IMAGINC OF THE HEAD AND NECK: A STATE-OF-THE-ART OVERVIEW 0030-6665/95 50.00+ .20 THYROID AND PARATHYROID GLAND PATHOLOGY Role of Imaging David M. Yousem, MD, and Alice M. Scheff, MD THYROIDIMAGING Lesions of the thyroid gland represent some of the most heteroge- neous abnormalitiesin the head and neck. Depending on the clinical pres- entation and suspected abnormality, the clinician may order nuclear med- icine scintigraphy, ultrasonography, computed tomography (CT), or magnetic resonance(MR) imaging as the first imaging modality. Clearly, the work-up for an infant with a midline cystic mass in the lower neck is entirely different from that for a 40-year-otdmale with a solitary palpable nodule in the thyroid gland. Therefore,it is important to consider lesions of the thyroid gland according to neoplastic, congenital, and inflamma- tory categories.Within each of these categories,imaging is discussed. Neoplasmsof the Thyroid Gland To assist the clinician faced with a solitary nodule in the thyroid gland, imaging has a well-defined role. Invariably, unless the classicap- pearanceof a benign condition is present,fine-needle aspiration or biopsy is required, becausethe appearanceof most thyroid malignancies may From the Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania OTOLARYNGOLOGIC CLINICS OF NORTH AMERICA VOLUME 28. NUMBER 3 . IUNE 1995 621 622 YOUSEM&SCHEFF simulate benign processes, and vice versa. For example, calcification oc- curs in 13% of all thyroid lesions, including 17% of all malignancies and 11"/" of all benign processes.s3In a similar fashion, cystic components of a lesion occur in 24% of thyroid masses; 38% of malignancies and 62o/"of benign masses may be wholly or partly cystic.s3Most adenomas and car- cinomas have solid components also. Hemorrhage may be found in pap- illary carcinomas, degenerated nodules, or goiters. It is difficult to pin- point a specific histologic diagnosis despite these imaging findings. Thyroid cancers are a mixed group of lesions. The most common histologic subtype, however, is papillary carcinoma, which occurs in 55% Io 75"/" of thyroid malignancies.4l,58Mixed papillary-follicular carcinomas (follicular variant of papillary carcinomas) are generally placed in the pap- illary carcinoma category, because they behave clinically similarly to pure papillary carcinomas. Purely follicular carcinoma occurs in 10% to 15o/", anaplastic carcinoma in 5'h to 15"/o,and medullary carcinoma in 5% of cases. Anaplastic carcinoma and medullary carcinoma do not take up io- dine usually, but may be thallium or gallium avid. Somatostatin receptor scintigraphy may also detect medullary carcinoma.s Medullary carcinoma may present in association with the multiple endocrine neoplasia (MEN) slmdromes and serologically may express calcitonin. Other histologic di- agnoses to consider in thyroid malignancies are non-Hodgkin's lym- phoma and metastases. The detection and characterization of a thyroid neoplasm is not the only role of imaging. Imaging should also evaluate for infiltration of ad- jacent soft tissue, paraspinal musculature, and adjacent vessels. The pres- ence or absence of adenopathy is important both for its prognostic impli- cations with thyroid cancer and for distinguishingbenign from malignant processes. Papillary carcinoma is the lesion with the greatest likelihood of spread to lymph nodes, and the nodes may be tiny, cystic, hemorrhagic, or calcified. After the lymph nodes, papillary carcinomas may metastasize to the lungs, bone, or central nervous system (CNS). The coexistence of parathyroid abnormalities in patients who have neoplasms or masses of the thyroid gland is also important. Parathyroid adenomas or hyperplasia may accompany medullary carcinoma as part of the MEN IIa or MEN IIb complex (along with pheochromocytomas). Ultrasound Ultrasound, because of its accessibility, low cost, noninvasiveness, and ability to distinguish cystic from solid lesions, is often the first mo- dality employed to evaluate a thyroid mass in the euthyroid patient. Good-quality ultrasound requires transducers with frequencies of 7.5 to 10 MHz.13.s8 This allows excellent detail of the superficial gland and enough penetration to evaluate as deep as the spine. When a solid lesion is hyperechoic, the likelihood of malignancy is only 4"/o.53If a solid lesion is isoechoic, the likelihood of malignancy in- creases to 26"/".If it is hypoechoic, the likelihood of malignancy is 63o/o.53 Papillary carcinoma most commonly presents as a solid hypoechoic(77"/") rather than isoechoic Qa%) lesions3 (Fig. 1). On the other hand, follicular THYROID AND PARATHYROID GLAND PATHOLOGY 623 Figure1. Unusual hyperechoic papillary carcinoma of the thyroid gland. This longitudinalview of theright thyroid gland