Читать книгу Anterior Skull Base Tumors - Группа авторов - Страница 75

Anterior skull base (ASB) tumors can be classified into three groups according to their site of origin: (1) sinonasal neoplasms involving or extending through the anterior cranial base; (2) neoplasms which arise from the bony framework of the base itself; (3) neoplasms originating from adjacent intracranial structures. With few exceptions, most of these tumors have a non-specific appearance on CT and MRI, which limits the role of imaging in terms of characterization. However, treatment planning (transnasal endoscopic surgery in particular) mostly depends upon the tumor map, exploiting the potential of modern cross-sectional imaging. As a result, the radiologist who has to evaluate a neoplasm involving the ASB needs to be fully aware of all the technical solutions available and the specific strengths/weaknesses of the different imaging techniques. Knowledge of radiological anatomy (and its variants) is also essential, which includes the ability to translate the CT appearance of structures into the equivalent MR signal (and vice versa). These main prerequisites have to be combined with up-to-date knowledge of treatment options and surgical procedures in order to be able to create a reporting checklist covering all the aspects that are essential for clinical decision making.

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The diagnostic work-up of neoplasms involving or arising from the anterior skull base (ASB) aims to achieve key data for treatment planning. Among these, four factors play a crucial role: the histological type, presence/absence of clinical signs of advanced disease, patient characteristics (age, comorbidities), and assessment of the local, regional, and distant extent by imaging [1–3].

With the exception of a few distinct benign histologies, such as osteoma, juvenile angiofibroma, inverted papilloma, and meningioma, the majority of sinonasal and skull base tumors have non-specific imaging features. Therefore, though imaging can help to narrow the differentials, the diagnosis of a suspicious lesion requires biopsy. In addition, besides discriminating benign and malignant neoplasms, histopathological diagnosis has a second key role: the histologic type may determine surgical versus non-surgical management [1].

While clinical findings of advanced local disease, such as neurological or ophthalmological signs, may suggest that a conservative surgical approach is unlikely to be indicated, it is only by exploiting the potential of high-resolution imaging that the detailed local extent and the resectability of a tumor can be accurately determined [4].

Fig. 1. Intestinal-type adenocarcinoma (T), CT (a), and TSE T2 (b) in the coronal plane. When a thin lamella is surrounded on both sides by a signal different from air, MRI shows the lamella as a thin hypointense line. This pattern is shown in the pneumatized roof of the right orbit (1) where the thin wall appears as a black line between the hyperintense mucus (supraorbital cell) and the fat tissue within the orbit. Although CT clearly shows the wall of an ethmoidal cell (2), the presence of mucus on both sides permits a similar appearance on MRI. Similarly, the left vertical lamella of the cribriform plate is better delineated by CT (3). However, the hyperintensity of CSF (on the intracranial side) and mucus (nasal side) permit the identification of the lamella. On the contrary, the left lamina papyracea, bordered only by air on the ethmoidal side, cannot be seen by MRI. However, when the mineral content of the lamella has been reabsorbed, it becomes undetectable by CT. MRI may show this on condition that it is bordered on both sides by signal, as it appears for the medial orbital wall (5) and the vertical lamella of the right middle turbinate (three parallel arrows). The lamella is displaced laterally by the neoplasm (T). A right infraorbital ethmoid cell is indicated (6).

Although anterior craniofacial resections still have a role in the treatment of some advanced malignant neoplasms, in the last 2 decades the indications for transnasal endoscopic surgery (TES) have been considerably expanded to include not only benign neoplasms, but also selected cases of malignant tumors extending through the ASB [5, 6], as well as neoplasms arising from the ASB itself. These advances have been made possible by a combination of three factors: increasing expertise in endoscopic procedures, developments in endoscopic technology, and improvements in diagnostic imaging techniques.