PAROTID TUMORS AND TUMORLIKE CONDITIONS
- Computed tomography and magnetic resonance imaging are instrumental in determining the intrinsic versus extrinsic origin of a parotid-region mass.
- Such imaging may provide critical information concerning the extent of a primary parotid tumor.
- Computed tomography and magnetic resonance imaging can often help to determine the relationship of an intrinsic parotid mass to the facial nerve.
- Perineural spread of parotid cancer is a critical factor in treatment strategy, and imaging helps to evaluate this important risk factor.
- When a parotid-region mass is due to metastases to parotid nodes, computed tomography and magnetic resonance imaging can help to identify parotid cancer or another etiology as a cause.
- Slowly progressive peripheral facial nerve palsy may be due to parotid cancer, and imaging should be employed whenever this occurs clinically.
- Computed tomography and especially magnetic resonance imaging are useful for surveillance imaging following treatment for parotid tumors while fluorine-18 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) has significant limitations in this regard.
A Dilemma: Parotid-Region Mass of Parotid or Nonparotid Origin
Mass lesions in the parotid region will be intrinsic to the gland in about 75% to 80% of patients. In the remainder, such masses will be due to periparotid adenopathy (Fig. 179.1 and Chapter 175) or masses arising from the mandible and/or temporomandibular joint (TMJ) (Chapter 103) and masticator space (Chapter 145), parapharyngeal space (Chapters 143–144) or temporal bone, and rarely the facial nerve (Fig. 179.2). Infiltrating systemic disease such as sarcoidosis, manifestations of autoimmune sialoadenitis and lymphoma (Chapter 178) can mimic a parotid epithelial-origin tumor if those conditions are not otherwise known to be present. Diagnostic imaging has a major impact on sorting out these possibilities and thereby significantly alters medical decision making in a high percentage of patients. The contributions are equally important in many cases of intrinsic glandular epithelial-origin neoplasms.
Parotid Gland Tumors
Major salivary glands contain several different groups of functioning and support cells. This leads to the variety of possible histologic neoplastic diagnoses discussed in Chapter 22. Precise histologic diagnosis by frozen section and needle biopsy may be difficult, especially with regard to distinguishing between benign and malignant neoplasms. Those planning care must be very aware of this limitation. Imaging features can sometimes help to anticipate malignancies, although the appearance may be misleading where masses appearing to have benign features on imaging end up being confirmed as malignant tumors. Because of this dilemma, both benign and malignant tumors are discussed in this chapter along with some of their more common potential mimics.
ANATOMIC AND DEVELOPMENTAL CONSIDERATIONS
The development of the parotid gland includes a phase where the glandular endodermal anlage wraps itself around developing lymphatic elements while crossing other vascular structures and components of the branchial apparatus that form part of the face and neck. The incorporation of these structures persists into adult life and explains masses of the parotid that are of lymph node origin, vascular origin, or branchial origin. Warthin tumor, also known as parotid papillary cystadenoma lymphomatosum, is a neoplasm uniquely associated with the parotid glands, perhaps in part related to this development.
The important anatomic considerations in parotid masses take into account the surrounding muscles, bones, vessels, and nerves that come in contact with the gland. Also, the parotid capsule attachments will direct the spread of both benign and malignant tumors. This anatomy should be reviewed in detail at this point, if necessary, in Chapter 175. The following anatomic relationships must be understood to properly ensure that imaging data is incorporated into the treatment of benign and malignant tumors:
- Facial nerve and the anatomic landmarks that relate to the surgical approach to facial nerve preservation; also, the relationships of the facial nerve to the fifth cranial nerve via the auriculotemporal nerve and greater petrosal nerve (Figs. 179.2 and 179.3)
- Attachments of the parotid capsule to the external auditory canal and external ear in general
- Deep relationships of the gland to the parapharyngeal spaces
- Parotid lymph nodes and their drainage patterns
- For very advanced lesions, the surrounding musculoskeletal relationships of the TMJ, mandible, temporal bone, and more central skull base to the parotid gland
The anatomic landmarks of prime importance in localizing the course of the facial nerve include the following:
- Descending facial canal
- Stylomastoid foramen
- Fat pad below the stylomastoid foramen (containing the nerve and vessels) (Fig. 179.3)
- Digastric groove and proximal portion of the posterior belly of the digastric muscle (Fig. 179.3)
- Tragal pointer (surgical jargon describing a part of the ear cartilage—the main trunk of the nerve typically lies about 10 mm inferior and 10 mm deep to the “pointer”) (Fig. 179.3)
- Superficial temporal artery and retromandibular vein (the nerve typically lies just lateral to the vein) (Fig. 179.3)
- Expected position of the division of the nerve into its major branches called the pes anserinus
The facial nerve itself is routinely visible throughout its course in the temporal bone and in the fat pad just below the stylomastoid foramen; it is occasionally visible in the gland on magnetic resonance (MR). It is relatively unimportant to see the nerve in the gland. It is definitely important to understand the surgical and imaging landmarks just described so that the relationship of disease processes to the facial nerve can be fully appreciated for diagnostic and treatment purposes.
Techniques and Relevant Aspects
Specific computed tomography (CT) protocols for various indications appear in Appendix A and are discussed in more detail in Chapter 175. CT data sets should be obtained with about 1-mm collimation reconstructed at 1- to 3-mm slice thickness depending on the clinical situation. Such acquisitions will be suitable for adequate multiplanar reformations. If sections are too thin, there may be an important loss in low contrast resolution that may cause a lesion to “hide” within the tissue density of the normal glands. Such occasionally poor contrast between a salivary gland mass and the normal gland can be daunting, and in any questionable case, magnetic resonance imaging (MRI) should be done to more confidently exclude a mass if one is strongly suspected clinically (Fig. 179.4). This issue makes MRI a better first choice for the evaluation of a parotid-region mass if there is no inflammatory component to the clinical presentation.
