THYROID: NODULES AND MALIGNANT TUMORS
- The incidental discovery of thyroid nodules must be reported, but this creates potentially large medicoeconomic burdens with little return in improved longevity and/or quality of life relative to the costs incurred. Therefore, follow-up plans must be rational and not based only on the nodule characteristics.
- Imaging with magnetic resonance and computed tomography is very useful to determine the primary tumor extent and regional nodal metastases.
- Fluorine-18 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) has a role that should be customized for each patient.
- Imaging aids significantly in treatment selection and surveillance for recurrence.
Thyroid Nodule Evaluation
Thyroid nodules are commonly detected on physical examination and even more commonly identified as incidental findings on computed tomography (CT), magnetic resonance imaging (MRI), radionuclide studies, and ultrasound examinations of the neck done for other purposes than evaluating the thyroid gland (Fig. 172.1). Risk factors that strongly suggest necessary biopsy or removal include a thyroid mass in a child or young adult, history of neck irradiation, family history of thyroid cancer, rapid growth, lymphadenopathy, or tracer accumulation on a fluorine-18 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) study (Fig. 172.1E–G). An older patient with a relatively soft, multinodular gland that has been stable for many years represents the other end of the spectrum. In the middle are sets of imaging criteria that are aimed at follow-up strategies in less clear-cut circumstances. In reality, many thyroid cancers will have no impact on a person’s life (Fig. 172.1). The socioeconomic and medical dilemma of how to manage incidentally discovered thyroid nodules is discussed in subsequent sections. It is not a simple issue at this point in time.
Thyroid Cancer Evaluation
Thyroid carcinoma is a relatively uncommon disease of highly variable clinical behavior. Many of these cancers will never threaten the life expectancy of the patient even if left untreated (Fig. 172.1A–D). Still, imaging is important for proper medical decision making, especially in cancers that have spread beyond the gland capsule.
ANATOMIC AND DEVELOPMENTAL CONSIDERATIONS
The developmental pathway of the thyroglossal duct cyst described in more detail in Chapter 170 contributes to rare cases of ectopic origin of mainly differentiated thyroid cancer (Figs. 170.5 and 170.6). Although rare, such cancer may occur in thyroglossal duct remnants or glandular ectopia anywhere along this migratory pathway from the foramen cecum at the upper tongue base to the isthmus of the gland.
During development, both lymphoid elements and squamous cells may be incorporated or are otherwise present in the gland, likely accounting for the rare primary squamous cell carcinomas of the gland that have occurred in the absence of a known primary tumor elsewhere in the head and neck. This may also be the source of primary thyroid lymphoma as well, although disease arises as a late complication of chronic thyroiditis.
Rests of normal ectopic thyroid tissue may be found in the lateral neck. These can be mistaken for metastatic disease. When found in cervical nodes, such deposits should most safely be considered metastases from well-differentiated thyroid cancer rather than a developmental condition.
The key anatomy of the thyroid gland and surrounding structures in the region that must be understood to adequately evaluate nodules and known thyroid cancers includes the following:
With regard to the evaluation of thyroid nodules:
- Normal texture of the thyroid gland at ultrasound and the normal appearance of lymph nodes at ultrasound (Figs. 172.1–172.4 and Chapters 4 and 149)
With regard to local extension and involvement of important surrounding structures:
- Complete understanding of the anatomy of the gland itself (Chapter 169) and the relationship of the thyroid gland to the organs including the larynx, trachea, hypopharynx, and esophagus (Chapters 169, 201, 209, 215, and 221)
- Developmental anatomy of the thyroglossal duct as it relates to the tongue base and visceral compartment of the neck (Chapter 170)
With regard to invasion of bony structures:
- Virtually never occurs in thyroid cancer, but adjacent cervical spine, clavicle, and manubrium of the sternum are rarely invaded in anaplastic carcinoma
With regard to possible routes of perineural and/or perivascular spread or functional deficits that may arise from involvement of surrounding nerves:
- Course of the vagus and recurrent laryngeal nerves, cervical sympathetic chain, and carotid sheath (Chapter 149); also, appearance of the larynx when the vagus or recurrent laryngeal nerves have been disrupted
With regard to regional lymph node involvement:
- Retropharyngeal nodes and cervical nodes (mainly levels 2 through 5 and in particular level 6) (Fig. 172.5); rarely involved are level 1, parotid area, posterior neck, and facial lymph nodes (Chapters 149 and 157)
Techniques and Relevant Aspects
The thyroid gland is studied in essentially the same manner as the infrahyoid neck is evaluated with MRI and CT. The principles of using these studies were reviewed in Chapter 149. Specific problem-driven protocols for MRI and CT are presented in Appendixes A and B.
