VETERINARSKI ARHIV 69 (5), 289-298, 1999

ISSN 1331-8055 Published in Croatia

Use of an animal model for the determination of radiation dose distribution in X-ray diagnostics

Miljenko Šimpraga¹*, Maria Ranogajec-Komor², Vladimir Butkovic³, and Darko Capak⁴

¹Department of Physiology and Radiobiology, Faculty of Veterinary Medicine, University of Zagreb, Zagreb, Croatia

²Ruder Boškovic Institute, Zagreb, Croatia

³Department of Radiology, Physical Therapy and Ultrasonic Diagnostics, Faculty of Veterinary Medicine, University of Zagreb, Zagreb, Croatia

⁴Clinic for Surgery, Orthopaedics & Ophthalmology, Faculty of Veterinary Medicine, University of Zagreb, Zagreb, Croatia

* Contact address:
Dr. Miljenko Simpraga,
department of Physiology and Radiobiology, Faculty of Veterinary Medicine, Univerity of Zagreb, Heinzelova 55, 10000 Zagreb, Croatia,
Phone: 385 1 23 90 170; Fax: 385 1 244 13 90; E-mail: simpraga@vef.hr

ŠIMPRAGA, M., M. RANOGAJAC-KOMOR, V. BUTKOVIC, D. CAPAK: Use of an animal model for the determination of radiation dose distribution in X-ray diagnostics. Vet. arhiv 69, 289-298, 1999.

ABSTRACT

The ever-increasing number of X-ray examinations in veterinary medicine calls for appropriate attention to be paid to radiation prevention and control. The initial premise for such an approach is the exact determination of the received radiation dose. Therefore, in order to increase the level of radiation prevention and control in X-ray diagnostics in veterinary medicine, we carried out a research focussing on establishing a model for the determination of radiation dose distribution in a patient's body with greater accuracy than is achievable with models used in human medicine. The research was carried out with two euthanised dogs of the German shepherd breed. For dosimetrical measurements the most common X-ray diagnostic examinations were selected: abdomen radiography in female and hip radiography in male dogs. For dose measurements a previously tested thermoluminescent system (TL) was selected, based on a TLD-700 detector and a TOLEDO 654 computerized reader. In addition to the testing of the dosimetrical system, homogeneity calibration was performed and radiation dose reproducibility was determined. On the basis of achieved results it can be concluded that the described animal model for the determination of dose distribution for X-ray diagnostic radiation yields reliable results and enables us to estimate the risks of ionising radiation associated with various X-ray diagnostic examinations, and thus to increase the level of prevention, and patient protection, in veterinary medicine.

Key words: X-ray diagnostics, dose distribution, animal model, dog

Introduction

According to the average number of radiologists and X-ray units per 1000 head of population, countries were classified into four groups. According to this classification, Croatia falls in the category with the highest health care level (ANONYMOUS, 1993). However, this level is not associated with corresponding care for radiation prevention and control. This was the reason why, in the last few years, actions were taken to procure new X-ray units which raised radiation protection and control in human medicine to a satisfactory level (NOVAKOVIC, personal communication).

In the same period, following amendment to the Veterinary Activity Act, the establishment of private veterinary clinics for so-called "small" practices was initiated (ANON., 1991). Very soon after having acquired initial experience, many veterinarians decided to include X-ray diagnostics services in the scope of their practice and proceeded to purchase new X-ray units. However, the high cost of new X-ray units and the obvious negligence of radiation prevention and control induced some of our colleagues to purchase second-hand X-ray units that had been rejected for use in human medicine.

In its first Article, the new Ionising Radiation Control Act provides that ionising radiation control measures are intended as safety measures designed to reducing threat to life and health risks in humans and in the environment, although no mention is made of animals whose health becomes a matter of concern only in the event of emergency (ANON., 1999). "Emergency" means any event associated with ionising or affecting the safety of nuclear plants caused by circumstances beyond control, which results in exposure to elevated levels of radiation of operators working with ionising radiation sources, of the population in general, or in radioactive contamination of the environment. However, a degree of hope for better care of animal patients subject to X-ray diagnostic or therapeutic treatment is offered by legally mandated measures for protection against ionising radiation, which should be enforced by separate regulations. These regulations should, in addition to human health care, take into account health care for veterinary medicine patients in a manner that would consider the reduction of health risks to both humans and animals on an equal level. After all, such an approach would be in compliance with general principles of the Animal Welfare Act, adopted in Croatia for the first time in 1999. (ANON., 1999a).

