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Background Cytology specimens are often used for biomarker testing in the setting of neoplasia. On occasion, formalin-fixed paraffin-embedded (FFPE) cell blocks unfortunately may not yield sufficient material for testing. Recent studies have suggested that residual supernatant fluid from cell block preparation is a valuable source of DNA: both cellular and cell-free DNA (cfDNA). In the present study, the use of cfDNA from supernatant is compared against DNA from FFPE materials. Methods cfDNA was extracted prospectively from residual supernatants of 30 cytology samples (29 neoplastic cases and 1 benign ascitic fluid from a patient with a history of melanoma). Samples were tested using clinically validated next-generation-sequencing platforms and the results were compared with data from paired FFPE cell blocks in a real-time prospective clinical setting. Thirteen samples were tested on an amplicon-based assay (Solid Tumor Hotspot), and 17 samples were tested using a comprehensive capture-based assay (UW-Oncoplex). Results Neoplastic content was estimated by mutational variant allele fraction, with a mean content of 24.0% and 25.8% in supernatant and FFPE, respectively. The variant concordance between paired samples was 90%, and identical results were detected in both supernatant and FFPE samples in 74% of cases. Conclusions This study confirmed that cfDNA from supernatant is a viable alternative to FFPE cell blocks for molecular biomarker testing using both amplicon-based and capture-based assays with potential for decreasing additional tissue sampling and faster turnaround time.

The 2019 ASCCP Risk Based Management Consensus Guidelines for prevention of cervical cancer promote clinical management recommendations aligned with our increased understanding of HPV biology and cervical carcinogenesis. They employ HPV-based testing as the basis for risk estimation, allow for personalized risk-based management by incorporating knowledge of current results with prior results, and streamline incorporation of new test methods as they are validated. They continue to support the principles of "equal management for equal risk" and "balancing harms and benefits" adopted in the 2012 version of the guidelines. These updated guidelines will be able to adjust for decreasing CIN3+ risks as more patients who received HPV vaccination reach screening age. Pathology organizations were closely involved in the development of these guidelines. Herein the pathologists who served as representatives to the 2019 ASCCP guidelines steering committee and workgroups, summarize the changes that are relevant to laboratories, pathologists, and cytotechnologists. Prior relevant screening and reporting recommendations that have not been widely and/or inconsistently adopted by laboratories are also discussed and considerations for modification of laboratory practices offered.

The Bethesda System for reporting thyroid cytopathology (BSRTC) predicts an incidence of malignancy of less than 5% in thyroid nodules with a benign diagnosis on fine-needle aspiration (FNA). However, recent series have suggested that the true rate of malignancy might be significantly higher in this category of patients. We reviewed our experience by performing a retrospective analysis of patients with benign thyroid FNA results who underwent thyroidectomy between 2008 and 2013 at a large academic center. Information including demographics, ultrasound features, FNA diagnosis, and surgical follow-up information were recorded. Slides were reviewed on cytology-histology discrepant cases, and it was determined whether the discrepancy was due to sampling or interpretation error. A total of 802 FNA cases with a benign diagnosis and surgical follow-up were identified. FNA diagnoses included 738 cases of benign goiter and 64 cases of lymphocytic thyroiditis. On subsequent surgical resection, 144 cases were found to be neoplastic, including 117 malignant cases. False negative, defined as interpretation error and inadequate biopsy of the nodule harboring malignancy, was 6%. When cases of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) were excluded from the analysis, false-negative rate was 5%. When microPTC cases were excluded, false-negative rate was 3% and was slightly less than 3% when both microPTC and NIFTP cases were excluded from the analysis. Retrospective review of neoplastic cases showed that 57% were due to sampling error and 43% were due to interpretation error. Interpretation error was more likely to occur in follicular patterned neoplasms (75%), while sampling error was more common in non-follicular variants of papillary thyroid carcinoma (non-FVPTC) (61%). With the exclusion of microPTC, interpretation errors were still more likely to occur in follicular neoplasms (79%) but there was no significant difference in sampling error between non-FVPTC (37%) and follicular patterned neoplasms (42%). Tumor size was larger in cases with interpretation error (mean = 2.3 cm) compared to cases with sampling error (mean = 1.4 cm). This study shows that the false-negative rate of thyroid FNA at our institution is not significantly above the rate suggested by the BSRTC. Interpretation errors were more likely to occur in follicular patterned neoplasms, while non-FVPTC was more frequently found in false negative cases due to inadequate sampling.

