Abstract
Positron Emission Tomography (PET) is considered as the fastest expanding diagnostic imaging system in the world, and has become one of the major tools in diagnosis and staging in oncology. Although all the European countries together do not have yet as many scanners as the USA, they have almost completed Phase One of the PET/Computer Tomography (CT) Services Plan. For example, the number of PET/CT scanners in England has increased by a factor of three during the last three years and the focus is now on the advanced PET/CT system. Combined PET and CT scanning is an advanced imaging modality that offers high patient throughput, but results in increased occupational radiation dose exposure. In addition, mobile PET/CT systems have been used to overcome space and money limitations. However, the working environment in a mobile scanner unit places particular constraints on space, patient handling and workflow. These factors stimulated us to investigate the occupational radiation exposure doses for a number of static and mobile PET and PET/CT services provided by NHS hospitals and private companies. The vital goal of this work was to determine staff doses in both static and mobile PET/CT environments and examine the influence of shielding and design by Monte-Carlo simulation. The PET/CT workflow for staff was divided into six phases that were given operational definitions, and start and end points. Using stopwatches, dose rate meters, electronic personal dosimeters (EPDs) made by Thermo and portable LaBr3 spectrometers and dosimeters made by Canberra, exposure time, dose rate, gamma ray spectrum and dose measurements were performed over a period of time varying between 1 day and 5 weeks by various technologists working in 3 static and 3 mobile PET/CT units. Measurements of exposure rate from more than 1250 patient procedures in total were collected and analysed. Experimental studies concluded that the occupational back ground was quite high (>3 muSv/8hours) in some locations and that injected patients were released with high dose rates exceeding 60 muSv/h in the close contact position (< 40 cm). In addition, in designs where the hot-lab room was located between the reception desk and control room without proper shielding, the exposure dose rate in the air exceeded 15 muSv/h at 120 cm from the wall of the hot-lab. The use of a portable LaBr3 detector was important because it has an excellent energy and timing resolution, superior intrinsic spatial resolution and high detection efficiency. Existing occupational doses recorded previously were analysed and provided information that was used in the experimental design of more detailed measurements. Although these results indicated that occupational background is very high in most units, all staff members currently working in Medical PET and PET/CT units are within the regulatory limits for non-classified occupational exposure (<6 mSv per year). However, many of the staff would exceed the classification level if the occupational background was not subtracted. The average exposure time for staff working in static and mobile environments was around 15 minutes and 25 minutes per patient respectively for all 6 defined workflow phases. The corresponding average exposure dose was 5.0 muSv (static) and 5.7 muSv (mobile) per patient. High exposure times and doses were seen to occur during the injection phase and the scanning phase, where staff members are in prolonged close contact with radioactive patients. In addition, on the mobile PET/CT unit, accompanying the patient to the toilet prior to the scan also incurred a high dose, in particular when patients needed additional assistance. It was found that at least 10% of the total dose was not attributable to any of the defined tasks in the workflow, and instead was attributed to unexpected occupational exposure as well as carelessness. Monte Carlo simulation was used to check the dose map inside the unit and to certify the shielding calculation and design. Also, the simulation was used to compare the staff effective dose per patient. Two types of tallies were used as a cross check and showed excellent agreement. Based on the findings of the experimental and numerical studies, a number of strategies for reducing occupational exposures in all workflow phases are suggested.