Accurate evaluation of bone invasion and the exact boundary of bone invasion by OSCC are important to make a precise mandible resection during surgeries. CBCT, which has a high spatial resolution and low radiation dose, has been widely used in oral and dentomaxillofacial regions. Although CBCT could be used in the bone invasion diagnosis, to date, no studies have investigated the accuracy of CBCT in the evaluation of bone invasion boundary. In our study, we compared the CBCT images and the histological slices to explore the possibility of using CBCT to preoperatively evaluate the bone invasion, the boundary of bone invasion and nerve invasion. We hope this study will provide evidence of using CBCT to evaluate bone invasion about the above questions.
Previous studies have compared the extent of bone invasion evaluated with panoramic radiograph or spiral CT images with those determined via histopathological examination. In a previous report, compared to histological results, the bone invasion presented on panoramic radiograph was smaller with 13mm in width and 2 mm in depth; and was smaller with 5 mm in width and larger with 3 mm in depth on spiral CT. A systematic review compared several modalities in detecting mandibular invasion by OSCC, 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 diagnosis efficiency of CBCT with conventional spiral CT, they found that the sensitivity and specificity were 91% and 60% for CBCT and were 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 previous studies. The high accuracy may be attribute to our study used in vitro samples. However, these results still demonstrate that CBCT is a reliable tool in 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. And considering the relatively accurate assessment of the extent of bone invasion using CBCT, a precise surgical guide plates maybe could be used in future. And in order to avoid recurrences, an 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. The nerve invasion could not be detected directly due to the poor presentation of soft tissues of CBCT. Nerve invasion was determined by discontinuity of mandibular nerve canal, and this is an indirect sign. For some OSCC, the infiltrated tumor cells may have reached into the nerve, but the mandibular nerve canal seems intact on CBCT images due to the special resolution was only 0.25mm.
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 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 also were 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[14-17]. 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 estimation of nerve invasion. The calculated underestimation of invasion was 2.97 mm on CBCT, which was lower than previously reported values. This suggests an enormous potential for narrowing the extent of mandibulectomy for mandibular preservation.