Atlas of thyroid cytopathology

INTRODUCTION — Fine-needle aspiration (FNA) biopsy and fine-needle capillary (FNC) sampling are the two most widely used diagnostic techniques for assessing the cytopathology of thyroid nodules to identify those patients who have nodules that should be excised. This topic will review the most common cytopathologic findings and the clinical entities they represent. Thyroid biopsy techniques (including core needle biopsies) and an overview of the diagnosis and management of thyroid nodules are discussed separately.

●(See "Thyroid biopsy".)

●(See "Diagnostic approach to and treatment of thyroid nodules".)

●(See "Evaluation and management of thyroid nodules with indeterminate cytology in adults".)

FNA CYTOLOGY CLASSIFICATION SCHEME — The National Cancer Institute Thyroid Fine-Needle Aspiration (FNA) State of the Science Conference ("Bethesda Conference") suggests the following six major cytologic categories, each of which indicates a different subsequent evaluation and management (table 1) [1]:

●I. Nondiagnostic

●II. Benign

●III. Atypia of undetermined significance (AUS)

●IV. Follicular neoplasm

●V. Suspicious for malignancy

●VI. Malignant

AUS, follicular neoplasm, and suspicious for malignancy are considered indeterminate cytopathologic categories by the Bethesda system as the nodule is not definitively benign or malignant. However, most experts suggest surgery for Bethesda 5 "suspicious for malignancy." Many analyses have not included Bethesda V when assessing indeterminate biopsies and molecular testing is not well validated for use in Bethesda V. (See "Evaluation and management of thyroid nodules with indeterminate cytology in adults", section on 'Evaluation and management'.)

The terms used by different cytopathologists to describe follicular thyroid nodules vary (table 2). It is essential that clinicians interpreting these reports be familiar with the terminology used by their cytopathologist.

Nondiagnostic — A nondiagnostic sample is inadequate (eg, insufficient number of follicular cells, specimen obscured by blood, cyst fluid only) [2]. The nondiagnostic rate varies from 3 to 10 percent at experienced centers. It is critical that the absence of malignant cells not be interpreted as a negative biopsy if no follicular tissue is obtained. For nondiagnostic biopsy results, we typically repeat the FNA biopsy under ultrasound guidance. This is consistent with the American Thyroid Association (ATA) guidelines [3]. A cutting-needle biopsy under ultrasound guidance may also help after repetitive nondiagnostic FNAs. (See "Thyroid biopsy", section on 'Other techniques'.)

Benign — Common entities in this category are:

●Normal thyroid tissue

●Nodules from adenomatous or multinodular goiters

●Chronic lymphocytic (Hashimoto's) thyroiditis

●Subacute granulomatous thyroiditis.

Normal thyroid tissue, nodules from adenomatous or multinodular goiters, and some true monoclonal tumors can have a macrofollicular pattern on cytology (picture 1A-B). This pattern may also be referred to as adenomatoid, hyperplastic, macrofollicular, or colloid adenomas (table 2). The risk of malignancy with this classification (based on long-term follow-up studies) is approximately 1 to 2 percent [1].

Benign follicular nodule — Cytology of macrofollicular nodules may show both intact macrofollicles and macrofollicles that have been broken apart, forming a flat sheet (picture 1A). The cells appear uniform in size and are not crowded together. Follicle size may vary, with a few microfollicles interspersed among the macrofollicles, especially if the sample was obtained from an area close to the capsule of the lesion. At high power, the follicular cells are seen to be small, uniform in size, flat, and non-crowded, and smeared colloid is seen in the background (picture 1B).

Macrofollicular nodules may have areas of cystic degeneration with cellular debris and hemosiderin-laden macrophages. There may be focal oncocytic (Hürthle cell) change and, rarely, multinucleated giant cells. Colloid, which stains blue on a Papanicolaou stain, is usually abundant in the background of macrofollicular lesions (picture 2). Colloid may also be obtained from colloid cysts, and those smears may have few or no intact follicular cells.

Surgical specimens of sporadic goiter often contain many such macrofollicles (picture 3). During needle aspiration, the follicles are frequently disrupted so that the colloid is smeared across the slide or occasionally is aggregated into droplets. The follicular cells lining the follicles may remain intact or break apart and form sheets.

Hashimoto's thyroiditis — Since the diagnosis of Hashimoto's thyroiditis can be made in most patients on the basis of the clinical presentation and laboratory findings, FNA should not be performed unless a discrete nodule is present.

