3. Squamous cell carcinoma. 3D isotropic balanced steady-state gradient echo (CISS) after administration of contrast agent, originally acquired in the axial plane (a) and reconstructed in the coronal plane (b). The slice thickness is 0.55 mm. a The nasoethmoid neoplasm (T) posteriorly invades the right Meckel’s cave (1, mc), replaced by tumor signal; 2 and 3 represent the right and left sphenoid sinuses, respectively. The right internal carotid artery (ica) is surrounded by tumor. Within the prepontine cistern both abducens nerves are imaged, close to the Dorello’s canal (VIn). ba, basilar artery. b In the coronal reconstruction, the tumor invades of the right cavernous sinus (T, dotted curved arrows). The tumor replaces the enhanced venous signal surrounding the right abducens nerve (VIn) and right intracavernous internal carotid artery (ica). On the left side, the normal abducens nerve and intra-extra-cavernous segments of the internal carotid artery are detected. mc, left Meckel’s cave; V3, mandibular nerve at foramen ovale; ch, chiasm. The asterisk (*) indicates the pituitary gland.
An additional group of critical structures that require proper high-resolution imaging is represented by the arteries running close to the ventral skull base, either below, through, or strictly above [14]. The anterior and posterior ethmoid arteries are among those running below and through the ASB. Unintentional damage during surgery can cause bleeding, and if the artery retracts into the orbit, a retrobulbar hematoma occurs. Both arteries originate from the ophthalmic artery inside the orbit. The position of their canals entering the ethmoid has a wide variability, which is more frequent for the anterior ethmoid artery, entering the ethmoid via the frontoethmoidal suture. In addition, its canal is very often dehiscent. Although the ethmoid artery canals can be identified in most patients using high-resolution CT or cone beam CT [15], the direct demonstration of the arteries running inside requires CT angiography (CTA). A similar result can be achieved by high-resolution 3D radiant echo T1W sequences (VIBE, THRIVE, LAVA). When obtained with the saturation of the fat, these MR sequences provide greater contrast resolution than CT, resulting in more precise delineation of the optic nerve, ophthalmic artery, and those extrinsic orbital muscles in close relationship with the anterior ethmoidal artery (superior oblique and medial rectus; Fig. 4).
Fig. 4. Intestinal-type adenocarcinoma. a 3D isotropic GE sequence after contrast administration. b CT after contrast agent administration. The neoplasm (T) arises from the anterior left ethmoid and contacts the nasolacrimal duct. The ophthalmic arteries (oa), optic nerves (on) and a right carotid artery aneurysm (a) are well depicted with both techniques. cgl, common ground lamella.
The vascular constraints become more complex when moving posteriorly, at the level of the planum sphenoidale and adjacent sphenoid sinus walls. This is the area where inadvertent carotid artery injuries during endoscopic skull base surgery can occur. The use of state-of-the-art CTA or MR angiography has been advocated in the delineation of the course of the internal carotid artery at this level. CT angiography is also indicated for detailing the course of the intracranial arteries running close to the ASB floor. Among these are the proximal branches of the anterior cerebral artery, like the orbitofrontal and frontopolar arteries which project toward the olfactory fossa, the anterior cerebral artery itself (A2 segment), and the anterior communicating artery.
Assessing the Regional and Distant Neoplastic Extent
The risk of lymphatic metastasis in sinonasal malignancies depends on the site, extent of tumor spread, and histology (Fig. 5). As the sinonasal tract is thought to have limited capillary lymphatics [16], the incidence of regional metastasis is low, ranging between 4 and 15% [17, 18]. When the neoplasm is confined within a sinus cavity, nodal metastasis is more frequent in tumors arising from the maxillary sinus than from the ethmoid. A greater incidence of regional involvement, up to 50%, is reported when lymphatic-rich areas adjacent to the sinuses are invaded, like the masticatory space or the skin. Among the different histologies, nodal metastases are more frequently observed in olfactory neuroblastoma (ONB; up to 43%), mucosal malignant melanoma, and squamous cell carcinoma [19, 20].
Fig. 5. Adenosquamous carcinoma. a The patient was examined for a large lymph node metastasis at level 2 with extranodal extent and no primary. The sternocleidomastoid muscle is infiltrated (black arrows). The right common carotid artery is displaced medially. A node-to-vessel contact of less than 180° is present (white arrows). b In the axial TSE T2 sequence, low-intensity tissue is detectable in the posterior ethmoid cells of both sides (black arrows on the right, white arrows on the left). At endoscopic surgery both ethmoid lesions were neoplastic; no connection between the two sides was demonstrated.
In addition to the upper jugular and facial nodes, sinonasal malignancies drain to the retro-latero-pharyngeal nodes (RLPN). While CT and MRI yield similar results in assessing cervical lymph nodes, MRI has been found to be significantly more sensitive than CT in detecting abnormal RLPN [21]. Findings suggesting RLPN metastasis include a short diameter greater than 5 mm or the presence of intranodal necrosis (a central area showing high intensity on T2W sequence and low intensity on postcontrast T1W sequences) [22]. Both cervical and RLPN metastases are a poor prognostic factor for decreased overall survival and locoregional control [23]. When MRI is used to evaluate the local tumor extent, contrast-enhanced CT or an ultrasound examination can be used to evaluate the lymph nodes of the neck. An advantage of the ultrasound examination is easy sampling of suspicious nodes via FNAC (fine-needle aspiration cytology).
Distant metastases are uncommon, ranging from 10 to 15%, and they depend on the histology [1, 24, 25]. Sinonasal mucosal melanomas have a worse prognosis than other head and neck sites [18, 26]. While in these patients the incidence at the time of diagnosis is <10%, up to 40–50% of patients will develop distant metastasis in the lung, brain, liver, and bone during follow-up [27]. PET-CT is indicated in the work-up of distant metastasis, particularly in very aggressive histologic types such as mucosal melanoma [28].
A Checklist Approach to Reporting Lesions Involving the ASB
When reporting the local extent of a lesion involving the ASB, the radiologist has to be aware of a series of critical anatomical structures, the invasion of which impacts treatment planning [4, 29]. Hence, a checklist approach should be adopted to standardize the report and reduce the
