Colloid nodule

• Flat honeycomb sheets of follicular cells which are evenly spaced showing uniform round nuclei with granular chromatin and scant cytoplasm.
• Colloid can appear in the form of a thin, translucent film with bubbles and linear cracks or in the form of thicker blobs.
• Macrophages are sometimes present, and, if abundant or accompanied by granular proteinaceous material, indicate cystic change.

• In addition to follicular cells, there are histiocytes, with varying numbers of lymphocytes and/or some neutrophils.
• May have poorly formed epithelioid granulomas.
• Presence of large multinucleated giant cells containing numerous nuclei is a helpful feature (sometimes these cells engulf colloid).
• Note: Smaller multinucleated giant cells are less specific and may be seen in Hashimoto thyroiditis and papillary thyroid carcinoma.

• Benign groups of follicular cells and scant colloid.
• Follicular cells may show varying proportions of Hurthle cells with round nuclei and abundant granular cytoplasm.
• Many lymphocytes in the background – mixed population, predominantly small lymphocytes.
• Sometimes germinal centre material may be seen. Sometimes multinucleated giant cells are also seen.

Atypical; Follicular Lesion of Undetermined Significance / Atypia of Undetermined Significance

• The FLUS category is a mixed one, covering a spectrum of findings.
• There is architectural and/or nuclear atypia; but not to the extent of being suspicious for malignancy.
• This is an example of FLUS/AUS with architectural atypia:
  • In some areas, the follicular cells show architectural atypia e.g. microfollicles, crowded sheets; while in other areas, some flat sheets are seen.

Atypical; Follicular Lesion of Undetermined Significance / Atypia of Undetermined Significance

• The FLUS category is a mixed one, covering a spectrum of findings.
• There is architectural and/or nuclear atypia; but not to the extent of being suspicious for malignancy.
• This is an example of FLUS/AUS with both architectural and nuclear atypia:
  • There are crowded sheets of follicular cells arranged in trabecular formations.
  • Nuclear atypia is seen, with enlarged nuclei with occasional nuclear grooves and rare nuclear pseudoinclusions.
  • There are insufficient definitive diagnostic features for papillary thyroid carcinoma.

Follicular Neoplasm

• Crowded sheets, microfollicles or trabecular arrangements.
• Follicular cells contain round nuclei and with granular chromatin and inconspicuous nucleoli.

Malignant; Papillary Thyroid Carcinoma

• Usually highly cellular aspirates, showing many monolayered syncytial sheets and sometimes papillary structures.
• Nuclei overlap each other within the syncytial sheets.
• Nuclei are often ovoid and enlarged, with fine powdery chromatin, occasional nuclear grooves and nuclear pseudoinclusions.
• Cytoplasm may be delicate or dense.
• Thick colloid (“bubble-gum” colloid), which are better appreciated in Romanowsky stains, may be present.
• Multinucleated giant cells may be noted in the background.
• Sometimes, psammoma bodies may be seen, especially in cystic PTC.

Malignant; Medullary Thyroid Carcinoma

• Cellular aspirates usually.
• Loose cell clusters and singly occurring cells.
• Cells appear epithelioid/plasmacytoid or spindled.
• Some binucleated or multinucleated cells may be present.
• Nuclei have ‘salt-and-pepper’ chromatin and inconspicuous nucleoli.
• Occasional nuclear inclusions may be seen.
• Sometimes, background amyloid may be identified.

Malignant; Anaplastic Thyroid Carcinoma

• Markedly pleomorphic nuclei; sometimes spindle cells (sarcomatoid appearance) and squamoid cells.
• Necrosis and inflammatory cells may be seen in the background.