Laboratory management should be an integral part of training in pathology residency and fellowships. Herein, we have outlined some basic laboratory management topics a graduating cytopathology fellow should be familiar with. An overview of regulatory agencies that have oversight over laboratory testing, cytopathology laboratory accreditation, pre-analytic, analytic and post-analytic quality assurance, billing/coding, basic statistics, verification/validation of testing, physician credentialing, board certification/maintenance of certification, and malpractice in cytopathology are addressed. This review is by no means all inclusive, but rather a guide to the basic management related topics to be covered during cytopathology subspecialty training.

BACKGROUND Pancreatic neuroendocrine neoplasms with a Ki-67 labeling index greater than 20% were reclassified in 2017 by the World Health Organization into well differentiated (WD) and poorly differentiated grade 3 neuroendocrine carcinoma (NEC). The authors describe the cytologic features of grade 3 WD pancreatic neuroendocrine neoplasms compared with grade 2 neoplasms and NEC. METHODS Fine-needle aspirates from 65 pancreatic neuroendocrine neoplasms were reviewed, and their cytomorphologic features were compared across grade 2, WD grade 3, and PD small cell type (PD-S), large cell type (PD-L), and type not otherwise specified (PD-NOS) neoplasms. RESULTS The 65 aspirates consisted of 19 grade 2 neoplasms, 32 WD grade 3 neoplasms, and 14 NECs (6 PD-S, 5 PD-L, and 3 PD-NOS). The medians Ki-67 proliferation index was 11% (range, 3.2%-17%) in grade 2 neoplasms, 40% (range, 21%-89%) in WD grade 3 neoplasms, 80% (range, 63%-95%) in PD-S neoplasms, 39% (range, 25%-61%) in PD-L neoplasms, and 70% (range, 30%-80%) in PD-NOS neoplasms. Both grade 2 and WD grade 3 neoplasms were associated with plasmacytoid morphology and smooth nuclear contours, but WD grade 3 neoplasms had significant increases in abundant cytoplasm (72% vs 17%; P = .007), nuclear tangles (75% vs 42%; P = .006), and apoptosis (86% vs 58%; P = .005). Compared with NECs, WD grade 3 neoplasms had increased plasmacytoid morphology (75% vs 7%; P < .001), smooth nuclear contours (94% vs 64%; P = .02), round nuclei (59% vs 21%; P = .01), and less pleomorphism (13% vs 50%; P = .004), molding (9% vs 79%; P < .001), and necrosis (13% vs 43%; P = .003). WD grade 3 neoplasms had less pleomorphism (13% vs 50%; P = .04), less necrosis (13% vs 60%; P = .04), and more plasmacytoid morphology (75% vs 20%; P = .03) than PD-L. CONCLUSIONS The prevalence of cytologic features differs in WD grade 3 pancreatic neuroendocrine neoplasms compared with grade 2 neoplasms and NECs, and these differences assist in the recognition of this newly classified entity. Cancer Cytopathol 2018;126:326-35. © 2018 American Cancer Society.

BACKGROUND Well-differentiated (WD) and poorly differentiated (PD) pancreatic neuroendocrine neoplasms are biologically distinct entities with different therapies and prognoses. WD neoplasms with elevated proliferation (Ki-67 > 20%) have been shown to have an overlapping histology with PD neuroendocrine carcinomas. This study compared expert cytomorphologic assessments of differentiation in pancreatic neuroendocrine neoplasms in a multi-institutional study. METHODS Fine-needle aspiration specimens from pancreatic neuroendocrine neoplasms (grade 2 [G2] and grade 3 [G3] according to the 2017 World Health Organization classification; n = 72) were diagnosed independently by 3 cytopathologists as WD or PD (poorly differentiated large cell type [PD-L] or poorly differentiated small cell type [PD-S]) purely on the basis of cytomorphology. Their diagnoses were compared with a final classification supported by immunohistochemistry (retinoblastoma (RB), death domain- associated protein (DAXX), and α thalassemia/mental retardation syndrome X-linked (ATRX) protein expression), targeted mutation analysis (Memorial Sloan Kettering–Integrated Mutation Profiling of Actionable Cancer Targets), prior history of G1/G2 histology, and consensus. RESULTS The rate of agreement on differentiation was 38% (15 WD cases and 12 PD cases) for the 70 cases included (55 WD cases [n = 19 G2, n = 31 G3, and n = 5 could not be graded] and 15 PD cases [n = 6 PD-S, n = 6 PD-L, and n = 3 PD, not otherwise specified). Two cases could not be classified by the employed methods. PD carcinomas had a higher rate of agreement (10 of 15 [67%]) than WD neoplasms (15 of 55 [27%]). Round nuclei and plasmacytoid cells were associated with agreement for WD cases, whereas apoptosis and angulated nuclei were associated with disagreement. Necrosis was associated with agreement for PD cases. CONCLUSIONS A purely morphologic approach to the distinction between G2 and G3 pancreatic neuroendocrine neoplasms based on cytology can be challenging, with disagreement found among experienced cytopathologists. Cancer Cytopathol 2017. © 2017 American Cancer Society.

