Thyroid carcinomas originating from follicular cells have the prognosis of heterogeneous diseases, but pathologists classify them all as malignant disease. Epidemiologists have issued a stern warning regarding over-diagnosis and overtreatment of patients with indolent thyroid
tumors that cause no harm to the patients. Review of the literature revealed that there were several proposals of borderline/precursor
tumors to some indolent
thyroid carcinomas. Thyroid
tumor of uncertain malignant potential (
UMP) was first proposed by Williams for encapsulated follicular pattern thyroid
tumors to solve problems due to observer variation. Rosai et al. proposed to rename papillary microcarcinoma (PMC) to papillary micro-
tumor as the overwhelming majority of them are of no clinical significance. Liu et al. proposed well-differentiated
tumor with uncertain behavior (WDT-UB) which covered WDT of
UMP (WDT-
UMP) and non-invasive encapsulated follicular variant of
papillary thyroid carcinoma (EFVPTC). The EFVPTC without invasion was renamed as non-invasive follicular
thyroid neoplasm with papillary-like nuclear features (NIFTP) by an international panel of pathologists. A new prognostic classification of thyroid
tumors was proposed by Kakudo et al., in which extremely low risk
tumors were grouped in a borderline
tumors category. The borderline/precursor thyroid
tumors included encapsulated
tumors [capsular invasion only follicular
carcinoma, encapsulated
papillary carcinoma without invasion, WDT-
UMP and follicular
tumor of
UMP (FT-
UMP)] and non-encapsulated
tumors (PMC). The
UMP and NIFTP were incorporated in the 4th edition WHO classification of thyroid
tumors as a new
tumor entity in chapter 2-2A: other encapsulated follicular patterned thyroid
tumors. Their behavior codes were decided to be 1 (borderline or uncertain behavior), and not 0 (benign), 2 (in situ
carcinoma) or 3 (malignant). These borderline/precursor thyroid
tumors are indolent
tumors biologically and should be treated more conservatively than as previously recommended for thyroid follicular cell
carcinomas [total
thyroidectomy (TTX) followed by radio-active
iodine (RAI) treatment] by western clinical guidelines.