There is considerable histologic variability in both FNA and histologic specimens from Hashimoto's thyroiditis. In surgical specimens, some sections are composed of normal-appearing follicles with minimal lymphocytic infiltration, while other areas show complete destruction of follicles with dense lymphocytic infiltration and lymphoid germinal centers (picture 4). Plasma cells are commonly seen, as are oncocytic changes in the remaining follicular cells, and fibrosis may be prominent.

Each of these findings may be the major cytologic feature in fine-needle aspirates. In some patients, the aspirates consist mostly of lymphocytes surrounding rare follicular cells (picture 5). Since blood lymphocytes contaminate most FNA specimens, the diagnosis of thyroiditis can be challenging. However, lymphocytes embedded within tissue are more fragile and tear easily when smeared (picture 6).

Some aspirates are devoid of any follicular cells and may be read as nondiagnostic. Nevertheless, a presumptive diagnosis of Hashimoto's thyroiditis can be made if other cytologic features are present, such as lymphocytes, plasma cells, germinal centers, and fibrotic follicles and if the clinical presentation is compatible with this disorder. One potential problem is that papillary thyroid cancer frequently has areas of lymphocytic infiltration, sampling of which can occasionally lead to false-negative FNA results interpreted as Hashimoto's. Another potential problem is that the oncocytic changes may result in a false-positive interpretation of an oncocytic tumor. In addition, it may be necessary to do flow cytometry when cytology specimens show only lymphocytes to exclude the possibility of a well-differentiated lymphoma.

Subacute granulomatous thyroiditis — Subacute, granulomatous thyroiditis (also called subacute or de Quervain's thyroiditis) is usually a clinical diagnosis. This disorder is typically a viral or postviral syndrome characterized by fever, malaise, and an exquisitely tender thyroid gland. Because the gland may be quite firm, concern about cancer may lead to FNA. Multinucleated giant cells, macrophages, and degenerated follicular cells are seen. (See "Subacute thyroiditis".)

Atypia of undetermined significance (AUS) — This category includes lesions that are not convincingly benign but do not have either definitive features of a follicular neoplasm and are not highly suspicious of malignancy (table 2). It is subdivided into nuclear atypia and other atypia. Other atypia includes architectural atypia (previously referred to as follicular lesion of undetermined significance), oncocytic atypia, and lymphocytic atypia, among others.

●Nuclear atypia – Mild nuclear atypia may be concerning for papillary thyroid cancer or follicular variant papillary thyroid cancer or other malignancies, with changes that are insufficient to identify the biopsy as suspicious (Bethesda V) or malignant (Bethesda VI).

●Architectural atypia – Architectural atypia refers to lesions that have a mixed microfollicular and macrofollicular pattern where the proportion of microfollicles and macrofollicles is similar. Such lesions may be benign adenomas, follicular thyroid cancer, follicular variant papillary cancer, or noninvasive follicular tumors with papillary-like nuclear features (NIFTP). There are insufficient microfollicles or cellular crowding to warrant the classification of follicular neoplasm (Bethesda IV).

●Oncocytic atypia – This refers to lesions with extensive oncocytic (Hürthle cell) change, but insufficient architectural atypia or cellular crowding to be considered an oncocytic neoplasm (Bethesda IV). Oncocytic neoplasms may be benign adenomas or carcinomas, but oncocytic atypia is seen most commonly in Hashimoto's thyroiditis and as reactive oncocytic change in a degenerating nodular or cystic component of a nodular goiter.

The risk of malignancy with this classification (based on surgical resection data) ranges from 13 to 30 percent [1]. AUS with nuclear atypia has a higher risk of malignancy compared with architectural atypia or oncocytic atypia [4,5]. In one meta-analysis of published reports citing risk of malignancy, the average malignancy risk was 46 percent for nuclear atypia, 22 percent for architectural atypia, and 13 percent for oncocytic atypia [5].

The original conference intended that no more than 7 percent of preparations would fall into the Bethesda III category, but the use of Bethesda III by cytopathologists has been variable, with some centers reporting its use in up to 29 percent of cytology readings [6]. In one multicenter analysis, the introduction of Bethesda III to diagnostic cytopathology reports (prior to the use of molecular testing) resulted in a dramatic increase in surgical referrals from 9 to 37 percent [7]. Following the introduction of the gene expression classifier, however, a 10-fold reduction in surgical referrals for indeterminate thyroid nodules was reported [8]. (See "Evaluation and management of thyroid nodules with indeterminate cytology in adults", section on 'Molecular markers'.)