Objectives: Anorectal cytology (ARC) is a widely used screening tool for anal cancer in human immunodeficiency virus (HIV)–infected men who have sex with men (MSM). Its diagnostic accuracy needs to be improved, especially for high-grade squamous intraepithelial lesions (HSILs). Methods: Using 100 HIV+ MSM with biopsy-proven anal HSILs, we correlated histologic/cytologic findings. Results: Upon review, HSIL cells were present in 58 cytology samples and absent in 42. Positive samples were higher in cellularity and contained transformation zones (P

Context All Food and Drug Administration–approved methods in the United States for human papillomavirus testing including the Hybrid Capture 2 human papillomavirus assay and the Roche cobas human papillomavirus test are approved for cytology specimens collected into ThinPrep media but not for specimens collected into SurePath solution. Objective To compare the performance of the Roche cobas and Hybrid Capture 2 tests for the detection of high-risk human papillomavirus using both ThinPrep and SurePath preparations as part of a validation study. Design One thousand three hundred seventy-one liquid-based cytology samples, including 1122 SurePath and 249 ThinPrep specimens, were tested for high-risk human papillomavirus DNA using the Roche cobas human papillomavirus test and the Hybrid Capture 2 human papillomavirus assay. For cases with discrepant results, confirmatory testing was performed using Linear Array human papillomavirus testing. Results One hundred and fifty-six (11.38%) and 184 (13.42%) of the 1371 specimens tested positive for high-risk human papillomavirus DNA using the Hybrid Capture 2 human papillomavirus assay and Roche cobas human papillomavirus assay, respectively. In addition, 1289 (94.0%) of 1371 specimens demonstrated concordant high-risk human papillomavirus results with a κ value of 0.72 (95% confidence interval, 065–0.78). There was no statistically significant difference in the percentage of positive high-risk human papillomavirus results between the 2 liquid-based preparations with either assay. Discordant results between the 2 assays were noted in 82 of 1371 cases (6%). Twenty-seven of 82 cases (32.9%) were Hybrid Capture 2 positive/Roche cobas negative and 55 of 82 cases (67.1%) were Roche cobas positive/Hybrid Capture 2 negative. Two of 20 Hybrid Capture 2–positive/Roche cobas–negative cases (10%) and 26 of 37 Roche cobas–positive/Hybrid Capture 2–negative cases (70%) tested positive for high-risk human papillomavirus by Linear Array. Conclusions Both assays showed good agreement and excellent specificity with either ThinPrep or SurePath preparations. The number of discordant results was relatively small. The performance of both assays was similar for ThinPrep specimens, but the Roche cobas test demonstrated higher sensitivity with SurePath specimens.

