Figure 2.56. Axial PET/CT fused image demonstrating intense FDG uptake in a parotid mass. Pleomorphic adenoma was diagnosed at surgery.
Figure 2.57. Axial contrast enhanced CT of a heterogenous parotid mass at the tail of the gland, with multiplicity and cystic or necrotic changes, diagnosed as a Warthin's tumor (arrow).
are thought to be the mechanism by which they tend to accumulate 99mTc-pertechnetate. The 99mTc-pertechnetate uptake and retention after lemon juice stimulated washout by the normal tissue is a good indicator of the diagnosis of War-thin's tumor (Miyake, Matsumoto, and Hori et al.
2001). This pattern is much less commonly seen by other lesions such as lymphoepithelial cysts, PAs, and oncocytomas. This technique allows visualization of Warthin's tumors as small as 9 mm (Miyake, Matsumoto, and Hori et al. 2001). By its peripheral location and cystic components, it can be mistaken for a necrotic lymph node or second branchial cleft cyst. The tail of the parotid region can be difficult to differentiate from adjacent cervical lymphadenopathy. However, coronal images can aid in determining the site of origin. If the lesion is medial to the parotid tail it is more likely cervical jugular chain lymphadenopathy and if it is more laterally located, it is more likely an exo-phytic tumor from the parotid gland (Hamilton et al. 2003).
These relatively rare tumors exist primarily in the parotid gland. Their imaging characteristics are that of PAs except that they do accumulate 99mTc-pertechnetate. They are also reported to have high 18F-FDG uptake. They are considered benign but may have some invasive features.
Mucoepidermoid carcinoma is the most common malignant lesion of the salivary glands. It is also the most common salivary malignancy in the pedi-atric population. One-half occur in the parotid gland and the other half in minor salivary glands (Jansisyanont, Blanchaert, and Ord 2002; Shah
2002). The imaging characteristics of mucoepider-moid carcinoma are based on histologic grade. The low-grade lesions are sharply marginated and inhomogenous, mimicking PA and Warthin's tumor. These well-differentiated lesions can have increased signal on T2 weighted sequences. The low-grade lesions are more commonly cystic (Madani and Beale 2006b). The high-grade, invasive lesions mimic adenoid cystic carcinoma and lymphoma or large heterogenous PAs or carcinoma ex-pleomorphic adenoma. They tend to have a lower signal of T2. Contrast enhanced studies demonstrate enhancement in the more solid components (Bialek, Jakubowski, and Zajkowski et al. 2006; Lowe, Stokes, and Johnson et al. 2001; Madani and Beale 2006b; Sigal, Monnet, and De Baere et al. 1992) (Figures 2.58 through 2.60). Therefore standard imaging cannot exclude a malignant neoplasm. Defining the tumor's extent is critical. Contrast MRI, especially in the coronal
or sagittal plane, is essential to identify perineural invasion into the skull base.
Adenoid cystic carcinoma has similar characteristics to mucoepidermoid carcinoma in that their imaging findings are based on histologic grade. Adenoid cystic carcinoma is the most common malignant neoplasm of the submandibular and sublingual glands, as well as the minor salivary glands in the palate. These tumors have a high rate of local recurrence, higher rate of distance metastases as opposed to nodal disease, and may recur after a long latency period (Madani and Beale 2006b). MRI is the imaging method of choice demonstrating high signal due to increased water content. Contrast enhanced fat saturated images are critical to evaluate for perineural spread, which is demonstrated by nerve thickening and enhancement (Madani and Beale 2006b; Shah 2002). CT can be helpful to evaluate bone destruction or foraminal widening. It is important to define the tumor's extent and identify perineural invasion into the skull base (Figures 2.61 through 2.65).
In the cervical soft tissues, lipomas are slightly more commonly seen within the parotid gland rather than peri-parotid. Lipomas of the salivary glands are uncommon (Shah 2002). The CT and MRI characteristics are those of subcutaneous fat with CT density very low (-100 H) and hyperin-tense on both T1 and T2. Lipomas tend to be echogenic on US. They may be uniform on imaging but may have areas of fibrosis. The heterogenous density from fibrosis, or hemorrhage, carries the additional differential diagnosis of liposarcoma or other neoplasms (Som, Brandwein, and Silver 1995) (Figures 2.66 and 2.67).
Figure 2.68. 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 2.67. Axial contrast enhanced CT through the sub-mandibular gland with fat density mass partially surrounding the gland. A lipoma was diagnosed.
Neurogenic tumors are uncommon in the salivary glands but when encountered are most commonly found in the parotid gland. The majority of the facial nerve schwannomas are on the intratemporal facial nerve with only 9% extra-temporal and in the parotid gland (Shimizu, Iwai, and Ikeda et al. 2005) (Figures 2.68 through 2.70). 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 then neurofibroma). Both are seen 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 hyperin-tensity relative to a central hypointensity (Martin et al. 1992; Shimizu, Iwai, and Ikeda et al. 2005; Suh, Abenoza, and Galloway et al. 1992). 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 the vast majority of schwannomas and neurofibromas are benign, they are reported as demonstrating increased uptake (hypermetabolism of glucose) of 18F-FDG
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