It should be noted that there may be substantial variability among cytopathologists in classifying FNA cytology in this category (AUS) [9]. In one study, second review changed the Bethesda classification in 32 percent of thyroid aspirates [10]. Thus, a second review of indeterminate biopsies (AUS or follicular neoplasm) may be warranted, especially at low-volume centers.

Follicular neoplasm — As many as 15 to 30 percent of all biopsies are classified as follicular neoplasms [2,11-14]. The category of neoplasm includes microfollicular or cellular adenomas. This pattern may also be referred to as cellular or indeterminate. Microfollicles are easily identified in specimens obtained by FNA (picture 7). Colloid is absent or scant, and the cells are more crowded than in macrofollicular nodules and may form clusters. Well-developed microfollicles, crowding, and scant colloid is the least worrisome pattern. More suspicious is the absence of follicle formation, cells arranged in clusters and clumps, and varying nuclear atypia and cellular pleomorphism (picture 8).

Microfollicular or cellular neoplasms may represent any of the following:

●Benign adenomas

●Autonomously functioning (benign) adenomas

●Well-differentiated follicular thyroid cancers

●Follicular variant of papillary thyroid cancers

●Follicular neoplasm with oncocytic features (previously called Hürthle cell neoplasm)

●NIFTP

Autonomously functioning thyroid nodules (which are virtually always benign) may rarely appear to be suspicious for a follicular neoplasm [15].

Follicular neoplasm with oncocytic features (previously called Hürthle cell neoplasm) may represent benign adenomas or oncocytic carcinoma. However, in one study, the risk of malignancy in microfollicular oxyphil cell lesions was twice that of nonoxyphil lesions (32 versus 16 percent) [16]. Oncocytic cells are large polyclonal cells with abundant oxyphilic cytoplasm. Sheets of oncocytic cells are seen in oncocytic cell adenomas (picture 9).

Focal oncocytic cell change can be seen in both degenerating macrofollicular lesions and Hashimoto's thyroiditis. Because of limited sampling, scant FNA specimens from these disorders may show apparent sheets of oncocytic cells and can result in a false-positive cytologic diagnosis of a oncocytic cell tumor.

Surgical excision and extensive sectioning are required to determine if a follicular neoplasm (microfollicular neoplasm) is benign or malignant. Microfollicular nodules that do not show vascular or capsular invasion after surgical excision are considered benign, while those that demonstrate even focal microscopic invasion are classified as follicular carcinomas (or follicular variant papillary cancers, or oncocytic carcinomas). Surgically excised microfollicular adenomas show well-developed microfollicles with considerably less colloid than is seen in macrofollicular nodules (picture 10). This pattern of well-developed microfollicles was previously referred to as a "fetal adenoma" as it is reminiscent histologically of the fetal thyroid. However, if invasion is seen, then, as noted, the pathologic diagnosis is follicular cancer (see "Follicular thyroid cancer (including oncocytic carcinoma of the thyroid)"). Lesions with less definite or no follicle formation are more likely to show vascular or capsule invasion.

Postsurgical evaluation reveals a malignancy rate of approximately 23 to 34 percent [1]. The difficulty in accurately diagnosing malignancy in a microfollicular adenoma is illustrated by a pathologic re-review of 185 follicular neoplasms excised 10 years previously [17]. Twenty-five percent had a change in diagnosis: 35 from benign to malignant and 11 from malignant to benign. The most common change in diagnosis was from benign to follicular variant papillary cancer. (See "Papillary thyroid cancer: Clinical features and prognosis", section on 'Follicular variants'.)

Several approaches are currently in use to improve upon cytology alone for the assessment of follicular lesions [18-22]. The routine use of molecular analysis on indeterminate FNA aspirates (follicular neoplasm, AUS) reduces substantially the number of patients who require diagnostic thyroid surgery. The use of molecular markers in the assessment of patients with indeterminate cytologic results is reviewed in detail separately. (See "Evaluation and management of thyroid nodules with indeterminate cytology in adults", section on 'Molecular markers'.)