Pancreatic neuroendocrine neoplasms with a Ki-67 labeling index greater than 20% were reclassified in 2017 by the World Health Organization into well differentiated (WD) and poorly differentiated grade 3 neuroendocrine carcinoma (NEC). The authors describe the cytologic features of grade 3 WD pancreatic neuroendocrine neoplasms compared with grade 2 neoplasms and NEC.Fine-needle aspirates from 65 pancreatic neuroendocrine neoplasms were reviewed, and their cytomorphologic features were compared across grade 2, WD grade 3, and PD small cell type (PD-S), large cell type (PD-L), and type not otherwise specified (PD-NOS) neoplasms.The 65 aspirates consisted of 19 grade 2 neoplasms, 32 WD grade 3 neoplasms, and 14 NECs (6 PD-S, 5 PD-L, and 3 PD-NOS). The medians Ki-67 proliferation index was 11% (range, 3.2%-17%) in grade 2 neoplasms, 40% (range, 21%-89%) in WD grade 3 neoplasms, 80% (range, 63%-95%) in PD-S neoplasms, 39% (range, 25%-61%) in PD-L neoplasms, and 70% (range, 30%-80%) in PD-NOS neoplasms. Both grade 2 and WD grade 3 neoplasms were associated with plasmacytoid morphology and smooth nuclear contours, but WD grade 3 neoplasms had significant increases in abundant cytoplasm (72% vs 17%; P = .007), nuclear tangles (75% vs 42%; P = .006), and apoptosis (86% vs 58%; P = .005). Compared with NECs, WD grade 3 neoplasms had increased plasmacytoid morphology (75% vs 7%; P.001), smooth nuclear contours (94% vs 64%; P = .02), round nuclei (59% vs 21%; P = .01), and less pleomorphism (13% vs 50%; P = .004), molding (9% vs 79%; P.001), and necrosis (13% vs 43%; P = .003). WD grade 3 neoplasms had less pleomorphism (13% vs 50%; P = .04), less necrosis (13% vs 60%; P = .04), and more plasmacytoid morphology (75% vs 20%; P = .03) than PD-L.The prevalence of cytologic features differs in WD grade 3 pancreatic neuroendocrine neoplasms compared with grade 2 neoplasms and NECs, and these differences assist in the recognition of this newly classified entity. Cancer Cytopathol 2018;126:326-35. © 2018 American Cancer Society.

Well-differentiated (WD) and poorly differentiated (PD) pancreatic neuroendocrine neoplasms are biologically distinct entities with different therapies and prognoses. WD neoplasms with elevated proliferation (Ki-67 20%) have been shown to have an overlapping histology with PD neuroendocrine carcinomas. This study compared expert cytomorphologic assessments of differentiation in pancreatic neuroendocrine neoplasms in a multi-institutional study.Fine-needle aspiration specimens from pancreatic neuroendocrine neoplasms (grade 2 [G2] and grade 3 [G3] according to the 2017 World Health Organization classification; n = 72) were diagnosed independently by 3 cytopathologists as WD or PD (poorly differentiated large cell type [PD-L] or poorly differentiated small cell type [PD-S]) purely on the basis of cytomorphology. Their diagnoses were compared with a final classification supported by immunohistochemistry (retinoblastoma (RB), death domain- associated protein (DAXX), and α thalassemia/mental retardation syndrome X-linked (ATRX) protein expression), targeted mutation analysis (Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets), prior history of G1/G2 histology, and consensus.The rate of agreement on differentiation was 38% (15 WD cases and 12 PD cases) for the 70 cases included (55 WD cases [n = 19 G2, n = 31 G3, and n = 5 could not be graded] and 15 PD cases [n = 6 PD-S, n = 6 PD-L, and n = 3 PD, not otherwise specified). Two cases could not be classified by the employed methods. PD carcinomas had a higher rate of agreement (10 of 15 [67%]) than WD neoplasms (15 of 55 [27%]). Round nuclei and plasmacytoid cells were associated with agreement for WD cases, whereas apoptosis and angulated nuclei were associated with disagreement. Necrosis was associated with agreement for PD cases.A purely morphologic approach to the distinction between G2 and G3 pancreatic neuroendocrine neoplasms based on cytology can be challenging, with disagreement found among experienced cytopathologists. Cancer Cytopathol 2018;126:44-53. © 2017 American Cancer Society.

