## 4 Discussion
These errors are usually rare under ideal conditions and may not have a significant impact on the overall error measure, especially if error measures such as the MAD are used. Although the percentages were low, the figures and tables of some studies showed a certain percentage of measurement errors [9; 10; 17; 32]. Radiologists using automated BAA, which can completely replace experts, tend to change the bone age value very rarely: 40% of users never change it, while 43% change less than 5% of the cases [33]. If bone age assessment is taken over by automated methods, there is a risk for large measurement errors, even when used under supervision. These types of measurement errors can be filtered out by obtaining identical results from repeated measurements [19], overcoming the effects of slightly different hand positions and the effects of using different X-ray equipment with different settings. Since it is unethical to repeatedly expose the same children to X-rays, previous studies have used retrospective longitudinal X-ray series taken, for example, at 1-year intervals, to establish a level of precision [19; 31]. Another study by [PERSON] et al. used differences between the left and right hands of normal children [34]. They found the upper limit of its between-image precision error to be 0.18 years, which was in good agreement with previous studies concerned with the BAA rating precision [19; 31]. To the best of our knowledge, our study is the first to compare the automated BAA results of both hands in various groups of patients visiting a tertiary hospital. Recently upgraded with a self-validation mechanism, BoneXpert rejects the images if the bone age deviates from the average bone age determined by tubular bones [32]. Even when using a self-validation mechanism, the method failed to reject cases where wrong bone ages were assigned, with 1.5% of patients showing an absolute difference between the automated method and manual assessment exceeding 1.8 years. Considering the self-validation mechanism proposed by BoneXpert, an automated BAA method approaching only the holistic hand image may have limitations in filtering out large measurement errors. Our hybrid model addressed the limitations of GP and TW3 by focusing on regions highly pertinent to changes in bone maturity and employing more refined maturity stages than TW3, leading to a robust and precise bone age estimation [11]. Our model integrated bone age assessments using detailed ROIs alongside holistic images. In comparison, the current commercial automated BAA system, BoneXpert version 3.0 (launched September 2019), evaluates bone age using both GP and alternative TW2 methods [31], similar to our approach. BoneXpert utilizes a feature extraction technique that reconstructs the border of 15 bones, including metacarpals, phalangeal bones, distal radius, and ulna [35], distinguishing it from our model. The model has been upgraded to improve user convenience and accuracy, including data augmentation techniques and the automatic hand bone alignment technique. Our study had a few limitations. First, the reference standard was based on the GP atlas, which was inherently limited to children. Differences in the nutritional and ethnic makeup of children in the 1930s and 1940s were used to generate standards. Additionally, although previous studies have reported racial differences among growth patterns in certain age groups [36; 37], cases were limited to patients of a single ethnicity. Finally, it would have been better to compare each ROI bone age of both hands, considering that skeletal maturation can vary within the same age. Our study showed an excellent overall correlation of BAAs between both hands using the model and provided the possibility of using the right-hand BAA as a validation tool. Perceptible differences between each hand may indicate a large measurement error and thus may be a signal for manual supervision. Conceptualization, S.O. and C.H.K. ; formal analysis, S.O. and K.-C.L. ; investigation, S.O. and K.-C.L. ; methodology, S.O. and J.J.L.