Magnetic Resonance Imaging
Specific MR protocols for various indications appear in Appendix B. MRI should be done with 3-mm sections and a field of view of 12 to 16 cm. Diffusion-weighted imaging (DWI) may be used since such imaging may contribute information about the likely benign or malignant nature of the mass. However, it is unlikely that any serious clinical decision will be based on such data relative to that from the clinical setting, anatomic imaging, and biopsy.
Since it is not predictable when contrast might be useful, a parotid-region study is generally done with acquisitions before and after contrast injection. Contrast-enhanced magnetic resonance (CEMR) studies are clearly useful when cranial nerves V and VII must be evaluated, if the lesion is aggressive, and/or if the cervical nodes are to be evaluated. Fat suppression is very useful on both the T1- and T2-weighted sequences.
Radionuclide studies are not used routinely for the evaluation of parotid gland masses. Those using technetium, iodine, and fluorine-18 2-fluoro-2-deoxy-D-glucose (FDG) are used in highly selected circumstances (Fig. 179.5).
Standard scanning techniques as described in Chapter 4 are used.
Conventional sialography is no longer used to evaluate parotid-region masses.
Pros and Cons
Magnetic Resonance and Computed Tomography
MR and CT are the most used imaging studies to evaluate a parotid-region mass. Such expensive studies may not be necessary if the mass is discrete and freely mobile. The clinician should be aware that on physical examination, superficial masses may be easily misinterpreted as a subcutaneous lesion and result in an unpleasant surprise during resection. Pleomorphic adenomas are notorious for such presentation. If there is a hint of a parotid inflammatory condition clinically in the presence of a mass, then CT is preferred over MR.
Ultrasound (US) may be used to determine whether superficial lesions are intrinsic or extrinsic to the parotid gland and to follow such masses if it is likely a reactive node (Figs. 179.1D and 179.3G–H) or if its benign nature is established by biopsy and definitive surgery is not elected. US may be used to guide percutaneous biopsies of the parotid gland that cannot be done by palpation alone. It also improves the diagnostic yield of the specimen by ensuring that the sampling is from the mass and not surrounding normal gland. The combination of US and biopsy can be a very quick and cost-effective way to evaluate a superficial mass that is likely to be benign or related to an enlarged periparotid or intraparotid lymph node. The US is limited by the skill of the operator and limited penetration between the bony structures of ramus of the mandible and mastoid portion of temporal bone.
Radionuclide studies are not used routinely for the evaluation of parotid gland masses. The constraints on its utility are discussed in more detail in Chapter 5 in general and with regard to glandular neoplasms in Chapter 22. Technetium, iodine, and FDG activity are frequently seen in normal glands (Fig. 5.9). FDG uptake is variable in both benign and malignant salivary gland neoplasms, somewhat limiting the utility of fluorine-18 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) studies in patients with salivary gland masses (Fig. 179.5).
Define the origin and extent of a parotid-region mass: Is it intrinsic or extrinsic to the parotid gland? If the margins of a mass are not clearly defined during physical examination, CT or MR should be done. This will help to determine whether the point of origin of the mass is intrinsic or extrinsic to the gland. Since “parotid masses” are frequently of nonparotid origin (Table 175.1), the relationship of the mass to the gland markedly alters the management in as many as a third of patients presenting with a parotid-region mass (Figs. 179.1 and 179.2). As a result, imaging can quickly narrow the diagnostic possibilities by excluding the extrinsic causes of parotid-region masses.
Determine whether the lesion present is solitary versus multiple and unilateral versus bilateral: The imaging determines whether the abnormality is a unilateral or bilateral, and whether it is multifocal on one or both sides (Table 179.1). A bilateral abnormality may be diffuse and is likely to represent systemic processes such as viral infection (human immunodeficiency virus [HIV], mumps), lymphoma, or autoimmune sialoadenitis (Chapter 177), and, for the purposes of this chapter, mass-inducing complications of systemic disease such as lymphoepithelial cysts in HIV and frankly neoplastic complications such as lymphoma arising in chronic autoimmune sialoadenitis (Figs. 179.6A–F and 179.7). Imaging can reveal more localized multifocal masses such as Warthin tumors (Fig. 179.6I–L), parotid adenopathy due to regional metastatic skin cancer (Fig. 179.1F–H), and a primary parotid carcinoma with parotid adenopathy (Fig. 179.8).
Define the relationship of an intrinsic parotid mass to the facial nerve and its major branches: At the minimum, the imager should be able to determine the superficial or deep relationship of the mass relative to the main trunk of the facial nerve (Figs. 175.8, 175.9, 179.3, and 179.8–179.12). With more effort, it is possible to refine the location of the parotid mass in relation to the surgical landmarks commonly used to identify the trunk of the facial nerve during dissection. As expertise grows, one can reasonably predict the position of the main branches of the facial nerve within the substance of the gland (Fig. 179.13).
Characterize the type of lesion by the degree of invasiveness present both within and beyond the parotid gland: One must determine the full extent of the lesion and its potential for invasiveness. The criteria used to evaluate for a malignant process include the margins of the lesion, internal signal characteristics on T2-weighted images, and perhaps behavior on DWI (Figs. 179.3–179.13). Metastatic adenopathy (Fig. 179.8) and perineural spread are almost sure indicators of a malignant neoplasm (Fig. 179.10).
In advanced cancers, findings such as bone erosion (Fig. 179.14), fixation to the deep neck (Fig. 179.15A) and skull base and carotid encasement (Fig. 179.15B) may dramatically alter the treatment approaches and prognosis.