A major reclassification occurred with the redesignation of noninvasive encapsulated follicular variant of papillary thyroid carcinoma as noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) due to its indolent nature. The aim of this study was to determine whether distinct cytomorphologic features could be identified on preoperative fine-needle aspiration (FNA) when NIFTP cases were compared with invasive follicular variant of papillary thyroid carcinoma (FVPTC) subtypes.Thyroid resection cases with the diagnosis of FVPTC from 2012 to 2016 were reclassified as NIFTP, invasive encapsulated follicular variant of papillary thyroid carcinoma (IEFVPTC), and invasive FVPTC subtypes. Corresponding FNA specimens were retrieved and retrospectively reviewed. A univariate analysis using Fisher's exact test was performed to determine any differences in the frequencies of various cytomorphologic features among NIFTP, IEFVPTC, and FVPTC cases. A multivariate analysis was performed to identify any independent salient features that would be helpful in differentiating NIFTP from its invasive counterparts.The study population consisted of 93 cases, including 51 cases of NIFTP, 21 cases of IEFVPTC, and 21 cases of infiltrative FVPTC. Demographics such as age, sex, and tumor size were comparable across the 3 groups. A predominantly microfollicular pattern, an absence of nuclear pseudo-inclusions, and less frequent nuclear elongations and grooves were significantly more likely to be associated with NIFTP versus its invasive counterparts. The absence of nuclear pseudo-inclusions and the presence of a microfollicular pattern were the only independent predictors of a NIFTP diagnosis.This study demonstrates that NIFTP cases have distinguishing cytomorphologic characteristics in comparison with invasive FVPTC cases. Therefore, a preoperative cytologic evaluation provides clues that can aid in the distinction between NIFTP and its invasive counterparts. Cancer Cytopathol 2017;125:865-75. © 2017 American Cancer Society.

Anorectal cytology (ARC) is a widely used screening tool for anal cancer in human immunodeficiency virus (HIV)-infected men who have sex with men (MSM). Its diagnostic accuracy needs to be improved, especially for high-grade squamous intraepithelial lesions (HSILs).Using 100 HIV+ MSM with biopsy-proven anal HSILs, we correlated histologic/cytologic findings.Upon review, HSIL cells were present in 58 cytology samples and absent in 42. Positive samples were higher in cellularity and contained transformation zones ( P .05). Cytology was able to predict HSILs in 36%, 48%, 68%, and 78% of patients with one, two, three, and four or more high-grade lesions. HSIL cells were identified in all cytology samples initially reported as HSILs or atypical squamous cells, cannot exclude HSIL and in 34 samples reported as low-grade squamous intraepithelial lesions or less. Notably, among this last category, 15 (44%) were keratinized-type HSILs.Our findings should improve the ARC detection rate for anal HSILs, helping to implement ARC as the primary screening tool for anal cancer.

Hurthle cell changes are frequently seen in reactive/hyperplastic conditions and can be extensive, resulting in the formation of nodules that must be distinguished from their neoplastic counterparts, Hurthle cell adenomas and Hurthle cell carcinoma. The primary goal of thyroid FNA in the management of Hurthle cell nodules is to separate those for which surgery is indicated, i.e., Hurthle cell adenomas and carcinomas, from those that can be managed conservatively, i.e., adenomatous nodules with oncocytic changes and Hashimoto’s thyroiditis. The distinction between Hurthle cell adenomas and carcinomas is not possible on cytology because of the inability of cytology to establish the presence of capsular and/or vascular invasion. Therefore, rendering a diagnosis of Hurthle cell neoplasm implies that these lesions should be excised to exclude a malignancy. The criteria for diagnosing “follicular neoplasm, Hurthle cell type,” or “suspicious for a follicular neoplasm, Hurthle cell type” are moderately to markedly cellular aspirate that consists exclusively or predominantly (>75 %) of Hurthle cells with little or no colloid and lack of background lymphocytes. The Hurthle cells can be either small with high N/C ratio (≥50 %) (small cell dysplasia) or large with at least 2× variability in nuclear size (large cell dysplasia). The differential diagnosis includes other neoplasms such as oncocytic and tall cell variants of papillary thyroid carcinoma, medullary carcinoma, parathyroid adenoma, and metastatic renal cell carcinoma.

Lymphocyte-only aspirates pose a significant diagnostic challenge in the interpretation of thyroid fine-needle aspiration specimens. Differential diagnosis includes Hashimoto thyroiditis, lymphoma, intrathyroidal lymph node, intrathyroidal thymoma, and thyroglossal duct cyst. Hashimoto thyroiditis especially in the florid lymphoid phase usually presents as lymphocyte-only aspirates. During this phase, numerous hyperplastic lymphoid follicles are seen throughout the gland, and they often replace the thyroid parenchyma. Lymphomas of the thyroid typically occur in older patients, almost always arise in a background of Hashimoto thyroiditis and are of B-cell lineage. Primary lymphomas of the thyroid gland are mainly diffuse large B-cell lymphoma (DLBCL) and extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) and to a much lesser extent Hodgkin and small lymphocytic lymphoma. Given the limitations of establishing a lymphoma diagnosis with only cytomorphology, morphological evaluation is commonly augmented with flow cytometry or immunohistochemistry in the diagnostic workup of lymphocyte-only fine-needle aspirates from the thyroid.

