collect information from hospitals/counties serving about 95 percent of the population. The remaining 5 percent where computer records were not available were estimated from comparable hospitals. All information about the number of examination was kept in a database at NIRH. When one has collected the information about the annual number of x-ray examination, the task is to gather information about the corresponding doses. For conventional radiography these doses where in Denmark measured using Termoluminscent Dosimetry (TLD), solid-state dosimeter and dose area product (DAP) meters. Dose measurements were performed at hospitals, chiropractors and dentists.

Measurements was done with DAP meters from RMI/Gammex (2). This instrument has the capability to measure at two x-ray tubes at a time. The instrument sends information to a connected computer every 5 ms. This measuring set-up is illustrated in figure 1.

Figure 1. Illustration of a typical set-up for dose measurement.

2 P-7-24

The measured values are transferred to a database at NIRH along with information about the x- ray procedure and patient data. A program ”Diagnostic Dose” calculates the effective dose to the patient and

Figure 2. The interface program ”Diagnostic Dose” for MCNP.

the individual organs (figure 2). This program is developed at NIRH and is build as an interface to the program Monte Carlo N-Particle (MCNP) (3). The program uses a hermaphrodite phantom (figure 3) based on the ADAM/EVE phantoms to simulate the patient (4).

Figure 3. X- ray field on mathematical phantom

Twenty hospitals were selected in such a way that all types and sizes were represented. In each x-ray department were installed 3 DAP meters for approximately 3 weeks. In this period, doses from as many examinations as possible were measured.

The information about procedures involving Computed Tomography (CT) was derives from a questionnaire send to each of the x-ray department in Denmark. In the questionnaire the department were asked

3 P-7-24 to describe how they performed different procedures. They should indicate parameters like scanning length, high-tension, current-time product, scanning position etc. This information was put in a CT-DOSE program developed at NIRH. When the effective doses were calculated the collected effective dose from CT could be calculated by means of equation 1.

Chiropractors

As with the hospitals one has to known the total number of examinations and the distribution. The National Health Service supplied the total number of examinations. NIRH wrote to 35 chiropractors and asked them to record how many x-ray examinations of various types they performed during a period of 40 working days. This information together with the total number of examination gave the distribution of examinations.

From the 35 chiropractors 15 were asked to participate in the dose survey. They should measure doses from the following examinations:

10 lumbar spine 6 cervical spine 10 thoracic spine (only 5 chiropractors should measure this)

The doses were measured by means of TLD chips and information about the procedure and patient were filled into a form by NIRH. The measurements should be started 2 working days after receiving the TLD chips and consecutive patients undergoing the examinations mentioned above. The effective doses were calculated using the “Diagnostic Dose” program. When the effective doses were calculated the collective effective dose from chiropractors was calculated by means of equation 1.

Dentists

Measurements were performed with TLD chips for extraoral examinations, and with the PMX- III meter from RTI Electronics (1), for intraoral examinations. The doses were estimated for the following procedures:

Intraoral radiology. Panoramic radiology. Cephalometric radiography.

Figur 4a. Intraorale radiograph (lateral) Figur 4b. Intraorale radiograph (front)

The parameter measured was the entrance skin dose (ESD). The high tension used in the examination was noted. These parameters were used to calculate the effective dose from the various procedures. The effective dose calculations were based on the MCNP code and a special phantom of the human head. The number of examinations was obtained from different sources like the National Health Service and personal information. Part of the work was done by Department Of Radiology at the School of Dentistry in Copenhagen.

RESULTS The data processing is not finished yet, but some preliminary results will be presented. Figure 5

4 P-7-24 shows the effective dose distribution for the lumbar spine examination. The distribution fits a log-normal distribution with a long tail with relative few but high doses, which normally is found in such studies. Figure 6 shows the effective dose for the lumbar spine examination from various hospitals.

n = 373 Frequency (%)

0 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0

Effective dose (mSv)

Figure 5. The dose distribution for the lumbar spine examination

4 Effective dose (mSv)

2 Max Min 75 % quartile 25 % quartile 0

I Mean value J L T F K P S A B E N O Q R U C D G H M Hospital ID

Figure 6. Effective dose obtained in various hospitals for the lumbar spine examination

Figure 6 shows that there is not that big a difference in the mean effective dose for the twenty hospitals participating in this study. Table 1 shows some data collected in this study. Figure 7 shows the ten most

5 P-7-24 common examinations in percent of the total number of examinations in Denmark in the middle of the nineties. Figure 8 shows the dose contribution in percent of the collective effective dose from the “top five”.

Thorax Lumbar spine 2% 2% 3% 3% Knee 2% Hand & fingers 22% Table 1. Data on hospitals, patients and radiographs. Hip 5%Information Number Pelvis Mammography Participating5% x-ray departments 20 Thoracic spine No. Of patients in this6% study 4% 3.281 Average no. Of patient pr. department 164 Anklejoint No. Radiographs in this study 23.147Cervical spine No. Field-check pictures generated (figure 3) 46.294

Figure 7. The ten most common diagnostic x-ray examination in Denmark (middle of the nineties)

11%

Barium Enema 8% Urography Upper abdomen (CT) 11% Thorax (CT) Lumbar Spine 5% 5%

Figur 8. The five examinations contributing with the highest doses to the collective effective dose.

CONCLUSIONS The study is not finished yet so a final conclusion is not ready. But it looks like the collective effective dose will be a little lower than expected when considering similar studies. The contribution from CT and conventional radiography show the same ratio (40 percent from CT) as seen in other Nordic countries.

REFERENCES 1. European Commission, Council directive 97/43/Euratom of 30 june 1997 on health protection of individuals against the dangers of ionising radiation in relation to medical exposure, and repealing Directive 84/466/Euratom, The Official European Journal No. L 180/22 9-7-1997 (1997).

2. Gammex-RMI Inc, Manual for dose area product meter model 841-M modular system, Gammex-RMI Inc (1996).

3. Briesmeister, J. F., MCNP- A General Monte Carlo N-particle Transport Code, Version 4B, Radiation Safety Information Computational Center (RSICC) (1997).

4. Kramer, R., Zankl, M., Williams, G., Drexler, G., The calculation of dose from external photon

6 P-7-24

exposures using reference human phantoms and monte-calo methods. Part I, The male (ADAM) and female (EVA) adult mathematical phantoms. GSF Bericht S-885 8 (1982).

5. RTI Electronics AB, User’s Manual, RTI Electronics AB (1994).