Introduction: The diagnostic category of follicular lesion of undetermined significance (FLUS) was intended to allow selection of cases with low risk of malignancy from all smears with indeterminate, suspicious cytology (ISC), which can potentially take advantage from repeat fine-needle aspiration (rFNA). Aim of the study was a comparison of the risk of malignancy related to FLUS nodules and other nodules with ISC: suspected follicular neoplasm (SFN) and suspected malignancy (SM), as well as analysis of the usefulness of assessing ultrasonographic malignancy risk features (UMRF) in nodules with ISC.

Material and methods: We analysed UMRF, rFNA, and results of histopathological examination (H) in 441 FLUS, 135 SFN, and 72 SM nodules.

Results: The frequency of exposing cancer in H in FLUS nodules was 5.9%, and when cytological follow up was also included it was 2.9%. rFNAs made the diagnosis more precise in 72.7% of FLUS, and in 5.2% it was diagnosis/suspicion of cancer. The incidence of cancer in SFN nodules was 8.2%, in SM nodules with suspicion of papillary cancer — 61.1%, and in nodules with suspicion of other or unspecified malignancy — 53.8% (p < 0.0001 FLUS vs. both groups). The presence of calcifications is the only independent UMRF for nodules with ISC (OR 4.7). Features of importance are also microcalcifications (OR 3.8), especially in the SM group, and taller-than-wide-shape (OR 2.2). FLUS and SFN nodules are characterised by particularly low value of assessing suspicious margins; analysis of hypoechogenicity is of low value in SFN nodules, like suspected vascularisation in SFN and SM nodules.

Conclusions: The risk of cancer in FLUS and SFN nodules is lower than in SM nodules. rFNAs of FLUS nodules make the diagnosis more precise in more than 70% of cases and are effective in revealing cancers. UMRFs present variable diagnostic value depending on the subcategory of ISC. (Endokrynol Pol 2016; 67 (1): 23–34)