Follicular Variant of Papillary Thyroid Cancer

• Though recognized as a distinct entity for many years:
  • This common histologic subtype did not garner widespread acceptance into the cytopathologic nomenclature:
    • Until it was described in a case series by Chem and Rosai in 1977
• Characterized by its follicular architectural pattern:
  • The nuclear changes are variably present throughout the tumor
• Follicular variant papillary thyroid cancer demonstrates a follicular growth pattern:
  • But cytologically the tumors display the typical features of common-type papillary thyroid cancers, including:
    • Large, overlapping nuclei with hypodense chromatin
    • Nuclear pseudoinclusions, and nuclear grooves
    • Most also contain psammoma bodies

• Of the several histologic subtypes of papillary thyroid carcinoma:
  • The follicular variants are probably the most common
• The incidence of follicular variant papillary cancer has been increasing:
  • While that of the classical form has been decreasing
• In a single-center study:
  • Nearly 40% of the papillary thyroid cancers were follicular variants
• In a multicenter study:
  • 23% of papillary thyroid cancers were:
    • The noninvasive, well-circumscribed subtype:
      • Noninvasive follicular thyroid neoplasm with papillary-like nuclear features, NIFTP
• According to the 2017 World Health Organization classification of endocrine tumors:
  • Follicular variants of papillary thyroid cancer are subtyped as being either infiltrative or encapsulated with invasion:
    • Infiltrative:
      • The infiltrative subtype demonstrates invasive tongues of tumor infiltrating into non-neoplastic thyroid parenchyma and lacks a well-defined tumor capsule
      • The infiltrative subtype has a biological behavior and molecular profile that is more similar to classic papillary thyroid cancer than follicular cancers
      • The infiltrative subtype is more likely to have lymph node metastases and BRAF V600E mutations
      • While being less likely than follicular thyroid cancers to have distant metastases or RAS mutations
    • Encapsulated with invasion:
      • The encapsulated with invasion subtype demonstrates both a well-defined tumor capsule and either invasion of the tumor capsule or invasion into vessels within or beyond the tumor capsule
      • The encapsulated variants, particularly those with vascular invasion, have a tumor biology (often RAS mutation) and biological behavior (more likely to have distant metastases, less likely to have lymph node metastases) that is more similar to follicular thyroid cancer than to classical papillary thyroid cancer
      • It is the presence of either vascular or tumor capsular invasion that differentiates an encapsulated follicular variant of papillary thyroid cancer (capsular and/or vascular invasion present) from noninvasive encapsulated follicular variant of papillary thyroid cancer (NIFTP; no capsular or vascular invasion present)
      • In the past, encapsulated follicular variant of papillary thyroid cancer without evidence for either vascular or tumor capsule invasion were considered noninvasive variants of papillary thyroid cancer
        • However, noninvasive encapsulated follicular variants of papillary thyroid cancer have a very low malignant potential and are uniformly cured with lobectomy
        • Because of the very low malignant potential, this type was renamed noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP), emphasizing that this tumor can be managed as a neoplasm rather than a malignancy
        • Both the American Thyroid Association and the World Health Organization have endorsed this change in nomenclature
        • While thyroid surgery is required to distinguish NIFTP from the encapsulated with invasive subtype, therapy beyond thyroid lobectomy is usually not required (ie, thyroid-stimulating hormone [TSH] suppression and radioactive iodine ablation is not required)
        • Proper use of the NIFTP nomenclature requires adherence to strict histopathological criteria, which means that tumors previously classified as noninvasive encapsulated follicular variant of papillary thyroid cancer cannot be assumed to be NIFTP tumors
        • Because the entire tumor capsule may not have been sampled or preserved at the time of initial diagnosis, it is often not possible to determine if an archived tumor sample meets all the diagnostic criteria to be reclassified as an NIFTP tumor

#Arrangoiz #ThyroidSurgeon #ThyroidExpert #EndocrineSurgery #HeadandNeckSurgery #ThyroidCancer #FollicularVariantofPapillaryThyroidCancer #NIFT #Miami #Mexico #MountSinaiMedicalCenter