Measurements of external radiation of staff during and after bone scintigraphy in horses and evaluation of generated waste contamination

Transcription

1 Measurements of external radiation of staff during and after bone scintigraphy in horses and evaluation of generated waste contamination D. DIDIERLAURENT*, F. AUDIGIÉ and J.M. DENOIX CIRALE (Centre d Imagerie et de Recherches sur les Affections Locomotrices Equines) RN Goustranville, France * Corresponding author : ddidierlaurent@vet-alfort.fr SUMMARY Bone scintigraphy is a valuable technique in the diagnosis of lameness in horses. Dosimetry and measurements of waste contamination have been done to determine radiation hazards during bone scintigraphy exams in horses. For a single examination of an adult horse (500 kg bw) requiring GBq of technetium, dose rates have been detected according to different situations during the exam. In order to reduce radiation exposure of staff members, it is advisable to perform complementary imaging examination (radiography, ultrasonography) 24 hours after radiopharmaceutical injection, and to clean the stall 4 days after this injection. Keywords : bone scintigraphy - horse - technetium - safety. RÉSUMÉ Mesures de l exposition du personnel aux radiations pendant et après une scintigraphie osseuse chez le cheval et évaluation du risque dans l environnement. Par D. DIDIERLAURENT, F. AUDIGIÉ et J.M. DENOIX. La scintigraphie osseuse est une technique intéressante dans le diagnostic des pathologies locomotrices chez le cheval. Une dosimétrie et des mesures de contamination ont été réalisées pour déterminer l exposition aux rayonnements ionisants pendant une scintigraphie osseuse. Des débits d équivalents de dose ont été détectés selon les différentes situations rencontrées lors de l examen d un cheval adulte de 500 kg (injection de 5,027 GBq de diphosphonate marqué au technétium). Dans le but de réduire l exposition du personnel, il est recommandé de réaliser les examens complémentaires d imagerie (radiographie, échographie) 24 heures après l injection du radiopharmaceutique, et de curer les boxes 4 jours après cette injection. Mots-clés : scintigraphie osseuse - cheval - technétium - radioprotection. Introduction The purpose of this study was to determine the dose rate according to different situations during a bone scintigraphy in a 5 years french trotter (gelding kg body weight (bw) - height at the withers : 165 cm) injected with technetium (10 MBq.kg -1 ) associated to 3,3-diphosphono-1,2-propanedicarboxylique (DPD). In humans DPD was shown to present a urinary excretion lower than MPD (Methylene diphosphonate) and a better bone accumulation than MPD [2]. No reference is available in horses with this radiotracer. Therefore a radiation dosimetry was made to evaluate the time period required to wait before doing complementary imaging examination (radiography, ultrasonography), before returning the horse to the owner without radiation hazard, and before cleaning the stall. The duration of confinement of the horse in the center will be determined so that there will be no more detectable exposure rate for the public. Finally, the time period required to wait before cleaning the stall will be determined when no more contamination of biological waste can be detected, keeping in mind the ALARA (As Low As Reasonably Achievable) principle. Materials and methods A horse (500 kg bw) was injected with GBq (measured with a well counter - gas ionization chamber MEDI 404*) of technetium associated with DPD. Several measurements were performed. Gamma spectrometries with ECAM** gamma-camera without collimator were performed during the soft tissue phase (10 min after radiopharmaceutic injection), and during the bone phase (3 h after radiopharmaceutic injection) in the examination room. Measurements of environmental dose rates (EDR) (Hp 10) in µsv.h -1 during preparation of radiopharmaceutic behind leaded protections as shielded glove box (2.5 mm of lead), tungsten syringe shield (3 mm), protective floor screens (2 mm of lead), were done with a gas ionizing chamber (91 babyline***) and semi-conductor silicium detector (electronic dosimeter: Dosicard****) in contact with the horse during 3 days (hospitalization time). The detectors were in contact with the horse on the halter, proximolateral part of the left metatarsus, dorsal part of last left rib (region of the kidney) and medial aspect of the left stifle (region of the bladder). Measurements of activity of physiological fluid (60 ml)

