Non-neoplastic   Hyperplastic Nodular goiter   Follicular nodules in Chronic Lymphocytic Thyroiditis Neoplastic   Follicular adenomas   Follicular carcinomas   Follicular variant of papillary carcinoma

Benign or non-neoplastic diseases
The cytological diagnosis of a usual type of nodular goiter is no diagnostic challenge. An admixture of benign follicular cells, colloid and macrophages with or without hemosiderin allows an accurate cytologic diagnosis. However a hyperplastic goiter yielding a markedly cellular aspirate with follicular cells in large tissue fragments, scant or no colloid in the background and absence of degenerative changes is often interpreted as a follicular lesion (indeterminate diagnosis) and treated surgically. Despite the increased cellularity of the aspirate, the presence of regular follicles, honeycomb arrangement of uniform small nuclei, favor a non-neoplastic lesion [6, 7, 8] and should not be referred for surgical excision.

Nodular proliferations of follicular epithelium in Hashimoto's disease will yield a cellular aspirate exhibiting morphology very similar to that seen in cellular follicular neoplasms. It is the presence of lymphoid cells in the background that will allow the diagnosis of Hashimoto's disease. If the lymphoid cells are sparse or overlooked or even absent, it is not possible to recognize the nature of the lesion as Hashimoto's disease and will be interpreted as a follicular neoplasm [6, 7, 8] This is an important cause for a false positive diagnosis. However, hyperplastic goiters and Hashimoto's disease with nodular proliferations do not pose a problem in surgical pathology because of the benefit of having the entire specimen and an opportunity for a thorough examination.

The major problem in thyroid cytopathology is differentiating various types of neoplasms that demonstrate a follicular architecture cytologically grouped as indeterminate. These include follicular adenomas, follicular carcinomas and follicular variant of papillary carcinomas (FVPC). It has been stated repeatedly and reiterated in almost every reported review article on thyroid aspirates that the follicular patterned lesions cannot be seperated cytologically. The reasons? First, the accepted diagnostic criteria for differentiating follicular adenoma from carcinoma are histological, requiring examination of the capsule and the blood vessels for demonstrating invasive features. Since aspiration biopsies do not include capsule or blood vessels, it is felt that any attempt at cytologic differentiation is futile. Secondly, the cytologic samples of cellular follicular adenomas (microfollicular and trabecular types), atypical adenomas and well-differentiated follicular carcinomas present a spectrum of overlapping cytologic features that merge imperceptibly, making the differentiating process difficult if not impossible.

The category of follicular patterned lesions also includes follicular variant of papillary carcinoma by virtue of its follicular architecture. The specific cytologic criteria for follicular variant of papillary carcinoma [7] makes it possible to differentiate it from the follicular adenomas and carcinomas. In the absence of specific nuclear features in the cytologic samples, FVPC may be interpreted as a cellular follicular neoplasm (follicular adenomas and follicular carcinomas).

The indeterminate category as reported in the literature comprises up to 20% of the satisfactory aspirates [3, 4] . Surgical excision is recommended for all. The yield of malignancy in surgically excised specimens is reported to be 14% to 20%. Majority turn out to be follicular adenomas and 5% to 20% represent non-neoplastic entities [3, 4, 5, 9, 10] .

The questions are: Can cytology differentiate non-neoplastic follicular lesions from the neoplastic ones? Can follicular adenomas be separated from follicular carcinomas and follicular variant of papillary carcinomas? If the answer is yes, what is the diagnostic accuracy? Do all cases cytologically interpreted as follicular neoplasms need to be removed, only to find a small number of cancers? Given the low probabilities of cancer, can "benign neoplasms " be followed conservatively so that surgery can be avoided? Will this triage be acceptable to the clinicians, endocrinologists and surgeons? Will the pathology-cytology community welcome it?

The author has attempted to differentiate various cellular follicular lesions into specific diagnostic categories primarily because some endocrinologists who preferred not to recommend surgery for benign lesions. The experience gained over the last several years with a substantial number of cases and with histologic correlations, it is felt that with an adequate aspirate and utilizing certain diagnostic criteria [6, 7, 8] most follicular lesions can be triaged. First, there are certain features common to all follicular patterned lesions. Secondly architectural configurations of the tissue fragments will separate non-neoplastic from the neoplastic ones. Third, the nuclear morphology will help separate the follicular neoplasms further (Tables 2 and 3).

