# Estimating Specific Patient Organ Dose for Chest CT Examinations with Monte Carlo Method
###### Abstract  
Purpose: The purpose of this study was to preliminarily estimate patient-specific organ doses in chest CT examinations for Chinese adults, and to investigate the effect of patient size on organ doses. Methods: By considering the body-size and body-build effects on the organ doses and taking the mid-chest water equivalent diameter (_WED_) as a body-size indicator, the chest scan images of 18 Chinese adults were acquired on a multi-detector CT to generate the regional voxel models. For each patient, the lungs, heart, and breasts (glandular breast tissues for both breasts) were segmented, and other organs were semi-automated segmented based on their HU values. The CT scanner and patient models simulated by MCNPX 2.4.0 software (Los Alamos National LaboratoryLos Alamos, USA) were used to calculate lung, breast, and heart doses. \(CTDI_{\text{vol}}\) values were used to normalize simulated organ doses, and the exponential estimation model between the normalized organ dose and _WED_ was investigated. Results: Among the 18 patients in this study, the simulated doses of lung, heart, and breast were 18.15 \(\pm\) 2.69 mGy, 18.68 \(\pm\) 2.87 mGy, and 16.11 \(\pm\) 3.08 mGy, respectively. Larger patients received higher organ doses than smaller ones due to the higher tube current used. The ratios of lung, heart, and breast doses to the \(CTDI_{\text{vol}}\) were 1.48 \(\pm\) 0.22, 1.54 \(\pm\) 0.20, and 1.41 \(\pm\) 0.13, respectively. The normalized organ doses of all the three organs decreased with the increase in _WED_, and the normalized doses decreased more obviously in the lung and the heart than that in the breasts. Conclusions: The output of CT scanner under ATCM is positively related to the attenuation of patients, larger-size patients receive higher organ doses. The organ dose normalized by \(CTDI_{\text{vol}}\) was negatively correlated with patient size. The organ doses could be estimated by using the indicated \(CTDI_{\text{vol}}\) combined with the estimated _WED_. Monte Carlo simulations; organ dose; computed tomography; tube current modulation 18 Septemberdisplayed on a CT scanner. Eventually, the organ dose became the important indicator for radiation risk assessment, as suggested by the International Commission on Radiological Protection (ICRP), but the organ dose is not readily available in scan records or images [9]. Moreover, CT dose indicators, such as \(CTDI_{\text{Iod}}\) and DLP (i.e., quantities assessed in a standard water body phantom), do not take into account a patient's anatomy and composition, thus they do not provide information on an individual patient's organ doses. SSDE is considered to be the most straightforward approach that could be used to estimate patient dose from CT examinations [8]. However, SSDE could not represent radiation doses of a specific organ, and SSDE was primarily determined from fixed tube current (FTC) scans. Direct measurement of organ dose is limited by practical obstacles and ethical issues, and organ dose measurement using anthropomorphic phantom is both time and labor consuming. The current related research mainly relies on an MC simulation or organ dose estimation software [10; 11; 12]. Most of the above-mentioned methods for organ dose estimation are based on patient anatomy models representing average patient anatomy, called patient reference computational phantoms. These computational phantoms have tried to match the anatomy of patients who have different ages, genders, and body sizes, even including pregnancy. However, there are still gaps between computational phantoms and patient-specific anatomy. Moreover, most organ dose estimation software did not consider the individual tube current curve when the CT scan was performed with tube current modulation (TCM). Therefore, to improve the accuracy in the analysis of cancer risk after CT scans, further investigations of the doses administered to individual organs with patient-specific phantom and a corresponding tube current curve using CT images are preferable.