# Dose Estimation by Geant4-Based Simulations for Cone-Beam CT Applications: A Systematic Review
## 3 Results and Discussion
We can distinguish 5 different intertwined nodes sharing the most common terms, but which, in turn, are grouped into more specific topics. The annual tendency show that around 2010 (blue-green colors) the articles were related to \"breast imaging\", \"mammography\", and \"image-quality\", while, around 2015 (green-yellow colors), the main topic was \"dosimetry\" involving \"mc simulations\" and \"radiotherapy\" and \"geant4\" by 2017. The latest articles (orange-red colors) are related to \"validation\", \"cancer\", and \"radiation dose'. Regarding Table 3, the most common application of Geant4-based toolkit are dedicated to breast imaging, such as CBCCT [120, 121, 122, 123, 124, 90, 110]. [PERSON] et al. found a potential for radiation dose reduction estimating the normalized glandular dose coefficients (\(\text{DgN}^{CT}\)) that provide air kerma-to-mean glandular dose conversion factors by GATE software, version 8 [90]. GATE was also used in Reference [109] simulating a multi-energy spectrum to radiate same-size built-in calcifications and lump breast motifs under the condition of the same number of particles, while References [120, 121, 122, 123, 110] investigate the feasibility of Geant4 codes to estimate the dose in CBBCT. The second most common application corresponds to IGRT [116, 117, 110]. In the first publication in 2013, [PERSON] et al. present a novel analytical method that can be employed in a fast treatment planning system for kV photon dose calculations in IGRT [117]. They validate their results by simulations performed with the Geant4 toolkit and found that dose calculation times can be significantly reduced in convolution-based algorithms. In 2018, [PERSON] et al. evaluated organ doses in IGRT due to CBCT and therapeutic MV irradiation in head-neck, thorax, and pelvis districts through GAMOS simulation toolkit [101]. After  
Figure 3: Co-occurrence network of keywords. The thicker line indicates a strong association between those keywords, while thinner lines depict weak association  
verification by comparing with experimental quality indexes, they found that scattered radiation in therapy was one to two orders of magnitude larger than that diffused by CBCT. GAMOS toolkit was also used for a deeper understanding of late toxicity risk in external beam radiotherapy patients by simulating dose depositions integrated from different sources over the entire treatment plan in the reference [113]. Besides, using GATE simulation tool, dose deposited during MV-CBCT procedure was simulated and compared to measurements obtained with dedicated detectors in References [104; 105], being able to reconstruct imaged volumes and to estimate doses deposited by simple and complex irradiation fields. GATE was also employed by [PERSON] et al. to investigate the imaging radiation dose delivered to radiosensitive organs from IGRT [116] and the imaging radiation doses resulting from six different acquisition protocols for the adult and pediatric numerical XCAT phantoms [107]. In the first article, they calculated the absorbed doses of head-and-neck, chest, abdomen, and pelvis regions, in both standard CBCT scan mode (125 kVp, 80 mA, and 25 ms) and low-dose scan mode (125 kVp, 40 mA, and 10 ms). They found that the reduction in the radiation dose in the low-dose mode compared to the standard mode was about 20%. In the second article, they provide a platform for developing a new kV-CBCT scanning protocol in conjunction with a low-dose capability. A few years later, the same first author successfully demonstrated the feasibility of incorporating a tube current modulation method in a kV-CBCT system for IGRT to optimized dose reductions without degrading image quality [116]. Thus, the results from these studies would significantly facilitate decisions regarding the administration of extra imaging doses to radiosensitive organs and may reduce the occurrence probability of adverse effects on health derived from radiation exposure.