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Monte Carlo-Calculated Patient Organ Doses From a Diagnostic X-Ray CT

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T Ono

T Ono*, F Araki,

SU-E-I-46 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To calculate patient organ doses from a diagnostic X-ray CT, using Monte Carlo (MC) simulations.
Methods:A MDCT (Aquilion 16, TOSHIBA Medical Systems) was modeled with the GMctdospp (IMPS, Germany) software based on the EGSnrc MC code. The X-ray spectrum and configuration of a bowtie filter (Teflon) were determined from a half value layer (HVL) of aluminum (Al) and dose profile (off-center ratio, OCR) measured in air under the body irradiation (120 kV). The MC-calculated dose was calibrated with the absorbed dose-to-acrylic obtained from the chamber measurement at a central axis in a CT water phantom with 30 cm diameter and 50 cm length. The dose distribution in patient was calculated using the CT images on beam setting for pelvis. The internal organ doses in pelvis were calculated from DVH.
Results:The calculated Al HVL was in agreement with 0.3% to the value of 5.2 mm measured. The calculated profile in air matched to that measured within 5% in a range of 15 cm from the central axis. The calibrated CT absorbed dose-to-acrylic was in agreement within 5% with the chamber measurement at peripheral four points. The calculated mean doses for prostate, rectum, and bladder were 2.38 cGy, 2.40 cGy, and 2.84 cGy, respectively, under irradiation condition of 120 kV, 200 mA, and slice thickness of 2 mm. For bones of femur and pelvis, mean doses were 5.61 cGy and 6.36 cGy, respectively. The doses for bone increased by up to 2-3 times compared to those for soft tissue. The CT dose also became much higher near body surface. Prostate and rectum were exposed uniformly. Meanwhile, bones of femur and pelvis were non-uniformity depending on their density.
Conclusion:Monte Carlo calculations are useful for evaluation of patient organ doses from a diagnostic X-ray CT.

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