Design and Shielding Considerations for an Image Guided Brachytherapy (IGBT) Suite
Abstract
Purpose
Image Guided Brachytherapy (IGBT) in a dedicated suite allows applicator placement, planning and treatment delivery in a single room, minimizing the patient movement during the process. The design and shielding considerations for the new IGBT suite are presented.
Methods
The suite is designed primarily for High-Dose-Rate (HDR) brachytherapy using an Ir-192 source integrated with a CT-on-rails imaging system. The shielding design constraints are 0.02 mSv/week (1 mSv/year) for uncontrolled areas and 0.1 mSv/week (5 mSv/year) for controlled areas. For a 12Ci source, Γ =4.69 R/(mCih) at 1 cm, fmed=0.96 cGy/R, t=10 hr/wk (40 patients/week* 15 min/patient*(1 hour/60 minutes), workload = 0.5 Gy/Wk. To maintain conservatism, the workload was increased by 30% to account for additional non-HDR contributions, including CT imaging and low-dose-rate procedures. The shielding calculations are provided for the points at a perpendicular distance from the isotopic Ir-192 source at the center of the suite. To be on the conservative side, a use factor of 1 and occupancy factors of 1 (control room, staff office) and 1/5 (janitor’s closet/staff toilet) are used.
Results
For points specified in the calculation, the wall shielding requirements with normal concrete are 38.3 cm (control room), 42.1 cm (staff offices), 38.3 cm (staff toilet), 31.8 cm (janitor’s closet) and 40.5 cm (duct point). The vault door is comprised of 2 inches of lead sandwiched between ¼ inches of steel and duct penetrations will be lead-lined to ensure equivalent safety measures. The walls and ceiling of the new vault will be 2 ft concrete (60.9 cm) which is more than adequate barrier per the NRC recommendations.
Conclusion
A strictly conservative approach has been presented for the planned IGBT suite shielding. The proposed vault construction provides substantial safety margins and ensures compliance with regulatory limits for both occupational workers and members of the public.