Suspicious for malignancy — This category includes lesions with some features suggestive of, but not definitive for, thyroid cancer. Typically, nodules in this category have a 67 to 83 percent risk of malignancy [1]. This risk is sufficiently high to warrant surgical resection. It has been argued that molecular testing may help inform the type of procedure (lobectomy versus thyroidectomy), but this is controversial. In the United States, health insurance may not cover molecular testing in Bethesda category V.

Malignant — The malignant category includes papillary cancer, medullary cancer, thyroid lymphoma, anaplastic cancer, cancer metastatic to the thyroid, and lymphoma. It does not include follicular or oncocytic thyroid cancer. The diagnosis of follicular or oncocytic thyroid cancer is made on the basis of capsular or vascular invasion, which cannot be identified in specimens obtained by FNA. (See 'Follicular neoplasm' above.)

Papillary cancer — The cytologic diagnosis of papillary cancer is usually straightforward. The architecture is papillary, the cells are large, their cytoplasm has a "ground glass" appearance, nucleoli are prominent, and the nuclei demonstrate clefts and grooves and intranuclear cytoplasmic inclusions ("Orphan Annie eyes") (picture 11). Psammoma bodies (picture 12) and dense "sticky" colloid (picture 13) may also be seen. Although colloid is scant in this disorder, the colloid that is present appears to "stick" to debris and cell clusters, instead of smearing across the slide. (See "Papillary thyroid cancer: Clinical features and prognosis", section on 'Histology'.)

Epithelioid giant cells can be seen in papillary carcinomas but are also frequently seen in degenerating areas of macrofollicular nodules and in subacute granulomatous thyroiditis (picture 14). However, patients with the latter disorder rarely undergo FNA, because the clinical manifestations of subacute granulomatous thyroiditis are so characteristic that FNA is rarely indicated. (See "Subacute thyroiditis".)

In patients with papillary cancer, the surgical specimen typically shows a papillary structure, with or without follicles, and little or no colloid (picture 15). In follicular variant papillary cancer, only follicular development is seen. The diagnosis of papillary cancer then is based upon the cytologic features of the cells, the presence or absence of a capsule, and capsular or vascular invasion:

●A noninvasive, encapsulated, well-circumscribed follicular lesion with papillary-like nuclear features was previously referred to as a noninvasive follicular variant papillary cancer, subtype (NI-FVPTC), but is now called a NIFTP [23] and is considered to be benign.

●A follicular tumor subtype with papillary nuclear features and with either invasion (capsular or vascular invasion) or lacking a well-defined capsule is a follicular variant papillary cancer.

This nomenclature suggests that encapsulated, noninvasive follicular variant papillary cancers not be labeled a malignancy [23]. These tumors are treated similar to a follicular neoplasm that has not invaded its capsule or blood vessels and is called a follicular adenoma rather than a follicular cancer. Importantly, these NIFTP can only be diagnosed after surgical excision. (See "Papillary thyroid cancer: Clinical features and prognosis", section on 'Follicular variants'.)

Medullary cancer — Cytologists will usually recognize FNAs from medullary thyroid cancer as suspicious but will not always make the diagnosis preoperatively [24]. The cells frequently disperse and are characterized by eccentrically placed nuclei and cytoplasmic tails. The cytoplasm may be slightly granular and is usually configured as a tear drop or cytoplasmic tail. Immunocytologic staining for calcitonin confirms medullary cancer (picture 16). The cellular pleomorphism and lack of follicular development usually results in a recommendation for surgical excision even if the diagnosis is not made on FNA.

Surgical specimens from patients with medullary cancer of the thyroid show spindle-shaped and frequently pleomorphic cells without follicle development because these cells originate from the calcitonin-producing parafollicular C cells of the thyroid (picture 17). Occasional red cytoplasmic granules are seen, and the supporting stroma frequently stains for amyloid.

Anaplastic — Clinical suspicion and cytology demonstrating marked pleomorphism, bizarre giant cells, and spindle cells characterize anaplastic cancer. (See "Anaplastic thyroid cancer".)

Cancer metastatic to thyroid — Renal cell is the most common cancer metastatic to the thyroid. Colorectal, lung, breast, sarcoma, melanoma, and uterine cancer are among the other more common cancers that metastasize to the thyroid [25,26]. If uncertainty exists, thyroglobulin staining as well as other markers may help determine the origin of the cells. If the underlying cancer diagnosis is known, specific antibodies can be used for immunohistochemical analysis.

