Thyroid follicular lesion of undetermined significance: Evaluation of the risk of malignancy using the two‐tier sub‐classification

Abstract

The Bethesda 2007 Thyroid Cytology Classification defines follicular lesion of undetermined significance as a heterogeneous category of cases that are not convincingly benign nor sufficiently atypical for a diagnosis of follicular neoplasm or suspicious for malignancy. In our institution, we refer to these cases as indeterminate, and they are further sub-classified into two: (1) low cellularity with predominant microfollicular architecture and absence of colloid (IN(a)) and (2) nuclear features not characteristic of benign lesions (nuclear atypia) (IN(b)). We reviewed these indeterminate cases to document the follow-up trend using this two-tier classification. A search of the cytology records was performed for the period between January 2008 and June 2009. All thyroid fine-needle aspiration (FNA) cases were reviewed and the ones diagnosed as indeterminate were identified. Correlating follow-up FNA and/or surgical pathology reports were reviewed. The percentage of cases showing a malignancy was calculated. One hundred and seventy-one indeterminate cases were identified, representing 2.8% of the 6,205 thyroid FNA cases examined during the time under review (107 IN(a), 64 IN(b)). Records of follow-up procedures were available in 106 (61%) cases. Malignancy was identified in 27% of all indeterminate cases. This was disproportionately more in the IN(b) (56%) compared to the IN(a) (7%) cases. A diagnosis of "IN(a)" does not carry the same implication as that of "IN(b)". The IN(b) category needs a more aggressive follow-up than the IN(a) category and may justify an immediate referral for lobectomy. Despite the vague morphologic criteria for this diagnostic category, the indeterminate rate remains relatively low and falls within the NCI recommendation (<7%).

Analiza znaczenia klinicznego niejednoznacznych wyników biopsji tarczycy ze szczególnym uwzględnieniem kategorii ZPBN na podstawie 5 lat stosowania nowej klasyfikacji rozpoznań cytologicznych

“…As a consequence of the above-mentioned factors the risk of malignancy in FLUS nodules as assessed on the basis of postoperative histopathological examination ranges from 4% up to as much as 50% [12,[22][23][24][25][26][27][28][30][31][32][33][34][35][36][37][38][39][40][41], so it exceeds by several times the values suggested by NCI recommendations. Many authors indicate that the final malignancy results were observed several times more frequently in AUS than in FLUS [7][8][9][10][11][12][13][14]. In our material, in 1 of 3 cases of FLUS with features of nuclear atypia a papillary cancer was found in postoperative examination.…”

“…A particularly controversial problem is the degree of nuclear atypia (polymorphism) that allows the categorisation of aspirate as FLUS and not SM. Some authors even distinguish the separate category for smears with abnormalities in cellular nuclei, such as the presence of occasional nuclear grooves, an abnormal chromatin pattern, or nuclear overlapping and crowding [7][8][9][10][11]22].…”

“…Another assumption was that FLUS diagnosis would not be a direct indication for surgical treatment, in contrast to other ISC categories (SFN and SM). However, numerous reports indicate that FLUS may be related to higher than assumed risk of malignancy [7][8][9][10][11][12][13][14]. It is also unclear whether implementation of the new FLUS category does not significantly affect the malignancy risk related to other ISC categories (SFN and SM).…”

Low malignancy risk of thyroid follicular lesion of undetermined significance in patients from post-endemic areas

“…Similar data were shown by Olson et al (12) -48 and 27% respectively. Horne et al (29) found that malignancy risk was several times higher in the group of FLUS with nuclear atypia (56%) than in the group with low cellularity and predominant microfollicular pattern (7%). In our material, FLUS with nuclear atypia was rare and the lesions with architectural atypia predominated, which may be a consequence of epidemiological situation and still high incidence of non-neoplastic follicular lesions.…”

“…This percentage varies between 4.7 and 48.2%, so in some centres, it is several times higher than that suggested in the NCI guidelines. Thus, there are attempts to distinguish among FLUS category two groups -showing 'nuclear atypia' that relates to higher risk for malignancy, and the group with 'microfollicular architecture' in which the risk is lower (12,19,28,29,30). As early as in 2001, Kelman et al (28) found that in the group of indeterminate specimens, cytological/nuclear atypia confers a higher risk of cancer than microfollicular patterns without atypia -60 and 7% respectively.…”