1. Goran Goran.kolarevic@medikol.hr, Poliklinika Medikol, Croatia
Computed tomography (CT) unit is also used as a CT simulator in radiation therapy (RT), primarily for planning external beam radiotherapy. From the perspective of radiation protection, diagnostic CT scanners and CT simulators have identical radiation characteristics. The emitted photon beam is filtered at an X-ray tube voltage of 140 kVp. After passing through the patient, the primary beam is almost completely absorbed and attenuated by the rotating detector array and gantry components. Therefore, only the scattered radiation component is considered when calculating the required thickness of shielding barriers. Shielding calculations were performed using the Dose Length Product (DLP) method, which is based on the product of the Computed Tomography Dose Index (CTDI) and the total scan length, together with scatter factors obtained from experimental measurements on different CT systems. The CT scanner was installed at the center of a bunker measuring 7.5 m toward the control room, 5.0 m toward the corridor and adjacent examination rooms, and 3.5 m in height. The weekly workload is 125 patients, including 75 abdominal examinations (DLP = 850 mGy•cm) and 50 head examinations (DLP = 900 mGy•cm).
The design dose constraints recommended by NCRP Report No. 147 are 5 mSv per year (0.1 mSv per week) for the control room and 1 mSv per year (0.02 mSv per week) for adjacent areas, assuming occupancy factors of 1 for examination rooms and 0.125 for the corridor. The air kerma due to scattered radiation at a distance of 1 m from the isocenter was assumed to be approximately 1 mGy per CT examination, corresponding to 125 mGy per week. Applying the inverse square law, air kerma values at the shielding barriers were determined and corrected for occupancy factors. Subsequently, the transmission factor (B) and the required number of tenth-value layers (nTVLs) were calculated, where one tenth-value layer (TVL) is defined as the thickness of shielding material required to reduce the photon beam intensity to one-tenth of its initial value. The results indicate that the required lead shielding thicknesses are 2.1 mm for the corridor, 3 mm for the examination rooms and ceiling, and 2 mm for the control room. The corresponding concrete shielding thicknesses are approximately 21 cm, 30 cm, and 20 cm, respectively. These findings confirm that CT facilities require thicker shielding barriers than other diagnostic X-ray installations. The effectiveness of the shielding is finally verified by radiation surveys performed on the outer surfaces of all shielding barriers, confirming compliance with the design dose constraints and identifying any potential shielding deficiencies.
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05.07.2026.
Contemporary Materials 2026 - Savremeni Materijali
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