The dramatic increase in the use of PET/CT and SPECT/CT techniques has given rise to concern regarding the level of radiation exposure of both healthcare staff and patients. The radionuclides used in PET techniques are generally generated in cyclotrons where the use of appropriate shielding techniques and procedures can successfully limit the risk of radiation. For the patient, a judicious choice of the parameters used in the CT scan itself can significantly reduce radiation dose, without any compromise in the quality of the diagnostic information generated exposure.
by Dr Pat Zanzonico
The use of positron emission tomography (PET) and computed tomography (CT) has grown dramatically. Over the period 1985 to 2005, the annual number of nuclear medicine procedures carried out in the USA increased three-fold (to 20 million) whilethe number of CT procedures increased twenty-fold (to 60 million) [1]. Moreover in 2005, more than 1,700 PET and PET-CT scanners were in use in the USA and another 500 - essentially all PET-CT units - were sold. More than 1.3 million patients underwent PET and PET-CT exams [2]. Fluorine-18 (used to radiolabel fluoro-deoxyglucose (FDG)) and other PET isotopes (e.g. carbon-11, nitrogen-13, and oxygen-15) are short-lived and produced almost exclusively in cyclotrons. The expanding role of PET has thus necessitated the widespread distribution of cyclotrons. Combined single-photon emission computed tomography (SPECT) and CT have also advanced clinically [3].
The cyclotron remains the most widely used accelerator for producing radionuclides, with two-thirds of the 230 cyclotrons worldwide being associated with compact medical devices [4]. Neutrons and gamma-rays can activate (i.e. produce radioactivity in) the various cyclotron components, contributing to external exposure of relevant personnel; air-borne activation products may also be inhaled causing internal exposure [5]. In “negative-ion” cyclotrons, the beam extraction efficiency approaches 100% and such activation is minimised. Composite shielding materials and automated radionuclide handling (isolation from the target and radiopharmaceutical preparation) have contributed to an excellent radiation safety record in cyclotron facilities. Radiation doses to personnel working in such a facility are typically well below the regulatory limits - 5,000 mrem per year for occupationally exposed individuals and even below ALARA (as low as reasonably achievable) design goals (500 mrem or 1/10th of the regulatory limit) [6]. Following a radioassay in a dose calibrator, the radiopharmaceutical syringe is placed in a shield and transported in a lead carrier to the patient injection area. The contact exposure rates can be substantial - from 50 milliroentgen per hour (mR/h) at the plunger to 500 mR/h at the needle [7] - and can result in significant finger/hand exposures when manually handling the syringe. The radiopharmaceutical is injected via a venous catheter. For 10-15 mCi of 18F-FDG, the finger doses can be as high as 3 mrem per patient procedure [8], with whole-body doses up to 1.5 mrem per procedure, 5 mrem per day, and 1,000 mrem per year; these can be reduced by to 80% with experience [9].
FDG and other short-lived PET radiopharmaceuticals will be imaged within one to two hours of injection. To minimise potentially variable muscle uptake of 18F-FDG, the patient should remain sedentary between injection and imaging. Thus, an “uptake room” is required. The design of uptake rooms is often challenging because of space limitations, and shielding is generally required to maintain doses in the adjoining areas below regulatory limits. Uptake rooms are not required for the longer-lived SPECT radiopharmaceuticals; patients generally either remain in the waiting room if scanning is to be performed the same day, or return home if scanning is to be performed on a later day. The SPECT or PET scanner room will require shielding both because of activity in the patient as well as scattered X-rays from the CT.
The shielding of SPECT-CT and PET-CT facilities presents special challenges because of the diverse radiation and sources involved [10]. Because the 0.511-MeV annihilation photons are more penetrating than lower-energy diagnostic radiation, shielding may be required in floors, ceilings and walls. Among the factors to consider are:
• the radionuclide and its physical properties (ie half-life and the abundances and energies of emitted radiations);
• the administered activity;
• radiopharmaceutical pharmacokinetics (ie uptake and excretion);
• scan duration;
• CT scan parameters;
• the workload, that is, the number of patients scanned per week;
• any existing structural and instrument shielding;
• the occupancy factor, that is, the fraction of time a point of interest (or “reference point”) is occupied by staff, the general public or other exposed cohorts;
• the point-of-interest distance, that is, the distance from the radiation source to the reference point;
• and the shielding design-goal dose limit for the point of interest.
Design-goal limits vary, depending on the cohort specified. For occupationally exposed individuals, the Maximum Permissible Dose (MPD) is 5,000 mrem per year [6]; this is also the MPD at a point of interest within a restricted or a controlled area (an area to which access is limited to protect individuals against risk of radiation exposure). For the general public, the MPD is 100 mrem per year [6]; this is likewise the MPD for a person at a reference point within an unrestricted area (an area to which access is neither limited nor controlled). While the 100-mrem per year or 2 mrem per week MPD is generally a reasonable shielding design goal for unrestricted areas, 500-mrem per year or 10 mrem per week (1/10th of the 5,000-mrem per year MPD) is commonly used as the design goal for restricted or controlled areas [11].
The National Council on Radiation Protection and Measurements (NCRP) has reviewed CT and other X-ray shielding requirements. The Task Group 108 of the American Association of Physicists in Medicine (AAPM) has likewise reviewed PET and PET-CT shielding requirements [10]; this review is general and adaptable to SPECT and SPECT-CT as well. Case studies in the literature present useful shielding-calculation examples when designing specific facilities [12]. Regardless of modality, the objective of a shielding calculation is to determine the thickness of the barrier (lead shielding) that is sufficient to reduce the dose at the designated reference point in an occupied area to a value less than or equal to the weekly shielding design goal. The required barrier thickness thus corresponds to the value of the broad-beam transmission function B(x) (the ratio of the dose behind a barrier of thickness x to the dose at the same location with no intervening barrier) yielding the design-goal dose limit. In CT shielding calculations, only scattered and leakage radiation are considered, since the primary X-ray beam is attenuated to much lower levels by the detector array and gantry hardware [10]. In any case, shielding calculations must be performed on a case-by-case basis by a qualified health or medical physicist.
There has been growing concern regarding the radiation dose associated with CT studies, particularly in paediatrics [1]. Accordingly, when the CT component of a PET-CT study is used for attenuation correction and anatomic localisation rather than diagnosis, the CT scan parameters should be adjusted to reduce the CT dose. In an adult, a diagnostic-quality CT delivers a dose as high as 2.2 rem, and an FDG (1.1 rem) PET-CT, a total as high as 3.3 rem [13]. However, a coarser-resolution attenuation-correction (AC) CT scan delivers a dose of only 0.60 rem. For children, the AC CT dose is only 0.1 rem [14].
Clearly, an appropriate design and workflow can maintain staff exposures well below regulatory limits in cyclotron, SPECT-CT and PET-CT facilities. A key component of the design of such facilities is shielding, with up to 12mm lead shielding for PET-CT scanner rooms and up to 18mm lead for uptake rooms (versus only 3mm lead or less for a CT scanner alone); SPECT-CT facilities will generally not require shielding beyond that dictated by the CT scanner. For SPECT- or PET-CT, judicious selection of the CT parameters can reduce the overall patient dose by over 50% without compromising diagnostic information.
References
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11. NCRP. Structural Shielding Design for Medical X-ray Imaging Facilities. National Council on Radiation Protection and Measurements (NCRP) Report No 147. Bethesda, MD: National Council on Radiation Protection and Measurements (NCRP); 2004.
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The author
Pat Zanzonico, PhD,
Memorial Sloan-Kettering Cancer Center
1275 York Avenue,
New York, NY 10021,
USA
