## 4 Discussion
CBCT is not normally used in tumour staging because it is not appropriate for soft tissue assessment [26] and it is too slow (5-40 s) for the administration of intravenous contrast medium with non-negligible motion artefacts [27,28]. However, it has been widely demonstrated that both the study of bone quality in CBCT and MSCT are superimposable [22,29] and that MSCT accurately identifies caries, root remnant, and periodontal disease [30,31]. For this reason, we felt it was reasonable to compare the accuracy of two-dimensional PAN imaging with the volumetric reference standard, whether represented by MSCT as in the current study or by CBCT in previous papers [9,23,24]. The study of infectious foci in anticipation of RT treatment is of fundamental importance for the prevention of long-term complications. The well-known effects caused by RT are vascular alterations of both soft tissue and bone, damage to salivary glands, and increased collagen synthesis resulting in fibrosis [32,33]. Tooth extraction in an irradiated field is a risk factor for both infection and osteonecrosis, and remediations prior to RT are thus strongly recommended so as to remove mobile or decayed teeth, teeth with periapical lesions, and teeth causing trauma or mucosal lesions [34]. These procedures should be performed at least fourteen days before the beginning of RT to allow tissue healing [35]. In our opinion, a rapid diagnostic technique able to adequately assess the prognosis of each tooth and the bone tissue is of primary importance in this regard. Due consideration must be given to the best diagnostic technique to evaluate dentoalveolar structures, as PAN has proved to be excellent in the identification of deep caries, root remnants, and advanced periodontal disease, but poor in the detection of AP lesions. Patients with OPC are already staged in the diagnostic workup on the basis of a head and neck MSCT scan. Therefore, the same radiation oncologist might not only perform the clinical examination, but also evaluate the maxillofacial MSCT scan together with his/her radiologist colleagues in order to decide whether the patient needs a dental examination or has to undergo directly to RT. In this way, not only patients in an advanced stage of the disease, but also those with no foci to be cleared could start therapy immediately, based on a shared assessment and with an obvious reduction of healthcare resources. Future studies should explore the clinical effectiveness of such a swift protocol that would avoid PAN in this setting. In addition, subjecting the patient to additional ionizing radiations without providing diagnostic benefits might not be acceptable from a radiation protection point of view. Any exposure to radiations is justified only when patients can benefit from it [36] and basing dental-periodantal screening on PAN may lead to more long-term complications since pathological teeth may not be extracted or restored before RT begins. Nevertheless, clinical settings in which maxillofacial MSCT cannot be performed absolutely need PAN to assess dentoalveolar structures. These situations include claustrophobic people, those suffering renal failure, since these patients are commonly diverted to no contrast magnetic resonance imaging because of its intrinsic high contrast resolution, and MSCT protocols with a thickness and/or reconstruction interval greater than 1 mm, thus not acceptable to appropriately identify small alterations of bone tissue surrounding teeth. Limitations of the study include the fact that we considered caries, root remnants, and periodontal alterations in advanced stages of disease and that we grouped all AP lesions regardless of their size, anatomical location, and bone cortical involvement. As has been previously demonstrated [9; 24], there is much difference in the ability of PAN to detect AP lesions larger or smaller than 4.5 mm, located in the upper or lower arch, and causing resorptions of cortical bones.