2 312 DIDIERLAURENT (D.) AND COLLABORATORS and estimation of the activity ratio (Activity ratio = Total activity estimated / Total activity injected with Total activity estimated = Sample activity x Total physiological volume / Sample volume) were performed with a gas ionization chamber (well counter MEDI 404*). Blood samples were taken 30 min, 1h30, 3h, 4h, 5h, and 24h after radiopharmaceutic injection; the activity ratios were estimated assuming a total blood volume of 40 L. Similar calculations were performed for 60 ml urine sample taken 2h and 24h after radiopharmaceutic injection, assuming a total volume of 3 L for miction. These measurements give informations about biological period of radiotracer. Measurements of biological waste contamination were performed with a gamma-camera (acquisition time: 60 s, matrix: 128x128 pixels), and with a 91 babyline** in contact with a bucket of urine excreted 2 hours after injection. Measurements of shavings contamination 48h after radiopharmaceutic injection with the gamma-camera were also performed. These measurements allow the establishment of the time period required to wait before cleaning the stall. Results Gamma spectrum as measured 10 minutes and 3 hours after radiopharmaceutic injection are presented in Fig. 1. EDR (Hp 10) measured in contact with a 91 babyline are below 1 µsv.h -1 behind leaded protections (shielded glove box, tungsten syringe shield, protective floor screens...) even in the laboratory. The EDR during radiopharmaceutic injection is between 10 and 35 µsv.h -1 when the operators are within 1 meter of the horse. The EDR, within a 3 m radius, three hours after injection is between 1.5 and 8 µsv.h -1. Results obtained with semi-conductor silicium detectors in contact with the horse are presented in Table I. Measurements of physiological fluids activity are presented in Table II and Table III. Measurements of biological waste contamination performed on a urine bucket emitted 2 hours after radiopharmaceutic injection are illustrated in figure 2. The measurements performed on a container of shavings collected 48 hours after radiopharmaceutic injection are illustrated in figure 3. Discussion Spectrometries performed during soft tissue and bone phases are relatively similar. The photopeak over 140 kev and a large amount of scattered radiation between 40 and 125 kev was detected. These results showed the need of an electronic dosimetry with an adequate response between 40 and 140 kev, for the staff working close to the horse. The EDR (Hp 10) in the laboratory were very weak using scintigraphic protective equipments, i.e. shielded glove box, tungsten syringe shield, protective floor screen. They were higher during radiopharmaceutic injection and soft tissue phase (up to 200 µsv.h -1 in contact of regions such as kidneys). During the second phase of the exam (3 hours later), FIGURE 1. Gamma spectrum collected in the exam room. The rectangular area represent the gamma measured to perform a scintigram (centered on 140 kev +/-15%). * MEDI Linearity ± 1% between 1MBq and 200 GBq for γ between 35 kev and 3 MeV - Medisystem - Guyancourt - France ** Siemens AG Erlangen - Germany. *** Babyline 91 - Linearity < ± 10% between 1 and 10 µsv.h -1, < ± 7% between 10 and 100 µsv.h-1 - Eurysis Mesures - Saint Quentin - France **** Dosicard - Linearity < ± 15% between 60 and 1.25 MeV, < ±30% between 50 and 2 MeV - Eurysis Mesures - Saint Quentin - France

