Imaging of head and neck cancer

Introduction

Head and neck cancers are a complex, heterogeneous group of diseases with a variety of treatment options. Surgery, chemotherapy, and radiation all have a role. Outcomes data are often ambiguous, and the same tumor may be treated differently at different institutions, depending on the local expertise and experience. Cross-sectional imaging plays a central role in assessing the extent of disease, and in planning treatment strategies.

Anatomy

A detailed understanding of cross-sectional anatomy in the head and neck is critical in interpreting pretherapy imaging studies. Figure 21-1 details the anatomy of the paranasal sinuses, nasal cavity, and anterior skull base. Figure 21-2 details the anatomy of the upper aerodigestive tract.

Imaging strategy

Computed tomography (CT), magnetic resonance (MR), and F-fluorodeoxyglucose positron emission tomography (FDG-PET) are complementary imaging modalities, each with unique advantages. CT is widely available, making it a workhorse modality. The very rapid acquisition time of a CT study with modern scanners, on the order of seconds, makes CT an especially useful modality for patients who are unable to lie still for the approximately 45 minutes required for an MR examination. CT also depicts bony anatomy and cortical bone erosion better than any other modality.

MR provides superior soft tissue contrast, and distinguishes between tumor, edema, and muscle with greatest accuracy. MR has the highest sensitivity for bone marrow infiltration and perineural tumor spread, and is superior for assessing the extent of disease in the oral cavity, where artifact from dental amalgam often obscures the soft tissues. In the sinonasal region, MR readily distinguishes among tumor, edema, and retained secretions, where CT is often unable to do so. In the larynx, MR is more sensitive for laryngeal cartilage invasion, and probably equally specific, depending on the criteria that are applied.

FDG-PET, when combined with CT, is the best exam for nodal staging, and is the most effective way to detect disease recurrence in the posttreatment neck. Limitations include false positives due to physiologic uptake, inflammation, and infection. False negatives include small tumors, tumors that are obscured by areas of normal physiologic uptake, and malignant tumors that are not FDG avid.

Assessing local extent of disease

Local extent of disease is in many cases the most important factor that determines the initial treatment plan. Radiologic imaging and clinical examination are complementary in assessing the local extent of disease. Superficial mucosal tumor is better seen by direct inspection or endoscopy than by cross-sectional imaging. It may be impossible to pass an endoscope beyond a bulky mass to see the full extent of a mucosal mass. Although fixation to deep tissues on clinical examination can provide some information on the deep extent of disease, this finding is confirmed and often better detected with cross-sectional imaging. Invasion of bone, intracranial extension of disease, and perineural spread of tumor are findings that are typically discovered by cross-sectional imaging.

Nodal staging

For any head and neck tumor, the size and extent of the primary tumor must be determined, and the regional lymph nodes must also be assessed. Determination of lymph node involvement may be based on morphologic features or on size criteria. Lymph node size is determined in the axial plane, which typically bisects the long axis of the lymph node, as most cervical lymph nodes have a craniocaudal orientation. One may measure either the longest dimension in the axial plane, or the shortest dimension in the axial plane. For either measurement, 10 mm is considered the upper limit of normal. The exception to this is the highest level IIA lymph node, that is, the jugulodigastric node. A normal jugulodigastric lymph node may measure up to 11 mm in the shortest dimension in the axial plane, or up to 15 mm in the longest dimension in the axial plane. Regardless of size, a lymph node is abnormal if it demonstrates necrosis or extracapsular tumor extension ( Figure 21-3 ).

Lymph nodes are grouped by region. Superficial lymph nodes, such as facial chain, periparotid, intraparotid, preauricular, and occipital lymph nodes, are described by location. Retropharyngeal lymph nodes, which lie medial to the internal carotid arteries near the skull base, are grouped into medial retropharyngeal lymph nodes (uncommonly seen) and the more common lateral retropharyngeal lymph nodes (nodes of Rouviere). Cervical chain lymph nodes are grouped into seven levels, determined by anatomic landmarks (see Figure 21-2 ). If a lymph node overlaps two regions, it is assigned to the level that contains the majority of its bulk. Level I lymph nodes are anterior to the posterior edge of the submandibular gland. These may be further subcategorized into Level IA (medial to the anterior belly of the digastric muscle) and Level IB (lateral to the anterior belly of the digastric muscle). Level II lymph nodes are found posterior to the posterior edge of the submandibular gland, lateral to the internal carotid artery, anterior to the posterior edge of the sternocleidomastoid muscle, and superior the inferior edge of the hyoid bone. These may be divided into Level IIA (anterior to the internal jugular vein or touching the internal jugular vein), and Level IIB (posterior to the internal jugular vein). Level III lymph nodes are bordered superiorly by the inferior edge of the hyoid bone, medially by the medial edge of the common carotid artery, inferiorly by the inferior edge of the cricoid cartilage, and posteriorly by the posterior edge of the sternocleidomastoid muscle. Level IV lymph nodes are bordered superiorly by the inferior edge of the cricoid cartilage, medially by the medial edge of the common carotid artery, posterolaterally by a line drawn through the lateral edge of the sternocleidomastoid and the lateral edge of the anterior scalene, and inferiorly by the clavicle. If the clavicle is visible in the axial plane, then a lymph node is said to be in the supraclavicular station. Because of varying anatomy between individuals, the level IV station may be quite small or even nonexistent. Level VA lymph nodes are located posterior to the posterior edge of the sternocleidomastoid muscle, from the skull base to the inferior edge of the cricoid. Level VB lymph nodes are located posterolateral to a line drawn from the lateral edge of the sternocleidomastoids to the lateral edge of the anterior scalene muscles, below the inferior edge of the cricoid, and above the clavicle. Level VI lymph nodes are below the inferior edge of the hyoid bone, medial to the common carotid arteries, and superior to the manubrium of the sternum. Level VII lymph nodes are in the superior mediastinum, medial to the common carotid arteries, below the top of the manubrium of the sternum, and above the brachiocephalic vein.