The risks of exposure, i.e. the genetic and somatic effects of radiation, are well known. Also, as the number of X-ray examinations conducted in veterinary medicine shows a progressively growing trend, it is necessary to take appropriate precautions with regard to radiation prevention and control. The initial premise for such an approach is the exact determination of a received radiation dose.

Dose determination, i.e. the dosimetry in X-ray diagnostics, is a very complex task since the intensity of this radiation varies considerably and the doses received by patients are very small. In human medicine, phantoms and various mathematical models have been used so far for this purpose (JACOBSON, 1993), as well as measurements of radiation doses on the surface of a patient's body (MILKOVIC et al., 1991). These models could be used in veterinary medicine too, but their main drawback is that the doses received by individual organs are estimated rather than actually measured.

This is why, in order to improve the level of radiation prevention and control in X-ray diagnostics, our research was focused on identification of a model for determination of radiation dose distribution in a patient's body with greater accuracy than is achievable with models used in human medicine.

Materials and methods

The research was performed on German shepherd dogs, a common breed in Croatia. Two ten-month-old littermates of different sexes were selected. The body masses of the female and the male animals were 18 and 23 kg respectively. The most common X-ray examinations were selected: abdomen radiography in females, and hip radiography in males. Radiography was performed by means of a 150 kV Philips Mueller unit DA 1001 (Hamburg, Germany), used for X-ray diagnostics at the Department of Radiology, Ultrasonic Diagnostics and Physical Therapy, Faculty of Veterinary Medicine, University of Zagreb.

For dose measurements a thermoluminescent system (TL) was selected, based on a TLD-700 detector and a TOLEDO 654 computerized reader (Pitman Instr., Weybridge, England). The selected system was previously tested in a standard x-ray field (RANOGAJAC-KOMOR et al., 1993). Calibration of the TLD system was carried out with radiation exposure at ¹³⁷Cs source, at a dose rate of 0.91 mGy/h and at 1 m distance. In addition to the testing of the dosimetrical system, homogeneity calibration was performed, i.e. the horizontal and vertical distribution of the X-ray unit radiation dose within and above an exposed area of 20 × 20 cm was measured. In addition to determination of homogeneity, on the basis of these measurements, the reproducibility of the radiation dose was determined and defined as a standard deviation - expressed in mSv and percentage - of the doses measured in three measuring cycles. In each measuring cycle 48 dosimeters were used (with 24 rubber holders containing 2 dosimeters each) under exposure conditions of 80 kV, 30 mAs and at a distance of 100 cm from the focus of the X-ray tube.

The first test was performed on the female dog which was euthanised with an intravenous application of 0,3 ml/kg body mass of T 61 (Hoechst, Munich, Germany). The animal was placed in lateral recumbency and the dosimeters were placed on the surface and inside the body, that is to say, two dosimeter packages at each measuring point. On the upper body surface, dosimeters were applied at the primary beam inlet, thorax, hip, muzzle, thyroid gland, elbow, thoracic vertebrae, knee, sacrum and tail root. On the lower side of the body, dosimeters were applied at the outlet of the primary beam, thorax, elbow, knee and hip. For the purpose of internal application of the dosimeters, the medial part of the abdomen was surgically opened and dosimeters were introduced into the small intestine, uterus, bladder, spleen and liver. Radiographs of the female animal were taken in profile (latero-lateral) recumbency in four measuring cycles. Test conditions were 70 kV and 25 mAs, and test material included conventional foils (calcium wolframate) and X-ray film FFO 110, 30×40 cm, at a distance of 100 cm from the X-ray tube focus (Fig. 1). Upon completion of the test the dosimeters were removed and the wound was reconstructed.