3 EXTERNAL RADIATION OF STAFF DURING AND AFTER BONE SCINTIGRAPHY IN HORSES 313 TABLE I. Dose equivalents debit (Hp 10) in µsv.h -1 and dose equivalents in µsv (dose equivalents are represented in bold) measured with dosicards placed on the horse. The dose equivalents debit are indicated with uncertainty of 15%. TABLE III. Activity measure (MBq) of a urine sample (60 ml) taken on a 3 L urine bucket. TABLE II. Activity measures of blood sample (60 ml) in MBq. dose rates were between 49 and 90 µsv.h -1. The dose rates measured the next day with semi-conductor silicium detectors on the horse showed debits between 2 to 7 µsv.h -1 depending on the region [3]. Therefore, the duration of 2 days hospitalization was necessary to reach zero values in contact with thoraco-lumbar region (kidneys), bladder, hindlimb and the head of the horse. Activity measurements of physiological fluids showed a quick decrease of the technetium blood concentration during the first 3 hours : technetium osseous fixation and kidneys elimination was very high at this stage. Therefore, radionuclide blood concentration disappeared quickly. Results showed an activity ratio of urinary concentration of approximately 10% (Table III) and a rapid decay of the radioelement in the sample. Two different biological periods were identified. The first one is very short and takes place during radiopharmaceutic bone accumulation (up to 2 hours after injection). During the second one, the radiopharmaceutic blood and urinary activity is low and reach zero 24 hours after injection as soon as the radiopharmaceutic is attached to the hydroxyapatite crystals on bone [2]. The law of radioactive decrease confirmed measurements of biological waste contamination. The regions of interest on the urine bucket showed that the activity decreased with physical half-life of technetium (6h) and measurements reached background noise 4 days later. The dose rates were 120 µsv.h -1 during the collection of urine 2 hours after injection, and 40 µsv.h -1 8 hours later. On shavings collected in the stall 48 hours after injection, the camera detected 2174 counts just after the collection, and reached background level 2 days later. Conclusion The different measurements showed that the dose rates were high during radiopharmaceutic injection and soft tissue phase. The basic radioprotection established permitted to limit dose equivalent, i.e. staying at distance from the injected horse; limiting the radiation time [4], avoiding the stall during the hours following radiopharmaceutic injection, performing maximum acquisitions without moving the horse;

4 314 DIDIERLAURENT (D.) AND COLLABORATORS FIGURE 2. Scintigraphic images of a urine bucket produced 2h after injection of radiopharmaceutic. Images are presented in chronological order : the date and time of images are indicated at the right bottom of each image. The last image represents background level detected by the camera (natural radiation). FIGURE 3. Scintigraphic images of a container of shavings collected 48 hours after radiopharmaceutic injection. Images are presented in chronological order : the date and time of images are indicated at the right bottom of each image. The third image represents background level detected by the camera (natural radiation).

5 EXTERNAL RADIATION OF STAFF DURING AND AFTER BONE SCINTIGRAPHY IN HORSES 315 immobilizing the horse with an appropriate tranquillizer; using leaded protections (lead aprons, shielded glove box, shielded syringe, protective floor screen...) to reduce average photon energy to personnel. The spectrum measurements demonstrated the need of using an electronic dosimeter that is capable of detection between 40 and 140 kev for workers monitoring. Twenty four hours after injection during further examination (radiography, ultrasonography), all the values detected at the surface of the horse were below 10 µsv.h -1. For hospitalization of horses, a two-day duration is sufficient to eliminate all hazards of external radiation of the public. Stall contamination came from the radioactive urine produced during the immediate hours after injection [2] as blood and urinary technetium concentration decreased very rapidly. The biologic period of the radiotracer was very short in the first 2 hours due to its renal clearance and then became longer due to its bone fixation. A period of 4 days is necessary to considerably limit the radiation caused by contaminated urine. Shavings contamination by faeces seems to be negligible in this study. To limit the hazard of contamination during exam and during the horse s hospitalization, latex gloves and clothes especially used for scintigraphy and overshoes are indispensable. References 1. DEVOUS M.D., TWARDOCK A.R. : Techniques and applications of nuclear medicine in the diagnosis of equine lameness. J. Am. Vet. Med. Ass., 1984, 184, SCHROTH H.J., HAUSINGER F., GARTH H., OBERHAUSEN E. : Comparison of the kinetics methylene-diphosphonate (MDP) and dicarboxypropan-diphosphonic acid (DPD), two radio-diagnostics for bone scintigraphy. Eur. J. Nucl. Med., 1984, 9, WHITELOCK RG. : Radiation hazard from horses undergoing scintigraphy using technetium-99m. Equine vet. J., 1997, 29, NEUWIRTH L, ROMINE C. : Ancillary equipment to increase and reduce radiation exposure in the equine nuclear medicine laboratory. Vet. Rad. & ultras., 2000, 5,