## 4 Discussion  
PAN showed excellent diagnostic accuracy in detecting deep caries, root remnants, and stage III periodontal disease, whereas it was not so accurate in the identification of AP lesions. In our opinion, the perfect accuracy of PAN in the evaluation of deep caries, root remnants, and stage III periodontal diseases was due to the advanced stage of the four pathological conditions that required remediation or extraction prior to radiotherapy. The loss of more than half of the dentine thickness in deep caries, loss of at least 75% of the crown in root remnants, and loss of bone to at least the middle third of the root in stage III periodontal diseases were all easily detected with PAN. Generally, the accuracy of PAN for the identification of AP lesions depends on the anatomic area, cortical bone involvement, and lesion size [19]. The air within the maxillary sinus, the presence of numerous roots infrequently orthogonal to the X-ray beam, the undulating morphology of the maxillary sinus floor, and the anterior portion of the zygomatic arch superimposed on dentoalveolar structures are all possible obstacles to  
Figure 3: An example of false-negative apical periodontitis. (**B**) Upper jaw MSCT imaging showing apical periodontitis affecting the left second molar. (**A**) In PAN, no periapical bone lesion was detected at the level of the periapex of the left second molar. Figure 2: An example of false-positive apical periodontitis. (**A**) Upper jaw MSCT imaging. No periapical bone lesion was detected (arrow). (**B**) In PAN, at the level of the periapex of the root of the left first premolar, the observers noted a radiolucent periapical image (arrow) characterised by supposed changes in bone structure that simulated an apical periodontitis lesion. the identification of AP lesions in the upper molar areas and to a lesser degree in upper premolar/canine areas. The evaluation of upper and lower incisive areas is also hard to carry out due to the morphologic diversities of chin/mental fossa and superimposition of the cervical spine, skull base, hard palate, and nasal bone/cartilage/air. On the contrary, AP lesions are well recognizable in lower premolar/canine and molar areas since the superimposition of extraoral anatomic structures is limited and roots are more orthogonal to the X-ray beam, although the projection of nerve canals and foramen may correspond to root apexes [9]. Furthermore, periapical radiolucency has to involve around 30-50% of the bone mineral loss to be radiographically recognizable [20]. Such value represents the needed threshold for the radiographic detection of AP lesions. The involvement of cortical bone facilitates the detection of AP lesions leading to earlier attainment of the bone demineralization threshold value since mineral content is greater in cortical than cancellous bones. For the same reasons, AP lesions involving a large amount of bone tissue are easier to identify than smaller ones [21,22]. Studies conducted by [PERSON] et al. [23] and [PERSON] et al. [9,24] showed that the accuracy of PAN in the identification of AP lesions was 53.8%, 65.0%, and 71.3%, respectively. In these studies, the reference standard for the assessment of AP lesions was cone beam computed tomography (CBCT). All teeth examined by [PERSON] et al. [23] had a history of endodontic infection confirmed by clinical examination, whereas the studies by [PERSON] et al. [9,24] investigated untreated and endodontically treated teeth with clinical/surgical diagnosis of the lesions. Another comparative study by [PERSON] et al. [25] between PAN and periapical radiography indicated that AP lesions detected on PAN were 58.0%, although the authors did not mention how AP lesions had been confirmed. In the current study, the accuracy in diagnosing AP lesions on PAN was only 72.1%, thus confirming the undefined role of PAN in identifying AP lesions. Our study assessed the accuracy of PAN in cancer patients, which is why the reference volumetric technique could only be MSCT.