high signal of the myxoid material (Kinoshita and Okitsu 2004; Moonis et al. 2007). While most PAs demonstrate the benign and nonaggressive features of smooth margins and homogenous enhancement, the more aggressive and invasive features may be seen with carcinoma ex‐pleomorphic adenoma, which is seen in areas of previously or concurrently benign PAs. Carcinoma ex‐pleomorphic adenomas can result in distance metastatic foci, including the brain (Sheedy et al. 2006). Heterogeneous signal within PAs can indicate a concurrent high‐grade malignancy, which can be low on T1 and T2 (Kinoshita and Okitsu 2004). FDG PET can be variable but tends to have increased uptake (Figure 2.62). Benignancy cannot be determined by imaging and therefore the differential includes primary parotid malignancy, metastases, and lymphoma as well as benign Warthin tumors (Shah 2002; Madani and Beale 2006b; Thoeny 2007).
Figure 2.60. Axial contrast‐enhanced CT of the maxillofacial soft tissues with a cystic mass interposed between the left submandibular gland and sternocleidomastoid muscle, consistent with a type 2 branchial cleft cyst.
Figure 2.61. Axial contrast‐enhanced CT of the parotid gland with a heterogenous mass with cystic changes. A pleomorphic adenoma (arrow) was diagnosed at surgery.
Figure 2.62. Axial PET/CT fused image demonstrating intense FDG uptake in a parotid mass. Pleomorphic adenoma was diagnosed at surgery.
Warthin tumor
Papillary cystadenoma lymphomatosum, or Warthin tumor, is the second most common benign lesion of the salivary glands. These tumors are typically well marginated but inhomogenous and found in the parotid tail of the superficial lobe and 15% present as bilateral or multicentric disease (Madani and Beale 2006b). There is by CT a heterogenous density and very mild enhancement (Figure 2.63). They typically have small cysts but do not demonstrate calcifications. The differential diagnosis therefore includes lymphoepithelial cysts, primary neoplasms, metastatic disease, and lymphoma. MRI signal on T1 is generally low but may be heterogenous, and T2 is either high signal based on its more cystic features or heterogenous. If the tumor is primarily solid, the imaging characteristics may mimic a PA with relatively homogenous hypoechoic architecture. Contrast with Gd follows CT characteristics. FDG uptake can be high on PET imaging. Warthin tumors contain oncocytes and 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 Warthin tumor (Miyaki 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 tumors as small as 9 mm (Miyaki et al. 2001). By its peripheral location and cystic components, it can be mistaken for a necrotic lymph node or second branchial cleft cyst (Ikeda et al. 2004). 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 exophytic tumor from the parotid gland (Hamilton et al. 2003).
Figure 2.63. 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 tumor (arrow).
Oncocytoma
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.
Malignant Tumors
Mucoepidermoid carcinoma
Mucoepidermoid carcinoma is the most common malignant lesion of the salivary glands. They are also the most common salivary malignancy in the pediatric population. One half occur in the parotid gland and the other half in minor salivary glands (Jansisyanant et al. 2002; Shah 2002). The imaging characteristics of mucoepidermoid carcinoma are based on histologic grade. The low‐grade lesions are sharply marginated and inhomogenous, mimicking PA and Warthin 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 heterogeneous 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 (Sigal et al. 1992; Lowe et al. 2001; Bialek et al. 2006; Madani and Beale 2006b) (Figures 2.64 through 2.66 ). 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
Adenoid cystic carcinoma has similar characteristics as 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 tumors' extent and identify perineural invasion into the skull base (Figures 2.67 through 2.71 ).