Involvement of cervical lymph nodes by metastatic disease is reported in 20–50 % of patients with well-differentiated thyroid cancer. Fine-needle aspiration biopsy (FNA) is usually required to confirm or rule out metastasis because reactive lymphadenopathy is common in the neck region. Because of a small but substantial percentage of false-negative cases, thyroglobulin (Tg) assay of aspirate from cervical lymph nodes has been advocated to supplement FNA of suspicious lymph node in patients with proven or suspected differentiated thyroid cancer. A high sensitivity (95 %) and specificity (95 %) in the detection of nodal metastases from differentiated thyroid carcinoma have been reported with Tg assay. However, Tg assay of FNA should not replace cytologic evaluation because of the potential of both false-positive and negative results. In addition, standardization in terms of sample collection and analysis is still lacking.

Numerous histiocytes can be seen in a variety of hyperplastic and neoplastic benign and malignant thyroid nodules undergoing cystic degeneration. Typically, such cystic degeneration is seen on smears of fine needle aspiration as numerous foamy macrophages, hemosiderin-laden macrophages, and scant colloid. Thyroid cysts are mostly benign. However, cysts are well-known causes of false-negative results. Risk factors for malignancy include large cysts, recurrent cysts, cysts in young males, and history of radiation. If macrophages only are present on a smear, other causes of cystic neck mass should be excluded.

Total thyroidectomy followed by surveillance of the neck and thyroid bed for recurrent disease is the mainstay of treatment for primary differentiated thyroid cancers. Ultrasound-guided fine needle aspiration (FNA) of the thyroid bed determines if the new lesion is recurrent thyroid cancer or residual hyperplastic thyroid tissue. The features of cytologic features of malignancy are usually similar to that in the primary tumor. Direct extension from head and neck squamous cell carcinoma as well as metastasis from a distant site can mimic thyroid bed recurrence of thyroid cancer. Benign entities may include hyperplasia of residual normal thyroid, normal parathyroid gland, benign lymph nodes, and benign reparative changes such as fibrosis and suture granulomas.

Data mining is a process of pattern and relationship discovery within large sets of data. Because of the large volume of data generated in healthcare settings, it is not surprising that healthcare organizations have been interested in data mining to enhance physician practices, disease management, and resource utilization. This chapter discusses a variety of data mining techniques that have been used to develop clinical decision support systems, including decision trees, neural networks, logistic regression, nearest neighbor classifiers. In addition, genetic algorithms, biologic and quantum computing, and big data analytics as well as methods of evaluating and comparing the different approaches are also discussed.

Secondary neoplasms of the thyroid gland are uncommon, but clinically they can be indistinguishable from primary thyroid tumors. The possibility of metastasis should always be considered whenever a patient presents with a thyroid nodule and they have a history of malignancy elsewhere in the body or when the cytologic picture is not consistent with or suggestive of common thyroid neoplasms. The most common primary sites of metastasis to the thyroid are kidney, lung, and breast, but the thyroid gland may also be involved by direct extension from malignancies of the head and neck region. It is important to distinguish secondary thyroid tumors from the primary tumors because patients who have metastasis to the thyroid have a poor prognosis in general, and most die shortly after the confirmation of distant metastasis. Immunohistochemistry is very helpful in the differential diagnosis.

A common pitfall in the diagnosis of thyroid nodules is the inadvertent sampling of parathyroid tissue, which may be difficult to distinguish from thyroid tissue on FNA due to similar cytologic features. Parathyroid cells are usually loosely clustered into small groups, often with a syncytial arrangement forming branching, loose, two-dimensional clusters. Microfollicular architecture is prevalent on ThinPrep preparations and this may contribute to overinterpretation of parathyroid tissue as follicular neoplasm of the thyroid. Parathyroid cells on ThinPrep preparations show round, centrally placed nuclei with stippled nuclear chromatin. The nuclei often appear smaller and darker on ThinPrep preparations than on corresponding FNA smears. The cytoplasm is scant to moderate. In addition to pathologic features, clinical features and radiologic appearance of the lesion are also helpful to differentiate between thyroid and parathyroid tissue. When the diagnosis is in doubt, parathyroid hormone immunohistochemical stain should be performed and positive PTH stain will confirm parathyroid origin.

