on PET imaging (Hsu et al. 2003). Although the calculated 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 2.71 and 2.72). Plexiform neurofibromas are also slow-growing and rare. They present with
Figure 2.72. Axial T2 MRI image corresponding to the case illustrated in Figure 2.71.
multiple cord-like masses and are also 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 similar to the neurofibromas and schwannomas including the "target sign" (Aribandi et al. 2006; Lin and Martel 2001). The target sign may also be seen by US as a hypoechoic periphery surrounding a subtle slightly hypere-choic center. There may also be slight increased through-transmission (Lin and Martel 2001).
Both primary and secondary lymphomas of the salivary glands are rare. Primary lymphoma of the salivary glands is the mucosa-associated lymphoid tissue subtype (MALT). MALT lymphomas constitute about 5% of non-Hodgkin's lymphomas (Jhanvar and Straus 2006). These lymphomas are seen in the gastrointestinal tract and are associated with chronic inflammatory or autoimmune diseases. The salivary glands do not typically contain MALT but may in the setting of chronic inflammation (Ando, Matsuzaki, and Murofushi 2005). The MALT lymphomas found in the gastrointestinal tract are not typically associated with Sjogren's syndrome. The MALT lymphoma, a low-grade B-cell type, tends to be a slow-growing neoplasm. Metastases tend to be to other mucosal sites, a demonstration of tissue tropism. The MALT lymphomas are amenable to radiotherapy but can relapse in the contralateral gland, demonstrating tropism for the glandular tissue (MacManus et al. 2007). In Sjogren's syndrome, there is an approximately forty-fold increased incidence of developing lymphoma compared to age-controlled populations. Of the various subtypes of lymphoma that are seen associated with Sjogren's syndrome (follicular, diffuse large B-cell, large cell, and immunoblastic), the MALT subtype is the most common at around 50% (Tonami, Munetaka, and Yokota et al. 2002). The parotid gland is the most commonly affected (80%). Less commonly the submandibular and rarely the sublingual gland may be involved. Clinically, it may present with a focal mass or diffuse unilateral or bilateral glandular swelling.
67Ga-citrate scintigraphy had been the standard imaging modality used to assess staging and post-therapy follow-up for lymphomas (Hodgkin's and non-Hodgkin's) for many decades. PET/CT with FDG is quickly becoming the standard for staging and follow-up for many lymphoma subtypes (Jhanvar and Straus 2006).
The imaging findings in salivary lymphomas, however, are not specific. CT may demonstrate focal or diffuse low to intermediate density mass with cystic areas and calcifications. MRI shows the soft tissue areas to be isointense to skeletal muscle on T1 images and hypointense relative to fat on T2 images along with diffuse enhancement post-contrast (Tonami, Munetaka, and Yokota et al. 2002). Although there may be cystic changes demonstrated by CT, MRI, or US, they are thought to be dilated ducts as a result of compression of terminal ducts (Ando, Matsuzaki, and Murofushi 2005). The US characteristics of MALT lymphoma may demonstrate multifocal hypoechoic intrapa-rotid nodules and cysts (which may be dilated ducts), and calcification as well. Large B-cell intra-parotid lymphoma has been described as a hypoechoic, homogenous, well-marginated mass exhibiting increased through-transmission (a characteristic of cysts) and hypervascularity (Eichhorn, Iakovos, and Ridder 2002). Although there are reports of hypermetabolism in MALT lymphomas, PET imaging findings are also not conclusive (Mac-Manus et al. 2007). Uptake in the tumor and a
background of chronic inflammatory changes of chronic sialadenitis may result in variably elevated uptake of FDG.
Secondary lymphomas (Hodgkin's and non-Hodgkin's) are also quite rare, with the histology most commonly encountered being of the large cell type. There is typically extraglandular lymphade-nopathy associated. The imaging features are also nonspecific, although there is usually no associated chronic sialadenitis (Figures 2.73 through 2.75).
Intraglandular lymph nodes are found in the parotid gland due to its early encapsulation during development. The sublingual gland and submandibular gland do not contain lymph nodes. The parotid and periparotid lymph nodes are the first order nodal site for lesions that affect the scalp, skin of the upper face, and external ear (Ollila and Leland et al. 1999). The most common malignancy to metas-tasize to the parotid nodes is squamous cell carcinoma, followed by melanoma and less commonly Merkel cell carcinoma (Bron, Traynor, and McNeil et al. 2003) (Figures 2.76 through 2.78).
The imaging findings are not specific. CT in early stages demonstrates the nodes to have sharp
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