Malignancy rate in thyroid nodules categorized as atypia of undetermined significance or follicular lesion of undetermined significance - An institutional experience

Abstract

Introduction: Atypia of undetermined significance (AUS) or follicular lesion of undetermined significance (FLUS) of Bethesda system for reporting thyroid cytopathology has emerged as most controversial category due to its heterogeneity and inconsistent usage. Initially associated risk of malignancy was estimated to be about 5-15%, but eventually different results have been obtained across institutions due to variable follow-ups and interpretation. The present study was conducted to evaluate the rate of malignancy along with appropriate management in this category.

Materials and methods: The study included AUS/FLUS cases diagnosed over period of 4 years on thyroid fine needle aspiration cytology (FNAC). Detailed clinical history and radiological examination were recorded along with follow-ups and management of all cases. Histopathology was available for cases which underwent surgery due to high clinical suspicion, worrisome radiological features or two consecutive AUS/FLUS reports.

Results: The incidence of AUS/FLUS was 6.4% with repeat FNAC done in 39.6%, immediate surgery in 49.2% and follow-up without intervention in 11.1% cases. The overall malignancy rate was 28.5% with most common malignancy being follicular variant of papillary carcinoma (FVPC). Malignancy rate was more in cases (24.1%) which morphologically showed atypical nuclear features in comparison to architectural atypia.

Conclusion: AUS/FLUS is associated with higher risk of malignancy than previously anticipated. Surgical intervention may have greater implication than repeat FNAC in appropriate management of this category. Substratification of this category according to nuclear features and architecture may be more useful in predicting malignancy risk and reducing the variability in the group.

Keywords: Atypia of undetermined significance; Bethesda system of reporting thyroid cytology; follicular lesion of undetermined significance; malignancy rate.

Figure 1

(a and b) FNAC thyroid shows mostly Hurthle cell change with few small follicles and abundant colloid in background (a: May–Grunwald–Giemsa; ×100, b: May-Grünwald-Giemsa; ×200); (c) Smears show few nuclei showing intra nuclear inclusions. Case was diagnosed as category AUS/FLUS of BSRTC (Pap stain x1000); (d) On histopathological examination, the case was diagnosed as follicular variant of papillary carcinoma (H and E stain x100)

Figure 2

(a) FNAC thyroid showing follicular cells arranged in sheets with few showing Hurthle cell and degenerative change (H and E stain x200); (b and c) Smears show few nuclei showing intra nuclear inclusions (black arrow) but features not sufficient enough to be diagnosed as papillary carcinoma. Case was diagnosed as category AUS/FLUS of BSRTC (b: H and E stain ×200, c: May–Grunwald–Giemsa; ×400); (d) Histopathological examination of same case showed papillary carcinoma (H and E stain x200)