Malignant Tumors Arising in the Maxillary Sinus
Malignant tumors that arise within the maxillary sinus and grow toward the nasal cavity to eventually reach the ASB are, in most cases, very advanced and rare (Fig. 14). If surgery is considered in these patients, craniofacial resection is indicated, usually with adjuvant radiotherapy (± chemotherapy) [40]. The checklist for assessing the 3D extent should include, once more, six “vectors of growth.” They will comprise the potential invasion of the anterior wall and premaxillary soft tissues (anterior vector), the orbital floor and PNS along the infraorbital nerve (cranial vector), and the extension through the floor of the maxillary sinus, alveolar process, and hard palate (inferior and medial vectors). When a maxillary malignancy invades the posterolateral maxillary sinus wall (posterior and lateral vectors) it results in a deep pattern of growth, which leads to destruction of the pterygoid laminae, invasion of the PPF, and spread into the infratemporal fossa/masticator space. Further extent leads the tumor to infiltrate the middle skull base structures via involvement of the greater wing of the sphenoid, and its foramina, or the superior orbital fissure. Once the neoplasm grows inside these spaces and structures, there is a good chance of PNS along the branches of the trigeminal nerve into Meckel’s cave or into the cavernous sinus.
Olfactory Neuroblastoma
ONB can be regarded as the prototype of a neoplasm arising from the skull base itself. In most cases, the site of origin of the neoplasm is the cribriform plate or the adjacent superior turbinate and the superior half of the nasal septum. From this restricted area, the neoplasm permeates the ASB floor, extends intracranially, and also grows in the nasoethmoidal cavity. The result of these two vectors is an “hourglass” pattern: a solid intracranial and intranasal mass with a “waist” at the ASB floor (Fig. 15). In the checklist, it is important not to overlook the findings contraindicating TES. These include: a large invasion of the frontal sinus, a massive extension to the cerebral parenchyma, spread of the tumor above the orbits, or erosion of the anterior facial skeleton [41]. Among the ONB imaging findings described, intratumoral necrosis and significant postcontrast enhancement should be considered [42]. The presence of a peritumoral cyst and meningeal dural tail have also been reported [43, 44]. Nevertheless, MRI and CT signal/density patterns are non-specific [42]. Careful evaluation of RLPN and cervical lymph nodes is recommended, since nodal metastases can be observed in up to 25% of patients [42]. PET-CT has been reported to modify staging in a relevant fraction of patients, particularly during follow-up [45].
Fig. 14. Squamous cell carcinoma. Axial planes: CT (bone window, a, d) and MRI (postcontrast VIBE, b; TSE T2, c). a The neoplasm (T) arises from the maxillary sinus. The walls are thickened (long standing inflammation?) with focal erosion of the anterior and posterior walls (black arrows). b MRI shows the tumor extent beyond both sinus walls (white arrows). The neoplasm invades the nasal fossa and the choana. c, d In the coronal plane, MRI clearly separates the solid neoplastic projection into the olfactory fissure from the blocked mucus within the posterior ethmoid cells (pec). A marked bone sclerosis (black arrows) surrounds the infraorbital nerve groove (ion), at the same time erosion of the maxillary sinus wall is present (curved white arrow). mpl, medial pterygoid plate.
Meningiomas
Although extracranial meningiomas may arise from the sinonasal tract and involve the ASB floor from “below,” direct extension of an intracranial meningioma is much more common. Because of its site of origin (from the olfactory groove, anteriorly, to the clinoids, posteriorly) and its pattern of growth, meningioma is both the prototype of a tumor arising in close contact with the ASB floor and the most frequent one (Fig. 16). According to the literature, TES demonstrates an inferior rate of complete resection of anterior cranial fossa meningiomas compared with open transcranial approaches [46]. Among the numerous elements that may explain such a limitation, three factors, belonging to the domain of pretreatment imaging, have been recently emphasized and also reported in a scoring system [47]. Basically, these factors are the degree of hyperostosis induced by the lesion at the level of the ASB floor, the resectability of the tumor once it extends into the cavernous sinus and involves the ICA, and the extent of the dural tail in the transverse plane compared to the length of the interfovea ethmoidalis distance.
Fig. 15. ONB. Coronal (a) and sagittal (b) TSE T1 after contrast administration. a Enhancing nasoethmoidal mass invading both sides of the ACF showing a “waist” at the passage between the ethmoid and the ACF. The tumor (T) shows intracranial intradural invasion, peripheral “cysts” are present (thin white arrows). The tumor extends into a blocked frontal sinus (white arrow in b).
Fig. 16. Meningioma of the olfactory groove/planum sphenoidale. CT, bone window. Marked reactive hyperostosis of the planum sphenoidale (black arrowheads) is induced by the meningioma (white arrows), which also causes enlargement and an “upward pulling” of the sinus (pneumosinus dilatans; curved arrow).
TNM Classification
The checklist outlined contains more detailed information than what is required to determine the T category according to the AJCC TNM classification. Hence, additional data are provided to the members of the multidisciplinary team who are involved in the process of selecting the most appropriate treatment planning. Information regarding regional and distant metastases, which are crucial, are usually easily inferable from different imaging studies (ultrasound, CT of the neck, PET-CT), and thus translatable in a synthetic classification. For all these reasons, reporting of TNM in radiological reports is not indispensable.
Conclusions
The radiologist who has to evaluate a neoplasm with a potential involvement of the ASB should carry “hand luggage” containing four different epistemic compartments. The first box contains the knowledge of the technical solutions available and the specific strengths/weaknesses of different imaging techniques. For example, even if MRI is superior to CT in solving several of the points delineated in the checklist for local staging, the integration of MRI and CT is advantageous, especially in challenging cases. For neck nodal staging, CT is frequently used. However, ultrasound can adequately evaluate the neck and permits sampling of suspicious nodes with ultrasound-guided FNAC. PET-CT has the greatest sensitivity in detecting distant spread for