Although architecturally, it fits into the category of follicular neoplasm, follicular variant of papillary thyroid carcinoma is classified as a subtype of papillary carcinoma because it behaves clinically like papillary carcinoma. The tumor is comprised predominantly of follicles; however, the characteristic nuclear features of classic papillary carcinoma are present. Papillary clusters are notably absent. The most common diagnostic dilemma is mistaking it for either goiter or follicular neoplasm. This is due to the fact that there is an overlapping diagnostic criteria for follicular lesions in general and also the paucity of diagnostic features of PTC in this variant, either because they are poorly developed or present only focally. When the predominant architecture is the presence of syncytial tissue fragments, especially in a background of colloid, the lesion can be misdiagnosed as nodular goiter.

Anaplastic thyroid carcinoma, although uncommon, is the most aggressive form of primary thyroid malignancy. The cellularity and the cytologic presentations of anaplastic thyroid carcinoma are variable and depend on the histologic type and the extent of tumor necrosis. Despite its diverse morphologic presentation, anaplastic thyroid carcinoma can be accurately diagnosed on FNA. False-negative diagnoses have been encountered due to low cellularity and obscuring inflammatory and necrotic debris. Its differential diagnosis includes other primary high-grade malignancies and metastases. Judicious use of immunocytochemistry may be helpful in the differential diagnosis.

Among the pediatric and adolescent population, thyroid nodules are uncommon with an incidence of 1–2 % but with a much higher risk of malignancy (14–40 %), when compared to that of adult population. Recent studies have shown that FNA, rather than diagnostic lobectomy, should be the initial diagnostic tool of choice for managing thyroid nodule in the pediatric population. Based on TBSRTC, thyroid FNA demonstrated a sensitivity and specificity of 94 % and 81 %, respectively, for evaluating thyroid nodules in the pediatric population. The majority of the malignancies encountered in the pediatric population are papillary thyroid carcinoma. Although the authors did not observe an increase in the frequency of the diagnostic category FLUS/AUS, a higher frequency of FLUS/AUS (35 %) has been reported in the literature due partly to the reluctance to diagnose malignancy in a young patient and partly to the low cellularity secondary to difficulty in sampling of these lesions. The use of molecular testing for equivocal thyroid FNA in pediatric population has not been extensively investigated.

Graves’ disease, an autoimmune disorder characterized by hyperthyroidism and a diffuse toxic goiter, is often treated by radioactive iodine 131I. Up to 35 % of patients with Graves’ disease, especially those with post 131I therapy, are found to have thyroid nodules which are often evaluated by fine-needle aspiration biopsy. Treatment with radioactive iodine can induce nuclear and cytoplasmic changes in follicular cells that could be mistaken as malignancy. The knowledge of a clinical history of Graves’ disease treated with 131I is of paramount importance in minimizing overdiagnosis of malignancy. On the other hand, patients with Graves’ disease can harbor thyroid malignancy, particularly, papillary thyroid carcinoma more than is seen in the general population. It is crucial to adhere strictly to the cytologic criteria in order to minimize over- and underdiagnosis of papillary thyroid carcinoma in patients with treated Graves’ disease.

Papillary thyroid carcinomas are the most common thyroid cancers and constitute more than 70 % of thyroid malignancies. The most common variants are the conventional type. However, many other uncommon variants have been described; the classification of these variants is based on the architectural patterns, such as follicular, solid, and cribriform-morular variants; the cell types, such as oncocytic, tall cell, and columnar cell variants; and the stromal reaction such as diffuse sclerosing, Warthin-like, and those with nodular fasciitis-like stroma. The common denominator for all the variants is the presence of nuclear features of PTC. The cytology may not be representative of the major morphologic pattern because of sampling error. In addition, a definitive cytologic diagnosis of PTC can sometimes be challenging if the typical nuclear changes of PTC were subtle or not readily apparent. It is advantageous in recognizing certain variants, namely, those with more aggressive clinical course, preoperatively since it may help the clinicians to plan for more aggressive treatment.

