Our research aimed mainly to evaluate the accuracy of CBCT for assessing the extent of mandibular invasion. We hypothesized that CBCT could help to narrow the area of surgical resection and increase the precision of mandibular resection. Meanwhile, we also studied the accuracy of CBCT for detecting nerve invasion, which we considered an experimental topic of interest. To date, no studies have investigated the accuracy of CBCT for predicting the extent of invasion. However, previous studies have compared invasion lengths and depths determined on orthopantomograms (OPG), bone scans, and CT images with those determined via histopathological examination.6 In a previous report, OPG underpredicted the width and depth of invasion by averages of 13 mm and 2 mm, whereas CT underpredicted the width of invasion by 5 mm and overpredicted the depth by 3 mm. Our study revealed an average underestimation of 2.97 mm by CBCT. In other words, CBCT provided a more accurate assessment of the extent of mandibular invasion.
CBCT has a high spatial resolution and reduces the radiation dose to patients; thus, it is useful for the prediction of mandibular invasion. Numerous studies have investigated the utilities of different modalities used to determine mandibular invasion. A systematic review by Rojas and colleagues in 2013 demonstrated that several modalities yielded highly accurate results when detecting mandibular invasion by OSCC, with sensitivity values of 94% for magnetic resonance imaging, 91% CBCT, 83% for CT, and 55% for panoramic radiography.7 Moreover, the authors of that review observed specificity values of 100% for magnetic resonance imaging, CBCT, and CT; 97% for positron emission tomography/CT; and 91.7% for panoramic radiography. In 2017, Czerwonka and colleagues compared CBCT with conventional CT and reported that the sensitivity and specificity values for CBCT were 91% and 60%, respectively, whereas those for CT with bone windows were 86% and 68%, respectively.8 These reports provide a basis for the use of CBCT in evaluations of tumor bone invasion. In our study, CBCT predicted bone invasion with 100% accuracy, a higher level than those reported in other studies. We suspect that this discrepancy was attributable to our use of in vitro samples, indicating that this method may increase the accuracy of CBCT. However, the results still prove that CBCT is a fairly accurate tool. In addition, the improved resolution of CBCT may allow us to make more precise surgical guides.
Notably, we found that CBCT could not predict inferior alveolar nerve invasion at a high level of accuracy. We attribute this finding to the lack of distinction between tumor invasion and the inferior alveolar neural tube in CBCT. However, more studies of CBCT are needed, and the evaluation of additional cases will enable the confirmation and verification of our results.
In our study, the bone specimens exhibited significant linear changes during histopathological examination. During histological processing, tissue shrinkage occurs as a consequence of the fixation and subsequent serial dehydration and rehydration procedures.9 Buytaert and colleagues reported a bone volume shrinkage rate of 17% during tissue processing.10 However, our study revealed more details of these changes, including shrinkage and enlargement. Previous reports have described the high significance of OSCC margin discrepancies after resection and specimen processing, as these might influence the adequacy of resection.11, 12 Therefore, bone shrinkage should be considered in studies involving the sectioning of bone for histopathological examination. Our finding may promote improvements in the accuracy of pathology-based research.
GP points played an important role in our research. The three GP points embedded in the samples not only enabled the pathologist and radiologist to focus on the same locations within samples, but also were utilized as markers to decrease the influence of shrinkage. As GP points are flexible and were inserted into the bottoms of the tissue holes, they could remain firmly in place until the specimen was sectioned. Accordingly, the GP points are superior to markers such as metallic pins, which shift easily during histopathology processing. Thus, GP points may be a very useful tool in imaging research. However, this method still has shortcomings. The pathological examination used 4-μm-thick sections, which were considerably thinner than the GP points. This defect could have led to errors in the merged images. Nevertheless, the differences between different planes that included GP points were very small. Although this technique is prone to error, it also yields substantial improvements.
As mentioned earlier, mandibular invasion by OSCC can be erosive or infiltrative.5, 13-16 The erosive pattern is characterized by a broad advancing boundary, with a well-defined interface between the tumor and the bone. Osteoclastic bone resorption and fibrosis are typically evident along the advancing boundary and support the absence of bone islands within the tumor mass. In contrast, the infiltrative pattern is characterized by nests and projections of tumor cells along an irregular advancing boundary, residual bone islands within the tumor, and haversian system penetration. The presence of features of both patterns suggests a mixed-pattern invasion. Unfortunately, we did not observe distinguishing features related to these invasive types on CBCT images. Therefore, the improvement of preoperative examination techniques remains a huge challenge.
The validation of medical imaging tools is an area of great clinical interest, and highly accurate coregistration between histopathological and radiological images in terms of the tumor boundaries can provide further clarity. The findings of this study suggest that researchers should consider bone shrinkage as a means of improving the accuracy of future bone studies. GP points can be utilized as markers to decrease the influence of shrinkage. Moreover, CBCT is a reliable and highly accurate method for predicting mandibular invasion, but is considerably less accurate in terms of estimating nerve invasion. The calculated underestimation of invasion of 2.97 mm by CBCT, which was lower than previously reported values, suggests an enormous potential for narrowing the extent of mandibulectomy for mandibular preservation.