Accurate evaluation of bone invasion and the exact boundary of bone invasion by OSCC is important for precise mandible resection during surgery. CBCT, which has a high spatial resolution and a low radiation dose, has been widely used in the oral and dentomaxillofacial regions. Although CBCT could be used in bone invasion diagnosis, to date, no studies have investigated the accuracy of CBCT in the evaluation of the bone invasion boundary. In our study, we compared CBCT images and histological slices to explore the possibility of using CBCT to preoperatively evaluate bone invasion, the boundary of bone invasion and nerve invasion. We hope this study will provide evidence for using CBCT to evaluate bone invasion with the above questions.
Previous studies have compared the extent of bone invasion evaluated with panoramic radiographs or spiral CT images with that determined via histopathological examination . In a previous report, compared to that obtained in histological results, the bone invasion presented on panoramic radiographs was smaller (13 mm in width and 2 mm in depth); it was also smaller on spiral CT (5 mm in width and larger with 3 mm in depth). A systematic review compared several modalities in detecting mandibular invasion by OSCC, and the results showed that the sensitivity of bone invasion diagnosis for magnetic resonance imaging, CBCT, spiral CT and panoramic radiography was 94%, 91%, 83%, and 55% and the specificity was 100%, 100%, 97% and 91.7% for magnetic resonance imaging, CBCT, positron emission tomography/CT and panoramic radiography, respectively . Czerwonka et al. compared the diagnostic efficiency of CBCT with that of conventional spiral CT and found that the sensitivity and specificity were 91% and 60% for CBCT and 86% and 68% for spiral CT .
In our study, the accuracy of CBCT in the diagnosis of bone invasion was 100%, which was higher than that in previous studies. The high accuracy may be attributed to our study using in vitro samples. However, these results still demonstrate that CBCT is a reliable tool in the diagnosis of bone invasion. For the bone invasion boundary, our study revealed an average underestimation of 2.97 mm using CBCT compared with histological slices. Considering the relatively accurate assessment of the extent of bone invasion using CBCT, precise surgical guide plates may be used in the future. To avoid recurrence, enlarged resection may be needed based on preoperative evaluation using CBCT. Moreover, in this study, we found that CBCT could not predict inferior alveolar nerve invasion with high accuracy. Nerve invasion could not be detected directly due to the poor presentation of soft tissues on CBCT. Nerve invasion was determined by discontinuity of the mandibular nerve canal, and this is an indirect sign. For some OSCCs, the infiltrated tumor cells may have reached the nerve, but the mandibular nerve canal seems intact on CBCT images because the special resolution is only 0.25 mm.
In our study, the bone specimens exhibited significant linear changes during histopathological examination. During histological processing, tissue shrinkage occurs as a consequence of fixation and the subsequent serial dehydration and rehydration procedures . Buytaert and colleagues reported a bone volume shrinkage rate of 17% during tissue processing . 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 [12, 13]. Therefore, bone shrinkage should be considered in studies involving the sectioning of bone for histopathological examination. Our findings 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 were also utilized as markers to decrease the influence of shrinkage. As GP points were 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 has shortcomings. For example, 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 various 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 [14,15,16,17]. 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 patterns on CBCT. 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 due to histopathological processing 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 for the estimation of nerve invasion. The calculated underestimation of invasion was 2.97 mm on CBCT, which was lower than previously reported values. This suggests enormous potential for narrowing the extent of mandibulectomy for mandibular preservation.