Table 2 - Features Common to all Cellular Follicular Lesions

Increased cellularity Sparse or absent colloid Absence of degenerative changes (Clean background)

Table 3 - Cytologic Parameters for Differentiating Follicular Lesions

Architecture of the tissue fragments of follicular cells   Sheets (honeycomb arrangement) versus syncytium   Regular follicles with evenly spaced nuclei versus irregular follicles with nuclear crowding Nuclear morphology   Normal-sized (7-9µm) in nodular goiter to variably increased in neoplasms   Uniform versus pleomorphic   Chromatin pattern   Parachromatin clearing   Micro-macronucleoli   Nuclear grooves   Pseudoinclusions

Cytologic Features of Follicular Adenomas
The aspirates of follicular adenomas composed of small to large follicles distended with colloid and lined by benign follicular cells with small nuclei (macrofollicular pattern) often present cytologic pattern indistinguishable from nodular goiter.

The cytologic features of a follicular adenoma with an admixture of different morphologic patterns such as macrofollicular, normofollicular and microfollicular/trabecular will depend on the area sampled and the cytologic diagnosis will vary from nodular goiter to follicular adenoma to follicular carcinoma. The sampling becomes a major issue especially when the nodule is very large. This type of adenoma tends to have more disparities in cytohistologic correlations [2, 6] .

The microfollicular and the trabecular type follicular adenomas are solid, yielding cellular aspirates that lack colloid and have a clean background. The cytology is characterized by increased cellularity, syncytial tissue fragments with or without follicular patterns and variably but uniformly enlarged nuclei. Nucleoli are generally absent.

Cytologic Features of Follicular Carcinomas
Despite the general point of view in the cytology community, the aspirates from a poorly differentiated follicular carcinomas clearly present malignant criteria, allowing a specific diagnosis to be made. The problem is with well-differentiated follicular carcinomas which in aspirates present cytologic pattern similar to cellular follicular adenomas. However in addition to the criteria for the cellular adenomas, follicular carcinoma cells demonstrate considerable increase in their nuclear size, pleomorphism, parachromatin clearing and the presence of nucleoli. Also the density of nuclei within the tissue fragments is much more intense in carcinomas than in adenomas.

An atypical adenoma will present cytologic pattern similar to that seen in follicular carcinomas. Since no capsular or vascular invasion is demonstrated in histologic specimens, an atypical adenoma becomes a histologic diagnosis and not a cytologic diagnosis.

Cytologic Features of Follicular Variant of Papillary Carcinoma
The follicular variant of papillary carcinoma is an important diagnostic entity that must be considered in the differential diagnosis of follicular patterned lesions. The characteristic and diagnostic nuclear morphology such as: variably enlarged nuclei, pale, dusty, powdery to finely granular chromatin, micro-macronucleoli, intranuclear pseudoinclusions and nuclear grooves, allow an accurate diagnosis (Table 4) [7]. However, these characteristic features may not necessarily be a universal pattern in any given case of follicular variant of papillary carcinoma. If the aspirated sample is from an area that lacks these cytologic features of papillary carcinoma, a diagnosis of follicular adenoma or a follicular carcinoma is usually rendered.

Summary of Cytologic Criteria for Differentiating the Follicular Patterned Lesions

The amount of colloid decreases from macrofollicular lesions to cellular follicular lesions. The architecture of the tissue fragments of the follicular epithelial cells is different in nodular goiter (monolayered sheets with honeycomb pattern, regular follicles with evenly spaced small nuclei) as compared to those seen in neoplasms [syncytial with and without follicular pattern, enlarged nuclei] (exception: syncytial architecture may be seen in Hashimoto's thyroiditis). The size of the follicular cell nuclei gradually increases from those seen in hyperplastic goiters to those of follicular neoplasms and much more so in follicular carcinomas. This fact is supported by morphometric studies as reported by several authors (exception: insular carcinoma where the nuclear size is around 9 µm ) The follicular cell nuclei tend to be uniform in size (7-9µm) in nodular goiters, mildly and uniformly enlarged in adenomas but considerably enlarged and pleomorphic in carcinomas. In follicular carcinomas, the nuclear chromatin tends to be coarsely granular with parachromatin clearing and with presence of nucleoli. The density and crowding of the nuclei within the tissue fragments is much more intense in follicular carcinomas. The presence of nucleoli usually indicates a high probability of invasive characteristics. The presence of characteristic nuclear features of papillary carcinoma will favor the diagnosis of its follicular variant.

Diagnostic accuracy and limitations
The cytologic interpretation is dependent on sampling, adequacy of the aspirate, technical quality of the smears, experience level of the pathologist/cytopathologist and willingness to go further in trying to separate the lesions. There are gray zones because of the overlapping patterns and 100% correlation can never be attained.