demonstrates a well-defined hyperechoic mass (ar,'ot4ls). The lesion is echogeniccompared with the surroundingthyroid tissue (I). Biopsyproved a papillary carcrnoma. carcinoma is solidly isoechoic n 52% and hypoechoic in 44o/" of cases. Anaplastic and medullary carcinomas and lymphoma are most commonly hypoechoic.l3, ls's3 Echogenic foci due to deposits of calcium may be seen iri primary sites and metastatic lymph nodes of medullary carcinoma'1s A lesion with echogenic foci (calcifications) in a solid nodule protruding into a cyst suggests cystic papillary carcinoma.ln If one looks at the margins of tumors on ultrasound, one finds that 16o/"of malignant lesions have sharply marginated, well-defined borders, and that irregular or ill-defined borders occur in approximately 60"/oof cancers.s3Unfortunately, irregular or ill-defined borders also occur in ap- proximately 45% of benign lesions.s3 A lesion surrounded by complete halo of echopenia makes it 12 times more likely to be benign than malig- nant (Fig. 2):n If the halo is incomplete, however, a benign etiology is only 4 times more likely than a malignant cause.s3 Ultrasound is usually not as helpful as cross-sectional imaging tech- nigues in the detection of soft tissue, esophageal, tracheal, and vascular infiltration. Lymph node metastases are also better imaged with other modalities. One strong point of ultrasound is the ability to perform guided bi- opsies. Fine-needle aspirations have over 95/" accutacy when directed by imaging. Ultrasound, because of its interactive real-time capability, is the Figure 2. Echopenichalo signifying benign lesion on ultrasound.This longitudinalview of the thyroidgland demonstrates an echopenichalo (arrows) around a largethyroid mass. An intact, completeechopenic halo suggests a benignprocess. study of choice in this regard. The needle tip can be followed ultrasono- graphically as it enters a suspicious abnormality. This advantage is es- pecially useful in postoperative patients or in patients without palpable abnormalities.ss Nuclear Medicine The agentsused for thyroid imaging include iodine 123(1123), iodine 131(I131), technetium(Tc 99m) pertechnetate,and thallium 201(T1201). The half-lives, whole-body dosesof radiation and thyroid doses of radi- ation are listed in Table 7.41'67 Scanningis performed 15 minutes after administration of technetium Table 1. ScintigraphicAgents Used for Thyroidlmaging Total Body Dose to Doseand Route Agent Half-Life Dose (rads) Thyroid Gland (rads) Administered | 123 13.6h 0.03 6-12 50-200pCi, PO I 131 8.05days 120-800 30-100pOi, PO Tc 99m 6.02 h 0.02 0.04-0.3 2-3 mCi,lV Thallium201 3 days 0.21 0.92 2-3 mCi,lV p",,".t'''",ute,4hoursdJ.;)"^:ff;::;Ht"lffi:Jttfr: and 24 to 72 hours after administration of I 131 agents.Scanning is per- formed 5 to 10 minutes after thalliurn 207 administration. Becausethe radiation energy of I L31 is so high (364keV), it is the preferred agent to image substernalthyroid glands or to image the whole body after thyroid ablalion in order to detect metastatic foci of thyroid cancer. The other agentshave energiesof 140keV (Tc 99m),1.59keV (I 1'23),and 80 keV (Tl 201).6? A major role of scintigraphy in the evaluation of a thyroid mass is the determination whether the lesion is "hot" (more uptake than the nor- mal thyroid gland), "warrn" (some activity but not as much as the normal gland), or "Cold" (hypofunctioning). The risk of cancer in a hot nodule is 1% to 4"/o,in a warrn nodule 8o/"to 10o/o,and in a cold nodule 15'/. to 25o/o.41,44,4e By Iar the majority (90%)of solitary hot nodules,on scintigraphy are benign in etiology, usually adenomas (Plummer's diseaseis hyperthy- roidism due to a solitary hot nodule) or hyperplasias that are expressing thyroid hormoneal(Fig. 3). The difference between an autonomous and a hypertrophic, functional hot nodule depends on the rgqponseto a thyroid suppressiontest. After a diagnostic course of thyroid hormone adminis- tra-ti,on,a lesion that is persistently hot on a Tc 99m pertechnetatescan is consideredan autonomous lesion, whereas a previously hot lesion that is now cold is considered hypertrophic.a Other sourcesof hot nodules are thyroiditis,- normal variation in thyroid functiory and ectopic tis-sue'41 A cold nodule is approachedmore aggressivelybecauseof the higher incidence of malignaniy. A biopsy or aspiration is often in the diagnostic algorithm. In a pitient'who has a prior history of head and neck irradi- Figure 3. Hot noduleon nuclearscintigraphy. This technetium99m pertechnetatescan dem- onitratesa solitarymass (m) with high uptake in the rightthyroid gland. Note that its increased activityleads to suppressionof the remainingthyroid gland. A hyperfunctioninghot nodule with hyperthyroidismis suggestiveof Plummer'ssyndrome.