13 General introduction and outline of this thesis 1 Table 1. The Bethesda System for Reporting Thyroid Cytopathology [16, 18, 27] 2009 Bethesda System 2017 Bethesda System 2023 Bethesda System Usual management Category ROM Category ROM if NIFTP ≠ CA ROM if NIFTP = CA Category ROM if NIFTP ≠ CA, mean % (range) ROM if NIFTP = CA, mean % I Nondiagnostic or unsatisfactory 1-4% Nondiagnostic or unsatisfactory 5-10% 5-10% Nondiagnostic 13% (5-20%) 12% Repeat FNAC with ultrasound guidance II Benign 0-3% Benign 0-3% 0-3% Benign 4% (2-7%)3 2% Clinical and ultrasound1 follow-up III Atypia of Undetermined Significance or Follicular Lesion of Undetermined Significance (AUS/FLUS) ~5-15% Atypia of Undetermined Significance or Follicular Lesion of Undetermined Significance (AUS/FLUS)4 6-18% ~10-30% Atypia of undetermined significance (AUS)4 22% (13-30%) 16% Repeat FNAC, MD1, diagnostic lobectomy1, or surveillance1 IV Follicular neoplasm or suspicious for a follicular neoplasm, specify if Hürthle cell type 15-30% Follicular neoplasm or suspicious for a follicular neoplasm, specify if Hürthle cell type 10-40% 25-40% Follicular neoplasm, specify if oncocytic type 30% (23-34%) 23% MD1, diagnostic lobectomy V Suspicious for malignancy 60-75% Suspicious for malignancy 45-60% 50-75% Suspicious for malignancy 74% (67-83%) 65% MD2, lobectomy or (near-)total thyroidectomy VI Malignant 97-99% Malignant 94-96% 97-99% Malignant 97% (97-100%) 94% Lobectomy1 or (near-)total thyroidectomy CA, carcinoma; FNAC, fine needle aspiration cytology; MD, molecular diagnostics; NIFTP, non-invasive follicular thyroid neoplasm with papillary-like nuclear features; ROM, risk of malignancy. 1: Introduced with the 2017 Bethesda System [18]. 2: Introduced with the 2023 Bethesda System [27]. 3: This ROM estimate is based on follow-up of surgically resected nodules. As most thyroid nodules with benign cytology do not undergo surgical excision, this ROM is likely skewed by selection bias. Based on long-term follow-up studies, the best ROM estimate for benign cytology is ~1% to 2% [23]. 4: This category can be further subclassified according to the type of atypia. The 2017 Bethesda System recognized cytologic atypia, architectural atypia, cytology and architectural atypia, Hürthle cell AUS/FLUS, and atypia not otherwise specified [18]. The 2023 Bethesda System recognizes nuclear atypia (previously cytologic atypia) and non-nuclear atypia. The ROM appears to be higher for AUS cytology with nuclear atypia [27].
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