An adequately cellular aspirate and well-prepared, well-stained smears will in most instances allow differentiation of follicular nodules of goiters and Hashimoto's thyroiditis from follicular neoplasms. The cellular follicular adenomas (microfollicular and trabecular types) have overlapping morphologic features with atypical adenomas or well-differentiated follicular carcinomas. Roughly 18-20% of the cases cytologically diagnosed as follicular adenomas, have demonstrated follicular carcinomas in histologic specimens (6). Up to 20% of the cases cytologically diagnosed as follicular carcinomas may not exhibit invasive characteristics in surgically removed specimens (depends on the extent of thoroughness in examination of the specimen and the number of sections submitted). These cases usually turn out to be cellular follicular adenomas and occasionally nodular goiter or Hashimoto's disease. There is a great potential for false positive cytologic diagnoses in cases of Hashimoto's thyroiditis. The presence of even a few lymphocytes in the background must not be ignored. Clinical data and thyroid antibodies will be helpful. Although a marked increase in the nuclear size of the follicular cells is an indication for possible malignancy, the exception to this rule is insular carcinoma where the nuclear size approximates that of a normal follicular cell nucleus. However, the neoplastic cells of insular carcinoma are discohesive, and contain coarsely granular chromatin, parachromatin clearing and nucleoli. The diagnosis of a follicular variant of papillary carcinoma enjoys a high degree of diagnostic accuracy in the presence of characteristic nuclear features (almost 100%). In the absence of these features a diagnosis of follicular adenoma or carcinoma is rendered. A follicular adenoma with a normofollicular architecture is often difficult to differentiate from nodular goiter. A macrofollicular adenoma, often referred to as colloid nodule, shows abundant colloid and sparse cellular material and cannot be differentiated cytologically from hyperinvoluted goiter (exception: pay attention to the nuclear morphology so as not to miss a macrofollicular variant of papillary carcinoma). Probabilities of malignancy is extremely low or none If a thyroid nodule that is being biopsied is larger than 2cm., sampling problems must be anticipated. Different areas of follicular adenomas often demonstrate different morphologic patterns. An area of normofollicular or macrofollicular pattern if sampled will be interpreted as nodular goiter, while solid, cellular areas with microfollicular and trabecular pattern will be interpreted as cellular adenomas. A combination of both patterns should suggest a diagnosis of a follicular adenoma. In a recent study (2), we found 3 cancers out of 21 cases in this group, a 14% incidence of cancer as compared to 1 cancer (papillary carcinoma) out of 22 cases of cellular adenomas, an incidence of 4% (Table 4). By lumping all of follicular adenomas (macro/microfollicular and cellular), and suspicious for follicular and FVPC in one group as cellular follicular neoplasms, the yield of cancer was 30% (Table 4). A nodule larger than 4cm is more likely to have degenerative foci with cystic change. The aspirates are often poorly cellular. Sampling may be an issue. It must be remembered that the hypercellularity of the aspirate is not indicative of a neoplastic process. Careful evaluation of the architectural patterns of the tissue fragments of follicular epithelial cells and the nuclear size in most instances allows the differentiation. Pre-requisite for proper cytologic evaluation of thyroid aspirates is an adequately cellular sample with well-prepared and well-stained smears. Direct smears of an aspirate are far better suitable for cytologic evaluation than other modalities. Wet fixation in a liquid medium will get rid of colloid and also hemolyse the red blood cells. The latter serve as a baseline for judging the nuclear size, an important parameter in the differential diagnosis of follicular lesions. In our experience the reported cytologic criteria (7,8) are best applicable to spray-fixed smears stained by Papanicolaou technique.

Table 4 - Cyto-Histo Correlations of 94 Cases with Cytologic Diagnoses of Follicular Neoplastic Lesions*

Cytologic Diagnosis	NO	NG/CG	HASH	FA	FCA	FVPC/PC	AC
Follicular adenoma	21	2		16	2	1
Cellular adenoma	22	5	1	15		1
Follicular neoplasm	30	5		16	3	5	1
Follicular carcinoma	13	2		1	8	2
Follicular variant of papillary carcinoma	8			1		7

NG—Nodular goiter
CG—Colloid goiter
Hash—Hashimoto's thyroiditis
FA—Follicular adenoma
FCA—Follicular carcinoma
FVPC—Follicular variant of papillary carcinoma
PC—Papillary carcinoma
AC—Anaplastic carcinoma

*Boboc L, Suterwala S, Kini SR, Zafar S, Wisgerhof M. Cytology can predict histology of follicular thyroid neoplasm. Presented at 74th Annual meeting of the American Thyroid Association, October 10-13, 2002

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