Lymphoma — Aggressive lymphomas are readily diagnosed based upon clinical findings and lymphocytes on FNA or core-needle biopsy. However, low-grade lymphomas may be difficult to distinguish from Hashimoto's thyroiditis. Virtually all thyroid lymphomas originate within Hashimoto's thyroiditis; as a result, sampling error is also a concern. Flow cytometry and immunocytochemical studies may be useful and may require larger fragments of tissue from a cutting-needle biopsy. The patient may be spared surgery if the diagnosis can be made on needle biopsy, although many pathologists prefer tissue obtained from an open biopsy. (See "Clinical presentation and initial evaluation of non-Hodgkin lymphoma", section on 'Analysis of biopsy material'.)

SUMMARY AND RECOMMENDATIONS

●Fine-needle aspiration – Fine-needle aspiration (FNA) biopsy and fine-needle capillary (FNC) sampling are the two most widely used diagnostic techniques for assessing the cytopathology of thyroid nodules to identify those patients who have nodules that should be excised. (See 'Introduction' above and "Thyroid biopsy".)

●Cytopathology classification – The National Cancer Institute Thyroid Fine-Needle Aspiration State of the Science Conference ("Bethesda Conference") suggests the following six major cytologic categories, each of which indicates a different subsequent evaluation and management (table 1) (see 'FNA cytology classification scheme' above):

•I. Nondiagnostic

•II. Benign

•III. Atypia of undetermined significance (AUS)

•IV. Follicular neoplasm

•V. Suspicious for malignancy

•VI. Malignant

●Nondiagnostic – A nondiagnostic sample is inadequate and should not be interpreted as a negative biopsy. For nondiagnostic biopsy results, we typically repeat the FNA biopsy under ultrasound guidance. (See 'Nondiagnostic' above.)

●Benign – Features of benign cytology include intact macrofollicles and macrofollicles that have been broken apart, forming a flat sheet (picture 1A). Colloid, which stains blue on a Papanicolaou stain, is usually abundant in the background of macrofollicular lesions (picture 2). Common entities in the benign category include normal thyroid tissue, nodules from adenomatous or multinodular goiters, chronic lymphocytic (Hashimoto's) thyroiditis, and subacute granulomatous thyroiditis (table 2). (See 'Benign' above.)

●Atypia of undetermined significance – Atypia of undetermined significance (AUS) includes lesions with mild nuclear atypia or other atypia (eg, architectural atypia [previously called follicular lesion of undetermined significance)] oncocytic [Hürthle cell] atypia, lymphocytic atypia). The risk of malignancy with this classification (based on surgical resection data) ranges from 13 to 30 percent. AUS with nuclear atypia has a higher risk of malignancy compared with architectural atypia or oncocytic atypia. Repeating the aspiration and the use of molecular testing, if available, substantially reduces the need for diagnostic thyroid lobectomy. (See 'Atypia of undetermined significance (AUS)' above and "Evaluation and management of thyroid nodules with indeterminate cytology in adults", section on 'Evaluation and management'.)

●Follicular neoplasm – Follicular neoplasm includes microfollicular or cellular adenomas. This pattern may also be referred to as cellular or indeterminate. Microfollicles are easily identified in specimens obtained by FNA (picture 7). Colloid is absent or scant, and the cells are more crowded than in macrofollicular nodules and may form clusters. Postsurgical evaluation reveals a malignancy rate of approximately 23 to 34 percent. The use of molecular analysis on indeterminate FNA aspirates (follicular neoplasm, AUS) reduces substantially the number of patients who require diagnostic thyroid surgery. (See 'Follicular neoplasm' above and "Evaluation and management of thyroid nodules with indeterminate cytology in adults", section on 'Evaluation and management'.)

●Suspicious for malignancy – This category includes lesions with some features suggestive of, but not definitive for, thyroid cancer. Typically, nodules in this category have a 67 to 83 percent risk of malignancy. (See 'Suspicious for malignancy' above.)

●Malignant – The malignant category includes papillary cancer, medullary cancer, thyroid lymphoma, anaplastic cancer, cancer metastatic to the thyroid, and lymphoma. It does not include follicular or oncocytic thyroid cancer. The diagnosis of follicular or oncocytic thyroid cancer is made on the basis of capsular or vascular invasion, which cannot be identified in specimens obtained by FNA. (See 'Malignant' above.)