# Dose Estimation by Geant4-Based Simulations for Cone-Beam CT Applications: A Systematic Review
###### Abstract  
The last two decades have witnessed increasing use of X-ray imaging and, hence, the exposure of humans to potentially harmful ionizing radiation. Computed tomography accounts for the largest portion of medically-related X-ray exposure. Accurate knowledge of ionizing radiation dose from Cone-Beam CT (CBCT) imaging is of great importance to estimate radiation risks and justification of imaging exposures. This work aimed to review the published evidence on CBCT dose estimation by focusing on studies that employ Geant4-based toolkits to estimate radiation dosage. A systematic review based on a scientometrics approach was conducted retrospectively, from January 2021, for a comprehensive overview of the trend, thematic focus, and scientific production in this topic. The search was conducted using WOS, PubMed, and Scopus databases, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. In total, 93 unique papers were found, of which only 34 met the inclusion criteria. We opine that the findings of this study provides a basis to develop accurate simulations of CBCT equipment for optimizing the trade-off between clinical benefit and radiation risk. one-beam computed tomography; monte carlo simulations; Geant4; radiation protection; radiation exposure; dosimetry; systematic-review; bibliometrics +
Footnote †: journal: Medical  
Although CBCT imaging dose is known to be smaller than typical CT scan, it cannot be neglected. Indeed, an overuse of CBCT procedures may increase the risk of the secondary side effects [43]. Lifetime attributable risks (LARs) of cancer incidence for several solid tumors have been classified by patient age and time of exposure [44]. Statistically significant dependence of relative risk (RR) of leukemia and brain cancer incidence following CT scans in childhood was first reported by [PERSON] et al. [45] showing a linear-no-threshold risk hypothesis [46]. LARs and RR of cancer incidence for various organs in standard- and low-dose CBCT modes from a single scan were analyzed showing that the highest LARs are for incidence of colon and bladder cancers, while the highest RRs are for incidence of stomach and liver cancers for the abdominal CBCT [47]. Therefore, accurate evaluation of the dose delivered from CBCT scans is crucial to provide physicians' awareness of radiation risks [48]. One of the most common applications of CBCT is image-guided procedures during proton therapy [49]. This technique improves the accuracy of radiation therapy but significantly contributes to an extra radiation dose to cancer patients, thus increasing the risk of second malignant neoplasms [50; 51] and other side-effects, as in Reference [52]. These effects are related to different factors as age at irradiation, type of irradiated tissue or volume, treatment technique, or previous radiological procedures [46; 53]. Besides, the potential risks of second cancers from scattered proton radiotherapy for childhood cancers is a cause of major concern [54]. Indeed, the use of low-dose mode to avoid unnecessary radiation exposure is recommended in the particular case of pediatric IGRT [47]. In response to the increasing awareness, large-scale efforts have been made to develop new methodologies for improving radiation protection standards [55; 56]. Monte Carlo (MC) simulations of the radiation transport and interaction are a powerful method for dose distributions calculations in complex geometries and the presence of extended spectra of multi-radiation sources [57; 58; 59; 60; 61; 62]. Not only can they accurately predict the distributions of absorbed doses but also other amounts of interest in radiation procedures, as well as the evaluation of the image quality [8]. During the last few years, MC techniques have played a major role in dosimetry; however, until its development, any comprehensive dosimetric study was thought to be impossible and dangerous.