Shielding a New CT Room Adjacent to an Office
A hospital is installing a 128-slice CT scanner in a room adjacent to a staff office on one side and a public corridor on the other. The scanner operates at 120 kVp, 400 mA, with a workload of 60,000 mA-min/week and a use factor of 1 for the floor and 0.25 for the walls. The office is occupied 40 h/week by 6 staff. The corridor is occupied by the public. The physicist proposes 2.0 mm Pb equivalent in the office wall and 1.5 mm Pb in the corridor wall. The state requires 0.1 mSv/week for controlled areas and 0.02 mSv/week for uncontrolled areas. The CT scatter fraction at 1 m at 120 kVp is approximately 0.003 of the primary beam, and the primary beam is always intercepted by the gantry.
1. Evaluate whether the proposed barriers meet the weekly dose limits.
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For CT, the primary beam is intercepted by the gantry, so only scatter is considered. Scatter dose at 1 m per mA-min is approximately the scatter fraction times the primary output. Using a typical CT output of ~0.1 mGy/mA-min at 1 m at 120 kVp, scatter at 1 m ≈ 0.003 × 0.1 = 3 × 10⁻⁴ mGy/mA-min. Weekly scatter at 1 m = 60,000 × 3 × 10⁻⁴ = 18 mGy/week. Apply use factor and occupancy: office wall U = 0.25, T = 1 (40 h occupied), distance d (say 3 m) gives 18 × 0.25 / 3² = 0.5 mGy/week unshielded. To reach 0.1 mGy/week requires a transmission of 0.2, which for 120 kVp is roughly 2–3 mm Pb depending on the attenuation curve - 2.0 mm Pb is marginal and should be verified against the actual attenuation data. The corridor: U = 0.25, T = 0.25 (public, 10 h/week), d = 4 m gives 18 × 0.25 × 0.25 / 16 = 0.07 mGy/week; to reach 0.02 mGy/week requires transmission ~0.28, which 1.5 mm Pb may not achieve. The physicist should use the manufacturer's actual output and scatter data and the correct attenuation curves, not a rule of thumb.
2. What additional considerations should the RSO raise before construction?
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(1) Use the actual scanner output (mGy/mA-min) and the manufacturer's scatter fraction, not generic values. (2) Confirm occupancy factors with the facility - the office may be occupied more than 40 h/week, and the corridor T may be higher if it is a waiting area. (3) Account for the control room and door penetrations; the door may govern the design. (4) Consider secondary radiation (leakage) in addition to scatter. (5) Verify the barrier with a post-construction survey and compare to the design goal. (6) If the office is a controlled area, 0.1 mSv/week applies, but staff should still be monitored if they could exceed 1 mSv/y. (7) Document the shielding calculation and retain it.