Fig. 1. Radiograph of the female German shepherd abdomen with applied dosimeters

Following the performance of tests on the female dog we proceeded with testing of the male dog, which had also been euthanised in the same way as the female one. Similar to the first test, dosimeters were applied on the surface of the body and inside, two dosimeters packed in rubber holders at each measuring point with the dog in dorsal recumbency. On the upper body surface dosimeters were applied at the primary beam inlet, at the beam inlet to the right and left hip, on the tip of the right and left knee, on the right and left axilla, at the beam inlet to the sternum, to the thyroid gland, eye and muzzle. On the lower part, dosimeters were applied at the primary beam outlet, to the neck below the thyroid gland, opposite the sternum and to the left and the right hip. After surgical intervention, dosimeters were applied within the primary beam area: the testes, prostate gland, femur and small intestine. For the purpose of measuring of the dose received by bone marrow, the femur had to be sawn with Gilly's saw and the dosimeter was inserted in the bone lumen. Radiographs of the hips were taken in sagital recumbency, in four measuring cycles at 80 kV and 30 mAs. The foils, film and distance from the X-ray tube focus were identical to those applied in the first test (Fig. 2). Upon completion of the X-ray we proceeded with osteosynthesis of the femur by means of an appropriate AO technique plate, followed by wound reconstruction.

Fig. 2. Radiograph of the male German shepherd hips with applied dosimeters

Results are expressed as mean values ±SD; as SD relative (SDrel.), and are presented either graphically or in tables.

Results

Testing of X-ray unit irradiation area

Testing of the homogeneity of an X-ray unit irradiation area of 20×20 cm was performed in three measuring cycles, each with 48 dosimeters. Measuring results are shown in Fig. 3.

Fig. 3. Horizontal (a) and vertical (b) X-ray unit radiation relative dose distribution inside and outside of the irradiated area in German shepherds

In the central beam, the mean value of the doses measured in three cycles with 6 dosimeters each, was: 2.07±0.14 mSv, i.e. with an SDrel. of

Dose distribution in abdomen radiography

The radiation dose distribution in abdomen radiography of the female dog was established in four measuring cycles with two dosimeters at each measuring point. Results are shown in Table 1.

Table 1. Dose distribution in X-ray examination in German shepherds

Results of radiation dose distribution in surface abdomen radiography in the female dog showed that the lowest SDrel. in 4 measuring cycles was that established for the primary beam inlet - 4.9%. Other values below 10% include the hip, muzzle and thoracic cavity outlet; values ranging between 10% and 20% were established for the eye, thoracic cavity inlet, thoracic vertebrae, elbow outlet and thyroid gland. Somewhat higher values were established for the elbow inlet (23%) and the tail root (27.9%) and the highest values were established for the knee outlet and inlet: 35.3%, and as much as 98.6% respectively. Measurement of dose distribution in internal organs shows that the lowest standard deviation in 4 measuring cycles was that established for the uterus (14.3%) and the highest for the bladder (21.8%). All other values ranged between these two limits.

Dose distribution in hip radiography

Dose distribution in male dog hip radiography was measured in four cycles with two dosimeters applied to each measuring point. Results of these measurements are shown in Table 1.

Results of dose distribution in surface radiography of the male dog hips show that the lowest SDrel. in 4 measuring cycles was established for the left hip inlet - 5.10%. Values between 10% and 20% were established for the primary beam inlet, right hip inlet, primary beam outlet and left axilla. Values between 20% and 30% were established for the sternum, right axilla and thyroid gland. Higher values were established for the muzzle, neck outlet, eye and right and left hip outlets, while the highest values were those established for the right and left knee. Dose distribution measurements involving internal organs show that the lowest standard deviation in 4 measuring cycles was established for the femur (7%) while values for the bladder, prostate gland, testes and intestine ranged between 21.2% and 26.8%.

Discussion

Results of investigation into the homogeneity of the X-ray unit irradiation area show that the differences between the maximum and the minimum doses in the horizontal and vertical distribution were 9.7% and 3.9% respectively. These results are in compliance with the standard requirements established for the X-ray unit operation, i.e. for the performance of X-ray diagnostic examinations (NOVAKOVIC, personal communication). Our measurements also showed that the mean value and standard deviation of all measurements amounted to 1.09±0.07 mSv, which ensures the reproducibility of X-ray unit irradiation in compliance with the requirements which must be met by X-ray units used for diagnostic examinations (NOVAKOVIC, personal communication).

Results of dose distribution in radiography of the female dog abdomen, in profile projection, established by means of the dosimeters applied at 21 points, showed that the SDrel in 4 measuring cycles was 4.9% in the primary inlet beam. When comparing this value with the standard deviation of the primary beam dose measured without animals, which was equal to 6.3%, we can conclude that the data reproducibility level is very high. Unlike these results, the dose measurements performed on the knee showed a high standard deviation. This was most probably due to the fact that during radiography the dosimeters applied to the knee were outside the homogenous irradiation area of 20×20 cm.