The approach with radionuclide studies depends on the aim of the examination. Most of the current usage is limited to known or suspected thyroid cancer by a combination of radioiodide and FDG-PET for cancer evaluation. Specifics of the use of this physiologic imaging in relation to anatomic imaging are discussed in general in Chapter 5. The use of radionuclide studies in thyroid cancer and nodule evaluation needs to be carefully integrated with specific clinical decision making on a case-by-case basis so that resources are appropriately expended.
Pros and Cons
Thyroid Nodule Evaluation
In at-risk populations, the main diagnostic tools for triaging patients for surgery are ultrasound and needle biopsy. Radionuclide scans are usually not helpful since many nodules are hypofunctioning or “cold” relative to normal thyroid tissue. Ultrasound is used to determine whether the nodule is solitary or one of several or many. If a nodule is >75% cystic, it is likely to be benign. Nodules <10 mm are typically not biopsied. A nodule >1 cm with >25% solid component may be followed with ultrasound or biopsied, but that is not entirely foolproof1 (Fig. 172.6A–D). Such a follow-up protocol creates substantial economic burdens on the health care system for many of the thyroid cancers that, even when discovered as such, would not affect the quality of life or longevity of the patient if left alone (Fig. 172.1). The debate over this problem is considerable and reasonable.2 It will not be settled for some time, but perhaps will be by some emerging technology that will still have a cost burden for a problem that is largely, in reality, a nonthreatening problem.
Additional ultrasound parameters can be added to the risk assessment about which nodules need to be biopsied, but these still must be kept in a clinical context.3–8 A nodule in a 15 year old certainly has a different meaning than one in an 80 year old clinically, but does it ethically?9 Such additional factors include vascular characteristics of thyroid nodules, with those with perinodal vascular prominence being less at risk for malignancy than those with more central hypervascularity. These flow characteristics may also be added to pathoanatomic features including a halo, microcalcifications, relative dimensions in cross section, and echogenicity to improve the predictive models (Figs. 172.1–172.3 and 172.6A–D).
Thyroid Cancer Evaluation
CT and MRI suggest malignancy when there is extraglandular extension and/or lymph node metastasis (Figs. 172.5–172.13). Such imaging is most useful for establishing the extraglandular extent of a tumor prior to surgery. These studies map tumor relative to the critical anatomy in the low neck and thoracic inlet. Regional neck and mediastinal nodes are also evaluated by CT and MRI. Level 6, superior mediastinal, and retropharyngeal nodes cannot be evaluated by physical examination.
MRI is the primary study for establishing the local extent of a known cancer because it is superior to CT at showing esophageal and tracheal wall invasion; this local extension will likely much more influence therapy than nodal staging (Figs. 172.5–172.11). Also, iodinated contrast is necessary for a reasonably definitive CT study, and this may interfere with plans for diagnostic and therapeutic use of iodine-based radionuclides.
Any cancer discovered by ultrasound >3 to 4 cm in size may be studied with MRI and/or CT because of the likelihood of capsular penetration unless ultrasound assures that the mass is completely surrounded by normal thyroid tissue. Any tumor potentially associated with vocal cord weakness, dysphagia, or airway symptoms should also likely be imaged with MRI and/or CT (Fig. 172.6).
The specific uses of MRI or CT include the following:
- Local spread: Invasion of the tracheal cartilages connecting membranes and the membranous posterior wall is best determined by magnetic resonance (MR) (Figs. 172.6–172.13). Invasion of the esophageal muscular wall is best determined by axial T2-weighted and contrast-enhanced MR images (Fig. 72.12). Invasion of the laryngeal framework is best determined by CT but may be detectable by MRI (Fig. 172.13).
- Neurovascular involvement: The relationship of tumor to the carotid artery, course of the recurrent laryngeal nerve, and other major neurovascular structures in the low neck and thoracic inlet is best determined by MRI and/or CT (Fig. 172.14).
- Regional nodes: MRI and CT are useful in level 6, superior mediastinal, and retropharyngeal groups, none of which can be palpated; of these, only level 6 is accessible to ultrasound (Figs. 172.6–172.13). CT has a modest advantage over MR in detecting nodal metastatic disease.