Thyroid gland
Bethesda system
Follicular neoplasm


Topic Completed: 16 December 2019

Minor changes: 16 December 2019

Copyright: 2014-2019, PathologyOutlines.com, Inc.

PubMed Search: Bethesda guidelines follicular neoplasm

Ayana Suzuki, C.T.
Andrey Bychkov, M.D., Ph.D.
Page views in 2019: 5,583
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Cite this page: Suzuki A, Bychkov A. Follicular neoplasm. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/thyroidfollicularneoplasm.html. Accessed July 4th, 2020.
Definition / general
  • Bethesda category IV, "follicular neoplasm / suspicious for a follicular neoplasm (FN / SFN)" is used for cases with a cellular aspirate comprised of follicular cells showing cell crowding or microfollicle formation (Thyroid 2017;27:1341)
  • Cases cytologically suspected for follicular adenoma and follicular carcinoma are included
    • Final diagnosis is based on tissue histology because capsular or vascular invasion are essential criteria
    • Follicular patterned lesions (nodular goiter, follicular adenoma, follicular variant of papillary carcinoma, follicular carcinoma and noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP)) have overlapping cytomorphologic features and cannot be accurately distinguished by fine needle aspiration alone
  • Some definitions and criteria for this category were slightly modified in the latest edition of the Bethesda system to accommodate a new entity: noninvasive follicular thyroid neoplasm with papillary-like nuclear features (Cancer 2007;111:306)
    • Criteria: cases with mild nuclear atypia (papillary carcinoma-like changes) were included in follicular neoplasm / suspicious for a follicular neoplasm
    • Reporting: recommended to mention the possibility of noninvasive follicular thyroid neoplasm with papillary-like nuclear features in the note (to avoid overtreatment)
Essential features
  • Includes cases with most of the follicular cells are arranged in cell crowding or microfollicle formation
  • Frequency 2.3 - 2.9%, resection rate 41.8 - 45.0%, risk of malignancy 25 - 40%
  • Most common histopathological diagnosis is follicular adenoma, followed by adenomatous nodule, follicular variant of papillary carcinoma and follicular carcinoma (Diagn Cytopathol 2018;46:148)
  • Impossible to distinguish follicular adenoma from follicular carcinoma by fine needle aspiration cytology
Terminology
  • Laboratory should choose one preferable term and use it exclusively for this category
  • Term "suspicious for a follicular neoplasm" may be more convenient than "follicular neoplasm" because some nodular goiter cases are included in this category
Clinical features
Diagnosis
  • Cellular aspirate comprised of follicular cells; most are arranged in an altered architectural pattern characterized by significant cell crowding or microfollicle formation
  • Sparsely cellular aspirates are excluded from this category and could be interpreted as atypia of undetermined significance or follicular lesion of undetermined significance (AUS / FLUS)
  • Cases that demonstrate suspicious or definitive nuclear features for papillary carcinoma are excluded from this category and should be classified as suspicious for malignancy or malignant, respectively
  • Follicular patterned aspirates with mild nuclear changes (swelling, contour irregularity or chromatin clearing) can be classified as follicular neoplasm / suspicious for a follicular neoplasm if true papillae and intranuclear cytoplasmic inclusions are absent
  • Invasive follicular variant of papillary carcinoma or noninvasive follicular thyroid neoplasm with papillary-like nuclear features can be represented in this category (Cancer Cytopathol 2016;124:767, Hum Pathol 2016;54:134)
Case reports
Treatment
  • Diagnostic thyroid lobectomy (Thyroid 2016;26:1)
  • Molecular testing may guide treatment
Cytology description
  • Moderate or marked cellularity
  • Atypical follicular cell architecture (cell crowding, microfollicles, trabecular and dispersed isolated cells)
    • Microfollicle is a flat group of < 15 follicular cells arranged in a circle that is at least two thirds complete (Am J Clin Pathol 2008;130:736)
  • Follicular cells are normal sized or enlarged and relatively uniform, with scant or moderate amount of cytoplasm
  • Nuclei are usually round and slightly hyperchromatic, with inconspicuous nucleoli
  • Some nuclear atypia may be seen, either enlarged, variably sized nuclei and prominent nucleoli or enlarged nuclei with nuclear contour irregularity and mild or focal chromatin clearing
  • Colloid is scant or absent but a small amount of inspissated colloid may be present within the microfollicle
  • Foamy histiocytes are not common unless the neoplasm is large
  • Hürthle cell predominant cases should be classified as follicular neoplasm, Hürthle cell type or suspicious for a follicular neoplasm, Hürthle cell type
Cytology images

Contributed by
Ayana Suzuki, C.T.

Microfollicle


 Contributed by
 Mark R. Wick, M.D.

Microfollicle



Images hosted on other servers:

Microfollicle

Follicular crowding

Trabecular pattern

Videos

Head and tail of the Bethesda system for thyroid

Thyroid cancer: fine needle aspiration, malignant or indeterminate results

Sample cytology report
  1. Dx / category: follicular neoplasm
    • Cellular aspirate of follicular cells with a predominantly microfollicular architecture, scattered isolated cells and scant colloid.
  2. Dx / category: suspicious for a follicular neoplasm
    • Although the architectural features suggest a follicular neoplasm, some nuclear features raise the possibility of an invasive follicular variant of papillary carcinoma or noninvasive follicular thyroid neoplasm with papillary-like nuclear features; distinction between these entities is not cytologically possible.
  3. Dx / category: suspicious for a follicular neoplasm
    • Cellular aspirate composed predominantly of crowded uniform cells without colloid. Features suggest a follicular neoplasm but the possibility of a parathyroid lesion cannot be excluded. Correlation with clinical, serologic and radiologic findings should be considered.
Differential diagnosis
Board review style question #1
Which lesion is not included in follicular neoplasm / suspicious for a follicular neoplasm category?

  1. Conventional papillary carcinoma
  2. Follicular carcinoma
  3. Invasive follicular variant of papillary carcinoma
  4. Nodular goiter
  5. Noninvasive follicular thyroid neoplasm with papillary-like nuclear features
Board review answer #1
A. Conventional papillary carcinoma. Lesions included in this category are follicular patterned lesions. Conventional papillary carcinoma is a papillary lesion with remarkable nuclear morphology.

Comment Here

Reference: Follicular neoplasm
Board review style question #2
Which histological diagnosis is most likely to correspond with this cytologic aspirate?



  1. Conventional papillary carcinoma
  2. Follicular adenoma or follicular carcinoma
  3. Invasive follicular variant of papillary carcinoma or noninvasive follicular thyroid neoplasm with papillary-like nuclear features
  4. Nodular goiter
  5. Parathyroid adenoma
Board review answer #2
C. Invasive follicular variant of papillary carcinoma or noninvasive follicular thyroid neoplasm with papillary-like nuclear features. Although the architectural features suggest a follicular neoplasm, the nuclei show grooves and powdery chromatin indicating papillary carcinoma

Comment Here

Reference: Follicular neoplasm
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