| Category | Meaning |
1 | Not assessable |
2 | No evidence of malignancy |
3 – AUS/FLUS | Unclear dignity |
4 | Suspicion of follicular neoplasia |
5 | Suspicious for malignancy |
| 6 | Malignant |
The Bethesda Classification
BTS I – non-diagnostic/non-informative: In this group, the cellular image does not allow for a reliable interpretation. Typically, about 2 – 20% of thyroid FNAs are not assessable.
BTS II – benign: These are cellular images that show no indication of malignancy. Clinical monitoring is recommended here, with a malignancy rate of 0 – 3%.
BTS III – Atypia of undetermined significance/Follicular lesion of undetermined significance: This group of findings is very heterogeneous. It includes cellular images that cannot be assigned to another suspicious group or the benign group. Often, limitations in assessability, such as blood overlays or low cell content, justify the use of this group, which also justifies the approach of re-puncture. However, malignancy rates vary greatly depending on the study (from 2% to 30%!).
BTS IV – follicular neoplasia: The cellular image of a follicular neoplasia can be either a benign (adenoma, adenomatous hyperplasia) or a malignant (follicular carcinoma) lesion, as these entities can only be differentiated by histological criteria (capsule breakthrough, capsule vessel invasion). According to the literature, the probability of malignancy is 15 – 30%. Generally, follicular neoplasia represents an indication for surgery.
BTS V – Suspicion of malignancy: These are cellular images suspected of a non-follicular carcinoma (e.g., papillary thyroid carcinoma), where a benign process cannot be definitively excluded based on the cellular image (e.g., inflammation with atypia). The malignancy rate is about 60 – 75%, and surgical clarification is recommended here.
BTS VI – malignant: In cytologies of this group, there is a malignant cellular image, where, if possible, a precise classification should be made in the diagnosis (e.g., papillary thyroid carcinoma). The probability of malignancy here is between 97% and 99%, and thus usually represents an indication for surgery.