Results achieved in radiography of the male dog hip correspond to those achieved in radiography of the female dog abdomen. That is, the standard deviation as a point of reference for the determination of reproducibility, in four measuring cycles at the left hip location, amounted to 5.1%, while at the knee location the standard deviation was very high since in this particular case this site was outside the homogenous radiation area.

On the basis of the foregoing, we can conclude that the dosimetrical system TLD-700 and the TOLEDO 654 computerized reader meet the requirements established for these kinds of measurement. Additionally, we can conclude that the irradiation area homogeneity and reproducibility of the applied X-ray unit is acceptable and meets the applicable standards.

As a result, we can conclude that the animal model described herein for determination of dose distribution in X-ray diagnostic examination is reliable and enables us to estimate, on the basis of the established surface radiation dose, the doses received by individual organs and to estimate the risks of ionising radiations associated with various X-ray diagnostic examinations, and thus to increase the level of prevention and protection of patients in veterinary medicine.

References

ANONYMOUS (1991): Zakon o zdravstvenoj zaštiti zivotinja i veterinarskoj djelatnosti. Narodne novine 52, 1428-1451.

ANONYMOUS (1993) Sources and effects of ionizing radiation. United Nations Scientific Committee on the Effects of Atomic Radiation. Report to the General Assembly, with Scientific Annexes. UN, New York.

ANONYMOUS (1999): Zakon o zaštiti od ionizirajuceg zracenja. Narodne novine 27, 813-820

ANONYMOUS (1999a): Zakon o dobrobiti zivotinja. Narodne novine 19, 505-510.

JACOBSON, R. (1993): A quality assurance phantom for diagnostic radiobiology. Radiat. Prot. Dosim. 49, 53-54.

MILKOVIC, Đ., M. RANOGAJEC-KOMOR, M. KRSTIC-BURIC, A. HEBRANG (1991): Mit Thermolumineszenz-Dosimetern gemessene Haudosen bei Thorax-Röntgenaufnahmen von Kindern und Jugendlichen. Atemw.-Lungenkrkh. 17, B67-B72.

RANOGAJEC-KOMOR, M., F. MUHIY-ED-DIN, Đ. MILKOVIC, B. VEKIC (1993): Thermoluminescence characteristics of various detectors for X-ray diagnostic measurements, Radiat. Prot. Dosim. 47, 529-534

Received: 7 October 1999
Accepted: 25 October 1999

ŠIMPRAGA, M., M. RANOGAJEC-KOMOR, V. BUTKOVIC, D. CAPAK: Korištenje animalnog modela u odredivanju raspodjele doze zracenja u rendgenskoj dijagnostici. Vet. arhiv 69, 289-298, 1999.

SAZETAK

Kako broj rendgenoloških pretraga u veterinarskoj medicini stalno raste, potrebno je povesti odgovarajucu brigu o prevenciji i zaštiti od zracenja, a polazna osnova za to je tocno poznavanje primljene doze zracenja. Zato smo, kako bi povecali razinu prevencije i zaštite od zracenja u rendgenskoj dijagnostici u veterinarskoj medicini, nacinili istrazivanja s ciljem da utvrdimo animalni model kojim bi odredivali raspodjelu doze zracenja u tijelu pacijenta preciznije nego modelima koji se koriste u humanoj medicini. Istrazivanja su obavljena na dva eutanazirana psa pasmine njemacki ovcar. Za pokus smo odabrali najucestalije rendgenske dijagnosticke pretrage: snimanje trbuha u zenke i snimanje kukova u muzjaka. Za dozimetrijska mjerenja odabrali smo termoluminiscentni sustav (TL) na bazi TLD-700 detektora i kompjuterizirani citac TOLEDO 654. Odabrani sustav je prije korištenja testiran, te je nacinjena kalibracija homogenosti i odredena reproducibilnost doze zracenja. Na osnovi dobivenih rezultata mozemo zakljuciti da je opisani animalni model za utvrdivanje raspodjele doze zracenja pri rendgenskom dijagnostickom zracenju pouzdan, te da nam daje mogucnost procjenjivanja rizika od ionizirajuceg zracenja pri razlicitim rendgenskim dijagnostickim pretragama. Korištenjem ovog modela povecat cemo razinu prevencije i zaštite pacijenata u veterinarskoj medicini.

Kljucne rijeci: RTG dijagnostika, distribucija doze, animalni model, pas