Imaging in Thyroid Cancer

Introduction

Although thyroid cancer represents only approximately 2% of all malignancies worldwide, it is currently one of the most rapidly increasing malignancies in the Western world. Thyroid nodules can be palpated in 2% to 6% of patients and can be detected on imaging in approximately 50% of the general population. Approximately 5% of these nodules represent cancer. With newly available imaging modalities, more and more asymptomatic and nonpalpable nodules are detected, making it imperative that primary care doctors and radiologists understand how to interpret these diagnostic studies, understand indications for further referral, and manage long-term survivors of thyroid cancer. Most patients who have thyroid cancer have well-differentiated cancers, with an excellent long-term prognosis. Some patients have well-differentiated cancers with a poor prognosis, and some have other less common types of thyroid cancers. The challenge for the clinician is to identify the patients who have cancers, to treat them according to the extent and aggressiveness of their disease, and to limit morbidity and mortality.

Epidemiology and Risk Factors

Over the last few decades, the incidence of thyroid cancer has increased dramatically. It is unclear whether this increased incidence is real or whether it may be attributable to increased detection by newer diagnostic imaging techniques. Autopsy studies have shown that up to 11% of adults have incidental cancers at the time of death. The average age of diagnosis of thyroid cancer is 47 years, and the average age of death is 74 years. The most common type is the papillary thyroid cancer (PTC); it accounts for approximately 80% of all thyroid cancers. Follicular thyroid carcinoma (FTC) represents approximately 12% of cancers and, in combination with PTC, composes the category of well-differentiated thyroid carcinomas. Grouped into the less common thyroid cancers are medullary (4%–6%), anaplastic (3%–4%), and others (≤5%), such as lymphoma, sarcoma, squamous cell carcinoma, and metastases to the thyroid.

The most significant environmental risk factor for developing thyroid cancer is prior exposure to ionizing radiation, which can be the result of either medical treatment or nuclear fallout (e.g., atomic bomb/testing survivors, nuclear energy accidents). The effects of ionizing radiation are most pronounced in children, especially those younger than 10 years old at the time of exposure. The latency period of developing cancer from this exposure is approximately 10 years for patients having external beam radiation exposure to less than 5 years for victims of the Chernobyl nuclear accident in the Ukraine. The increased risk persists for 30 to 40 years. Patients presenting with benign thyroid disease may also be at higher risk of harboring a malignant nodule; for example, a cold nodule found on radionuclide scanning in patients with Graves disease may be malignant 15% to 38% of the time, and complex cysts in thyroid disease may harbor a malignancy approximately 17% of the time. Given that, overall, two-thirds of thyroid cancer cases occur in women, there would seem to be a link between reproductive hormones and the development of thyroid cancer. Estrogen has been linked to genomic instability, which may explain how it exerts its mutagenic effects on the thyroid. There are no definite data on the role of dietary iodine and its role in the development of thyroid cancer. Certain genetic syndromes increase the risk for thyroid cancer or are associated with thyroid cancer, especially medullary thyroid cancer (MTC), which has been linked to several specific genetic abnormalities. In some cases, PTC follows a familial pattern. Several rare genetic disorders, including Cowden disease, multiple endocrine neoplasia (MEN), and Gardner syndrome, are also associated with a higher incidence of thyroid cancer.

Anatomy

The thyroid is a shield-shaped gland that consists of right and left lobes connected by the isthmus in midline, although occasionally, the isthmus can be absent. The thyroid isthmus is anterior to the trachea, usually overlying the first through the third tracheal rings. The thyroid gland typically terminates above the level of the clavicle; however, substernal extension into the superior mediastinum can occur. An accessory lobe, the pyramidal lobe, may be present in 50% to 70% of people and usually arises from the isthmus and extends superiorly. The visceral fascia, part of the middle layer of the deep cervical fascia, attaches the thyroid gland to the larynx and trachea. As a result, the gland or abnormalities related to it will move with the larynx during swallowing. The arterial supply to the gland is derived from two separate paired vessels. The inferior thyroid vessels come directly off of the thyrocervical trunk and supply the inferior part of the gland, as well as both the superior and the inferior parathyroids. The superior thyroid artery arises as the first branch of the external carotid artery and supplies the superior portion of the gland. Rarely, a small artery or pair of arteries may come directly from the aorta or brachiocephalic trunk, named the thyroid ima artery. There are three main paired veins that drain the thyroid. There is some variability in location and presence, but they generally drain into the internal jugular veins and innominate vein. The lymphatics of the thyroid consist of intraglandular and extraglandular components. Extraglandular lymphatics generally follow venous flow; the inferior portions of the lateral lobes drain along the tracheoesophageal groove into the central neck. The superior parts of the lobes drain toward the superior thyroid veins, and the isthmus may drain toward the delphian (prelaryngeal) lymph node or central neck nodes. More unusual, but clearly documented, are lymphatic pathways to the retropharyngeal region, accounting for metastases to the skull base. Based on clinical and anatomic review, the central lymphatics are generally considered the primary drainage pathways for thyroid cancers, with the lateral neck nodes being considered secondary levels of lymphatic spread. These facts are important from an imaging and a treatment perspective. The parathyroid glands are also closely related to the thyroid anatomically. They tend to lie on the undersurface of the thyroid and receive their blood supply from the inferior thyroid artery.

Pathology