Читать книгу Salivary Gland Pathology - Группа авторов - Страница 102

Neurogenic tumors are uncommon in the salivary glands but when encountered are most commonly found in the parotid gland. Most facial nerve schwannomas are on the intratemporal facial nerve with only 9% extra‐temporal and in the parotid gland (Shimizu et al. 2005), (Figures 2.74 through 2.76 ). These are difficult to preoperatively diagnose as they do not typically present with facial nerve dysfunction. As seen in other parts of the body, they tend to be sharply marginated and have an ovoid shape along the axis of the involved nerve, such as the facial nerve. The CT density is that of soft tissue but post‐contrast, both enhance (schwannoma slightly greater than neurofibroma). Both are noted as low signal on T1 and high on T2. MRI enhancement pattern follows that of CT. They may demonstrate a target sign appearance with peripheral hyperintensity relative to a central hypointensity (Martin et al. 1992; Suh et al. 1992; Shimizu et al. 2005). However, this sign is not pathognomonic and may be seen in schwannomas or neurofibromas. Increased uptake is seen on FDG PET in both diseases. The neurofibroma may be associated with Von Recklinghausen's disease. Although, most schwannomas and neurofibromas are benign, they are reported as demonstrating increased uptake (hypermetabolism of glucose) of 18F‐FDG on PET imaging (Hsu et al. 2003). Although the calculated standard uptake value (SUV) can be helpful in differentiating benign from malignant lesions, there is a significant overlap (Ioannidis and Lau 2003). There is difficulty in separating low‐grade malignant lesions from benign lesions (Ioannidis and Lau 2003), (Figures 2.77 and 2.78). Plexiform neurofibromas are also slow‐growing and rare. They present with multiple cord‐like masses and are far more common in the parotid gland relative to other salivary glands. By CT and MRI, they are sometimes described as a “branching” pattern or “bag of worms” secondary to the multiple lesions growing along nerve branches. They have CT and MRI signal characteristics like neurofibromas and schwannomas including the “target sign” (Lin and Martel 2001; Aribandi et al. 2006). The target sign may also be seen by US as a hypoechoic periphery surrounding a subtle and slightly hyperechoic center. There may also be slight increase through‐transmission (Lin and Martel 2001).

Figure 2.74. Coronal T1 contrast‐enhanced MRI demonstrating a mass in the left parotid gland with smooth margins. The mass extends superiorly into the skull base at the stylomastoid foramen (arrow). A benign schwannoma was diagnosed.

Figure 2.75. Coronal T2 MRI corresponding to the case illustrated in Figure 2.74.

Figure 2.76. Axial CT at the skull base displayed in bone window showing dilatation of the stylomastoid foramen with soft tissue mass (arrow). A benign schwannoma was diagnosed.

Figure 2.77. Coronal T1 contrast image showing a very ill‐defined mass with heterogenous enhancement in the parotid gland with skull base extension via the stylomastoid foramen. A malignant schwannoma was diagnosed.

Figure 2.78. Axial T2 MRI image corresponding to the case illustrated in Figure 2.77.