CT Room Shielding Next to a Paediatric Ward
A 300-bed hospital is installing a 128-slice CT scanner in a room adjacent to a 24-hour paediatric ward and a staff corridor. The unit will operate 100 patients per day, six days per week, at 120 kVp and 250 mAs, with one rotation per scan. The contractor has proposed 2.0 mm lead-equivalent shielding in the shared wall and states that “standard CT rooms use 2 mm lead.” The paediatric ward is occupied continuously; the corridor is occupied intermittently by staff and visitors. The hospital has no medical physicist on staff and is preparing its DRAP/PNRA licensing file. The contractor wants approval to close the walls next week. A radiation survey is planned only after installation.
1. Should the hospital approve the contractor’s 2.0 mm lead proposal now? Identify the information missing from the proposal and the decision you would make.
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Do not approve the wall closure on the contractor’s statement. “Standard 2 mm lead” is not a shielding calculation. Shielding depends on workload, scatter fraction, distance, occupancy, and the dose constraint for the area beyond the wall. The proposal omits the weekly workload in mA·min, the distance from the isocentre to the occupied area, the occupancy factor for the paediatric ward and corridor, and the design goal in mSv/week. It also omits the contribution from the control room, door, penetrations, and ceiling/floor, which are common weak points.
At 100 patients/day and six days/week, the workload is substantial, and a continuously occupied paediatric ward is a high-occupancy area. The correct decision is to withhold approval until an independent, qualified medical physicist or radiation-protection expert produces a shielding plan using a recognised methodology and the hospital’s actual workload. The plan should specify lead equivalence for each barrier and for the door, and should state the assumptions. A post-installation survey is necessary but cannot substitute for design; discovering a deficit after the walls are closed is far more expensive and may halt clinical operation.
Approve only a conditional construction sequence: block work may proceed, but final closure and finishing wait for the shielding plan and a pre-closure verification of lead thickness.
2. What shielding design inputs and acceptance criteria would you require, and how would occupancy affect the result?
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Require the physicist to document: weekly workload in mA·min or patient workload, kVp and mAs assumptions, scatter fraction at 1 m, distance to each occupied point, occupancy factor, use factor, and the dose constraint for public and occupational areas. For a paediatric ward, occupancy should be treated as 1 because children are present continuously. For the corridor, a lower occupancy factor may be justified only if staff presence is genuinely intermittent and the corridor is not a waiting area. The design should use the more restrictive public constraint for the ward and a separate occupational constraint for controlled staff areas.
Acceptance criteria should be expressed as a design dose rate at the boundary, not only as lead thickness. Require calculation for the shared wall, door, control room, and penetrations, plus a pre-closure inspection of lead sheet overlap and fixing. After installation, a radiation survey should confirm that measured dose rates are below the design goal at each occupied point under worst-case operating conditions.
The occupancy judgement matters because doubling occupancy can require an additional half-value layer or more. If the paediatric ward is the critical receptor, it should drive the wall specification even if the corridor is closer. A 2.0 mm lead wall might be adequate for a low-workload room, but the contractor has not shown that it is adequate here.
3. How should the hospital handle licensing and the post-installation survey, given that it has no medical physicist?
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The hospital should contract an independent qualified expert, not rely on the CT vendor or contractor for the final radiation-protection opinion. The licensing file should include the room design, shielding calculation, equipment specifications, operator qualifications, quality-assurance programme, and emergency procedures. The post-installation survey should be performed before the first clinical patient and repeated after any change in workload or room layout.
The survey should measure dose rates at the paediatric ward wall, corridor, control room, door, and all penetrations under a realistic high-workload protocol. It should also check that the scanner’s indicated dose and the survey measurements are consistent, and that the technologist’s viewing window and interlock function correctly. If a hot spot is found, the hospital should not begin routine scanning until it is corrected; a temporary restriction on workload is safer than an undocumented exposure.
Given the absence of in-house physics support, the hospital should also establish a standing radiation-safety committee and a service contract for annual physics testing. The licensing authority will expect a named radiation-protection officer and records, not just a one-off survey. Budget for this before commissioning, because the cost of delaying a CT suite is usually greater than the physics fee.