The 2007 National Cancer Institute State of the Science Conference proposed a category of follicular lesion of undetermined significance (FLUS) or atypia of unknown significance (AUS) for lesions that are characterized by too great a degree of architectural or cytologic atypia for definitive assignment to the benign category but insufficient findings for a diagnosis of follicular neoplasm or suspicious for carcinoma. Studies have shown that routinely subclassifying lesions in the FLUS category into two help to better stratify these cases for management, and uniform criteria have been defined for both subcategories. The first subcategory is for cases that demonstrate low cellularity as well as a predominantly microfollicular pattern and no or minimal colloid. These features elicit concern for a follicular neoplasm. The second subcategory is used for cases that show nuclear atypia, which elicit concern for a papillary carcinoma.

Hyalinizing trabecular tumor of the thyroid gland is a rare neoplasm of follicular origin that is characterized by an encapsulated nodule, a prominent trabecular growth pattern, and stromal hyalinization. This tumor typically behaves in a benign fashion but malignant cases have been reported. Hyalinizing trabecular tumors are usually well circumscribed or encapsulated with variability in color. The possibility that hyalinizing trabecular tumor represents a variant of papillary thyroid carcinoma has been considered given that the tumors frequently coexist and they share similar morphologic and nuclear features. However, on genetic analysis, hyalinizing trabecular tumor has been shown to be a discrete entity from papillary thyroid carcinoma. Hyalinizing trabecular tumors are frequently mistaken for papillary thyroid carcinoma, medullary carcinoma, or paraganglioma, but immunohistochemical stains are helpful in resolving this.

Poorly differentiated thyroid carcinoma, accounting for less than 10 % of all thyroid cancers, occupies an intermediate position between well-differentiated thyroid carcinoma and anaplastic thyroid carcinoma in regard to morphology and biologic behavior. The typical cytologic findings consist of predominantly small follicular cells with high nuclear-to-cytoplasmic ratio, marked nuclear crowding and overlapping, frequent mitotic figures, and irregular nuclei, frequently with evidence of necrosis. However, its differentiation from other thyroid neoplasms, in particular, follicular adenoma/carcinoma, and follicular variant of papillary thyroid carcinoma, may be difficult due to overlapping of cytologic features, such as microfollicular structures, irregular nuclei, and nuclear grooves. Therefore, when the cytologic findings do not permit a definitive diagnosis of poorly differentiated thyroid carcinoma, a diagnosis of “carcinoma of follicular origin” or “carcinoma not otherwise specified” should be rendered and not as “follicular variant of papillary thyroid carcinoma” and certainly not as “follicular neoplasm.”

Cystic change is commonly seen in papillary thyroid carcinoma. Macrophages can be seen in variable numbers with or without hemosiderin-laden pigments. Some cytologic features have been found to be useful in the differential diagnosis between cystic papillary thyroid carcinoma and cystic degeneration in a benign goiter. Atypical histiocytoid cells have been described in smears of cystic papillary thyroid carcinoma. These atypical histiocytoid cells have abundant vacuolated cytoplasm, nuclear pleomorphism, and nucleoli. They are larger and more atypical than benign histiocytes. The combination of macrophages, hemosiderin, cellular debris, and old blood in the background may obscure distinction between cystic papillary thyroid carcinoma and cystic goiters. Ancillary immunohistochemical markers and mutational analysis may be useful in challenging specimens.

The finding of thyroid tissue in a neck mass without distinct connections to the thyroid gland is uncommon but poses a diagnostic dilemma to the cytologist as to whether the finding represents a metastatic thyroid malignancy to the cervical node or ectopic thyroid tissue/benign thyroid inclusions in cervical lymph node. Some consider that all thyroid tissues found in the lateral cervical nodes represent nodal metastases from a primary thyroid carcinoma while others believe the existence of benign thyroid tissue inclusions in cervical lymph nodes. The finding of cytologically benign-appearing follicular cells with or without colloid does not necessarily imply a benign process since the pattern of growth of certain thyroid carcinomas can be so well differentiated as to simulate non-neoplastic thyroid tissue. On the other hand, the presence of cytologic and/or architectural atypia in follicular cells, even if accompanied by a lymphoid background, does not always indicate a metastatic thyroid carcinoma. Therefore, it is best to adopt a conservative stance with respect to a diagnosis of malignancy as long as definitive cytologic features of papillary thyroid carcinoma are not identified.