Attenuation Coefficients for Layered Ceiling and Floor Shields in PET/CT Clinics

INTRODUCTION Positron Emission Tomography (PET) is enjoying explosive growth due to its ability to accurately stage many types of cancer and follow the progress of treatments. The facilities present unique challenges in shielding design due to the nature of the study, and the desire to place the facilities near other imaging and treatment clinics. A typical PET patient will receive 555 MBq (15 mCi) of F labeled 2-Fluoro-2Deoxy-D-Glucose (FDG) and will rest in a darkened room (called a quiet room) for 45 minutes to allow the drug to localize in the lesions of interest. A normal clinic will have one to three quiet rooms. While in the quiet room, the patient will normally recline in a comfortable lounge chair. The distribution of the FDG in the patient will vary from patient to patient, but much of the isotope will be located in the brain and the bladder. After resting for approximately 45 minutes, the patient is asked to empty his or her bladder, and is placed on the scanner bed. Images are collected for 30 to 45 minutes on the PET or combined PET/CT unit. The patient is then released after the scanning is complete. Shielding design for PET/CT clinics is frequently difficult as many of the facilities are being retrofitted into existing imaging centers and hospitals where space is cramped. The patient quiet rooms, the hot laboratory, and the scanners are commonly positioned in close proximity to uncontrolled areas where non-occupational dose limits apply. Of particular concern are the ceiling and floor shields for the quiet and the scanner rooms because these are constructed by adding sheets of lead to the existing concrete ceiling and floor decks. Much of the cost of shielding in a PET/CT clinic is driven by the installation costs of these shields. The attenuation provided by these layered shields is difficult to calculate by point kernel methods. An underestimation of the shielding effectiveness will result in additional lead and higher cost.