# Dose Estimation by Geant4-Based Simulations for Cone-Beam CT Applications: A Systematic Review
## 3 Results and Discussion
The dose delivered, in both standard CBCT scan mode and low-dose scan mode, was studied in different treatment sites as the head-and-neck, chest, abdomen, and pelvis regions [116]. The reduction in the radiation dose in the low-dose mode compared to the standard mode was about 20%. The average absorbed doses in standard mode doses of the organs in the head and neck and chest regions 40.9-82.8 mGy and 43.0-74.8 mGy for the abdomen and pelvis region, while, for low-dose, ranged 16.1-18.9 mGy and 7.9-18.5 mGy, respectively. The variation in organ doses between the different imaging protocols is a consequence of the employed imaging parameters. The average organ doses from 360\({}^{\circ}\) scans for the head, thorax, and pelvis protocols were calculated with values between 6-8 mGy, 15-17 mGy, and 24-54 mGy, respectively [98]. Absorbed doses to organs located inside the FOV ranged between 6-54 mGy per 360\({}^{\circ}\) scan translating into cumulative doses from repeated daily CBCT imaging of 0.2-1.6 Gy. Organ doses in IGRT in head-neck, thorax, and pelvis districts were also evaluated due to CBCT and therapeutic MV irradiation [101]. The results showed that the scattered radiation during therapy (MV range) is larger than that diffused by CBCT by one to two orders of magnitude; therefore, it allows to optimize the dose delivery, and it also does not cause excessive dose absorption to the organs at risk. Similarly, the imaging organ doses for the six different CBCT scanning protocols doses were calculated to range from 0.1-24.1 mGy for the adult phantom, and up to 36.8 mGy for the pediatric phantom in Reference [107]. In contrast, the imaging dose to the pediatric phantom in the chest region turns out to be 1.5-2 times higher than that of the adult phantom. For the female organs in the pelvic region, such as the ovary, uterus, and vagina, the imaging doses for the pediatric phantom were calculated to be 1.6-2.24 times higher than those for the adult phantom. These results may shed light on this issue, providing reference data for clinicians to make better decisions on the related clinical procedures, particularly for the pediatric patients. Furthermore, the feasibility of implementing a tube current modulation method to provide optimized dose reductions without degrading image quality has been successfully proven in a kV-CBCT system for IGRT [102]. On the other hand, breast cancer is becoming increasingly common among young women [128]. CBBCT improves breast cancer detection and characterization providing high-quality images with the potential to better visualize overlapping breast tissue compared to mammography or ultrasound [129; 130; 131]. However, there is a lack of standardized operation criteria in CBBCT [109]. The mean glandular dose using the CBBCT automatic tube current settings can vary from 5.5-17.5 mGy, depending on the breast size and composition [121]. The glandular dose distribution inside the breast can vary by up to 50% of the mean value, while, for a typical mammographic acquisition, the glandular dose distribution varies from 15-400% of the mean. Lower dose values can be achieved if lower tube current settings are selected instead of those automatically suggested by the system, although the image quality may be reduced, as well. [PERSON] et al. proposed that it is possible to selectively image a VOI with a reduced breast dose and scatter effect [122]. Furthermore, [PERSON] et al. [118] found that the more energetic beams provide a more uniform dose distribution than at low energy. Dose values within a PMMA phantom ranged from 0.28-0.46 mGy (at 50 kVp), and 0.44-0.60 mGy (at 80 kVp), while the 50 kVp beam presented an almost double coefficient of variation (ratio between the standard deviation and the mean of the distribution) than that for the 80 kVp beam. A recent study proposed a CBBCT system with an offset detector that estimates the normalized glandular dose coefficients with a potential dose reduction [90].