13 Cross-sectional Imaging Abstract Cross-sectional imaging has become an integral part of the workup, treatment planning, and surveillance for patients with neoplasms arising within or otherwise involving the head and neck and directly impacts decision-making regarding neck dissections. This chapter is organized around imaging anatomy as it applies to neck dissection, imaging approaches to staging, and surveillance imaging. Much of the discussion revolves around in computed tomography (CT), but MRI and combined CT/PET strategies are also briefly touched upon. Keywords: CT, MRI, neck dissection, lymphadenopathy, head and neck cancer, squamous cell carcinoma Imaging plays an integral role in the staging, treatment planning, and surveillance for squamous cell carcinoma (SCC) and other malignancies occurring in the head and neck. Advances in CT and MRI in recent years have led to improvement in each of these imaging modalities as they apply to the management of head and neck cancer including their role in determination of the necessity of neck dissection. Modern imaging and its interpretation are challenging endeavors due to the complexity of the hardware and software technologies that form the basis of image generation. Although this chapter will cover some important technical considerations that impact image acquisition and interpretation, the bulk of the material will focus on how imaging can be applied to maximize diagnostic certainty while at the same time maintaining efficiency and cost-effectiveness. Discussion of neck anatomy as it applies to the search for, and description of, cervical lymph node disease is covered briefly herein and more extensively in other chapters. Rather than organizing this chapter around imaging modalities, an outline based on imaging features of nodal metastases across multiple imaging modalities will instead be followed. Given its high prevalence relative to other malignancies, the bulk of the text is based on data from studies of nodal disease attributable to SCC. Nevertheless, many of the concepts such as imaging features indicative of lymph node metastases are broadly applicable across a wide spectrum of neoplasms. In the face of ever-evolving imaging techniques applied to head and neck malignancy, the goal has been to collate peer-reviewed evidence from the past decade. That said, in some cases it has been necessary to cite earlier work upon which more recent investigations have built. After a short synopsis of imaging-based neck anatomy, the bulk of this chapter will focus on the imaging approach to neck staging and thereafter conclude with a discussion of the imaging approach to surveillance of the neck. Image-based classification of cervical lymph nodes in the context of head and neck malignancy has been widely adopted due to the ubiquity of imaging as part of cancer staging for most patients, the ability of imaging to detect clinically occult nodal metastases, and the high level of reproducibility with which modern cross-sectional imaging can localize metastatic nodes in relation to anatomic landmarks.1 In 2000, Som et al proposed the most widely utilized radiologic classification system for cross-sectional imaging assessment of cervical lymph nodes, which complements the clinically based classification espoused by the American Joint Committee on Cancer and the American Academy of Otolaryngology–Head and Neck Surgery.1 In this system, landmarks forming the borders of each nodal level are based on axial image sections. Level I includes lymph nodes superior to the hyoid bone, below the mylohyoid muscle, and anterior to the posterior borders of the submandibular glands. Levels II and III refer to lymph nodes that reside anterior to a transverse line along the posterior borders of the sternocleidomastoid muscles. Level II nodes are posterior to the posterior margin of the submandibular gland, lie between the skull base and inferior margin of the hyoid bone, and are positioned lateral to the medial border of the internal carotid artery. Below level II, level III describes a contiguous compartment that extends inferiorly to the lower border of the cricoid cartilage. Further inferiorly, level IV extends to the level of the clavicle. In contrast to levels II and III, level IV nodes lie anterior and medial to an oblique line drawn through the posterior edge of the sternocleidomastoid muscle and the posterior lateral edge of the anterior scalene muscle. Level V describes nodes that are dorsal to the posterior border of levels II to IV and lie anterior to a transverse line through the anterior edge of the trapezius muscles. Level VI refers to lymph nodes that lie inferior to the lower body of the hyoid bone, superior to the upper border of the manubrium, and medial to the medial borders of the common/internal carotid arteries. Level VII lymph nodes lie between the upper border of the manubrium and brachiocephalic veins and are also medial to the medial borders of the common carotid arteries. CT-based examples of the Som classification scheme are available in their original publication.1 Anatomic localization of suspected nodal metastases is important not only for staging and treatment planning, but can also provide guidance for further investigation if the site of primary neoplasm is not readily apparent. For example, nodal metastases isolated to level II would draw attention to the oral cavity or oropharyngeal mucosal space rather than the thyroid gland, whereas the finding of abnormal nodes confined to levels VI and VII would be much more likely to emanate from a thyroid primary. In cases in which the primary neoplasm is known, such knowledge can direct scrutiny to first-level nodal drainage areas and thus maximize the detection of early or subtle signs of metastatic involvement of cervical lymph nodes. Although size is frequently invoked in the discussion of lymph node metastases, it is a poor marker of metastatic involvement relative to other factors discussed later. At many centers, lymph node size is reported as long-axis diameter on an axial image section and may also include a long-axis measurement in another plane if such a measurement would impact staging. Long-axis measurements, in contrast to short-axis measurements that are employed at other locations such as the mediastinum, best replicate the clinical determination of palpable lymph node enlargement. Measurement in the long axis is the most frequently employed methodology across the literature and is the metric utilized in the TNM classification of the American Academy of Otolaryngology–Head and Neck Surgery. Underlying the limitations of lymph node size as a primary determinant of metastatic involvement is the frequency of pathologic confirmation of metastases in lymph nodes of normal size ( Fig. 13.1 Axial image from a contrast-enhanced CT scan in a patient with a history of laryngeal carcinoma reveals tumor recurrence (dashed arrow) along the ventral wall of the neopharynx reconstruction. A 4-mm lymph node within the nearby subcutaneous compartment was positive for metastases at resection despite its small size (solid arrow). Poorly defined margins and central low attenuation indicative of necrosis were prospectively suggestive of metastatic involvement.
13.1 Introduction
13.2 Imaging Anatomy
13.3 Imaging Approach to Neck Staging
13.4 Lymph Node Characteristics Related to Metastatic Involvement
13.4.1 Size
Fig. 13.1). As an example, in a study of subjects with head and neck SCC, Don et al found that 67% of metastatic nodes had a longitudinal diameter smaller than 1 cm.2 Lymph node enlargement may be a late manifestation of metastatic involvement that is proceeded by heterogeneous enhancement or other more sensitive imaging biomarkers.3 The specificity of lymph node size as an indicator of metastatic involvement is lacking due to the propensity of lymph nodes to become enlarged as a response to infection and inflammation. The existence of multiple disagreeing size criteria and differences in accepted size thresholds based on location and patient age further confounds the application of size to the staging assessment.3 For instance, the application of a 1-cm threshold within levels II and VI would yield a substantial difference in the specificity of nodal metastases, as nodes measuring above 1 cm are frequently encountered in level II of normal subjects. As such, many practitioners use a tiered threshold in which the size threshold for levels I and II is higher than that for more caudal levels. Asymmetry can serve as another useful discriminator regarding the significance of lymph node enlargement, whereby enlarged lymph nodes that are unilateral and/or confined to a single anatomic level are more likely to be true positives than cases in which nodal enlargement is more widespread (
Fig. 13.2). Despite the suboptimal sensitivity and specificity of size as a marker of nodal metastases, lymph node enlargement can be a useful tool toward identification of nodes requiring further scrutiny, but as an isolated feature it is not necessarily sufficient for the determination of metastatic involvement in the absence of additional markers of nodal metastases such as irregular margins or internal necrosis.
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