title: Cytologic features of focal papillary thyroid carcinoma arising within follicular adenoma: a masked cytomorphologic analysis of 17 cases.
abstract: OBJECTIVE
  Focal papillary thyroid carcinoma (PTC) arising within a follicular adenoma (PTCFA) represents a clinically significant, but rare, histopathologic subset of papillary carcinomas whose cytologic features have not been well described. This uncommon presentation of PTC may contribute to a subset of thyroid aspirates interpreted as 'atypia of undetermined significance/follicular lesion of undetermined significance' (AUS/FLUS).
  
  
  METHODS
  Seventeen fine-needle aspiration biopsy (FNAB) cases diagnosed as 'PTCFA' on corresponding surgical excision were identified from the archival records of 2 large academic medical centers. A control group of 40 FNAB comprised of 20 follicular adenomas (FA) and 20 PTC was identified (based on the corresponding surgical pathology diagnosis) for comparison. All 57 FNAB were reviewed in a masked fashion and scored for a series of 31 cytomorphologic features. The intraclass correlation between diagnostic categories and overall agreement between cytopathologists was statistically evaluated.
  
  
  RESULTS
  Aspirates of PTCFA were originally diagnosed as 'negative' (n = 3), 'AUS/FLUS' (n = 7), 'suspicious for a follicular neoplasm' (n = 3), 'suspicious for malignancy' (n = 3), and 'malignant' (n = 1). On masked review, the most common cytomorphologic features of PTCFA were a nonmacrofollicular cytoarchitectural pattern (71%), medium-large cell size (74%), and micronucleoli (79%). Intranuclear pseudoinclusions and a papillary architecture were absent in 85 and 88% of the cases, respectively. Relative to the 2 control groups, the PTCFA cases demonstrated overlapping features between FA and PTC for the majority of the 31 examined cytomorphologic features.
  
  
  CONCLUSIONS
  PTCFA represent a rare subset of PTC that is difficult to recognize as PTC by FNAB. Most cases exhibit overlapping features between a benign thyroid nodule and conventional PTC, and they are often interpreted as 'AUS/FLUS'.
authors.affiliation: Department of Pathology, Massachusetts General Hospital, Boston, Mass., USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Ono
authors.forename: D C
authors.initials: DC
authors.lastname: Wilbur
authors.forename: H
authors.initials: H
authors.lastname: Lee
authors.forename: J
authors.initials: J
authors.lastname: Yang
authors.forename: J F
authors.initials: JF
authors.lastname: Krane
authors.forename: M
authors.initials: M
authors.lastname: Bongiovanni
authors.forename: W C
authors.initials: WC
authors.lastname: Faquin
mesh_terms: D000236:Adenoma / Q000145:classification / Q000175:diagnosis / Q000473:pathology*; D000328:Adult; D044963:Biopsy, Fine-Needle; D002277:Carcinoma / Q000145:classification / Q000175:diagnosis / Q000473:pathology*; D002291:Carcinoma, Papillary; D005188:False Negative Reactions; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D018193:Neoplasms, Complex and Mixed / Q000145:classification / Q000175:diagnosis / Q000473:pathology*; D017410:Practice Guidelines as Topic; D011379:Prognosis; D012189:Retrospective Studies; D012306:Risk; D009626:Terminology as Topic; D000077273:Thyroid Cancer, Papillary; D013961:Thyroid Gland / Q000473:pathology*; D013964:Thyroid Neoplasms / Q000145:classification / Q000175:diagnosis / Q000473:pathology*; D016606:Thyroid Nodule / Q000145:classification / Q000175:diagnosis / Q000473:pathology*