# Estimating Specific Patient Organ Dose for Chest CT Examinations with Monte Carlo Method
## 4 Discussion  
As in other countries, the frequency of CT examinations in China is also increasing with time, and the chest scan accounts the largest proportion [31; 32]. The radiation risk has also aroused great concern, especially for the radiosensitive organs, such as breasts and lungs. Therefore, it is necessary to estimate the organ dose undergoing CT examinations as accurately as possible.  
As we know, the CTDI is just a simple robust indicator of CT scanner output, and not the patient dose [6]. Even though the \(CTDI_{vol}\) can be used to normalize organ doses from fixed tube current CT examinations, it is useless in TCM exams due to varying tube current, which is a function of patient attenuation and thus wholly patient-specific. Therefore, more accurate and practical methods of estimating organ doses in CT exams are still desirable.  
Organ doses were proved to be estimated using both \(CTDI_{vol}\) and \(WED\) with different models [15; 17]. In this study, there is also a good exponential fitting relationship between normalized organ doses and the \(WED\). The fitting curves of the heart and lung were similar, while the dose curves of the breast were quite different from them. The results were consistent with that of previous studies [14].  
The constant term of the organ dose model should be further studied by ethnicity. Compared with the other estimation model, the new fitting model calculated in this study was more consistent with the MC simulation results for the estimation of breast dose in Chinese patients. Although the dose to deep-seated organs does not vary greatly among different ethnicities, the difference in dose of superficial organs, such as breasts, between different ethnicities cannot be ignored. It might be due to differences in anatomical structure and the characteristics of physique between different populations. Therefore, it is necessary to study the structure of the Chinese breast and compare the differences between different populations. It is also valuable to establish a more accurate estimation model of Chinese organ doses through the retrospective studies of a larger number of patients considering different breast size and tissue composition. In addition to the influence from the patient size, part of the differences in the results may result from the difference of 120 kVp energy spectrum between the different scanners. However, the differences in energy spectrum between different scanners does not significantly affect the dose results [17].  
Although it may have the potential to provide a better estimate of organ dose for the Chinese population, this study still has many limitations and requires further exploration and concrete analysis. The developed and tested dose estimation model in this study has the capability to estimate organ dose more accurately for Chinese adults than the existing methods; it is limited to only three fully-irradiated organs and one type of CT examination, and the doses caused by the over-ranging were not investigated. Because the chest CT in this study can only provide a voxelized chest model for each patient, it is not capable of assessing doses to partially or indirectly irradiated organs, nor is it able to estimate an effective dose. Moreover, the patients in this retrospective study were selected from 567 patients in one hospital, but the range of body sizes was not complete. The overall average \(WED\) of women is smaller than that of men (\(p<0.001\)). Since this study was a preliminary verification and discussion, the number of the CT scanners and patients were limited. We have considered adding more patients and CT scanners for comparative analysis in the future. Since we carried out a retrospective study, another limitation of the study was that it refers to a single CT scanner model used at a specific kVp. Other kVp settings available in the CT scanners will be studied and discussed in the future.