Give me All lesion classification on FNAC
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I now have comprehensive data from multiple authoritative textbooks. Let me compile the complete answer:
FNAC (Fine Needle Aspiration Cytology) - All Lesion Classification Systems
FNAC reporting has been standardized by organ-specific classification systems. Below are all the major classifications used worldwide.
1. THYROID FNAC Classifications
A. The Bethesda System for Reporting Thyroid Cytopathology (BSRTC) - 2023 (3rd edition)
The most widely used international standard, developed by the National Cancer Institute. Six tiered categories:
| Bethesda Category | Diagnosis | Risk of Malignancy (NIFTP ≠ cancer) | Risk of Malignancy (NIFTP = cancer) | Usual Management |
|---|---|---|---|---|
| I | Nondiagnostic / Unsatisfactory | 5%-18% | 5%-20% | Repeat FNA with ultrasound guidance |
| II | Benign | 0%-4% | 2%-7% | Clinical and sonographic follow-up |
| III | Atypia of Undetermined Significance (AUS) / Follicular Lesion of Undetermined Significance (FLUS) | 6%-24% | ~13%-30% | Repeat FNA, molecular testing, or lobectomy |
| IV | Follicular Neoplasm (FN) / Suspicious for Follicular Neoplasm | 17%-28% | 23%-34% | Molecular testing or lobectomy |
| V | Suspicious for Malignancy | 58%-74% | 67%-83% | Near-total thyroidectomy or lobectomy |
| VI | Malignant | 94%-96% | 97%-100% | Near-total thyroidectomy or lobectomy |
NIFTP = Noninvasive Follicular Thyroid Neoplasm with Papillary-like nuclear features
- Source: Sabiston Textbook of Surgery, Table 73.3
Category VI (Malignant) includes: Papillary thyroid carcinoma, medullary carcinoma, poorly differentiated carcinoma, undifferentiated (anaplastic) carcinoma, squamous cell carcinoma, carcinoma with mixed features, metastatic carcinoma, and non-Hodgkin lymphoma.
B. BTA/RCPath "Thy" System (UK)
The British Thyroid Association / Royal College of Pathologists system:
| Code | Cytological Diagnosis | Management |
|---|---|---|
| Thy1 | Non-diagnostic | US assessment +/- repeat FNAB |
| Thy1c | Non-diagnostic - cystic lesion | Repeat or clinical correlation |
| Thy2 | Non-neoplastic (benign) | Correlate with clinical/US findings |
| Thy2c | Non-neoplastic - cystic lesion | Correlate clinically |
| Thy3a | Neoplasm possible - atypia (uncategorized) | Further US +/- repeat FNAB |
| Thy3f | Neoplasm possible - possible follicular neoplasm | Diagnostic hemithyroidectomy |
| Thy4 | Suspicious of malignancy | Diagnostic hemithyroidectomy |
| Thy5 | Diagnostic of malignancy (Malignant) | Surgical resection / near-total thyroidectomy |
- Source: Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol 1; Bailey & Love's Short Practice of Surgery 28th Ed.
C. Italian TIR System
| Code | Diagnosis | Risk of Malignancy |
|---|---|---|
| TIR 1 | Non-diagnostic | - |
| TIR 1c | Non-diagnostic cystic | - |
| TIR 2 | Non-malignant | <3% |
| TIR 3A | Low Risk Indeterminate Lesion (LRIL) | <10% |
| TIR 3B | High Risk Indeterminate Lesion (HRIL) | 15%-30% |
| TIR 4 | Suspicious of malignancy | 60%-80% |
| TIR 5 | Malignant | >95% |
D. Japanese System
| Code | Diagnosis | Risk of Malignancy |
|---|---|---|
| 1 | Inadequate | ~10% |
| 2 | Normal or benign | <1% |
| 3A | Follicular neoplasm A-1 (favour benign) | <15% |
| 3A | Follicular neoplasm A-2 (borderline) | 15%-30% |
| 3A | Follicular neoplasm A-3 (favour malignant) | 40%-60% |
| 3B | Indeterminate - Others | 40%-60% |
| 4 | Malignancy suspected | >80% |
| 5 | Malignant | - |
E. Australian System
| Code | Diagnosis | Risk of Malignancy |
|---|---|---|
| 1 | Non-diagnostic | 0%-10% |
| 2 | Benign | 0%-3% |
| 3 | Indeterminate / follicular lesion of undetermined significance | 5%-15% |
| 4 | Suggestive of a follicular neoplasm | 15%-30% |
| 5 | Suspicious of malignancy | 60%-75% |
| 6 | Malignant | >97% |
- All thyroid comparison data from: Scott-Brown's Otorhinolaryngology, Table 58.1
2. SALIVARY GLAND FNAC - Milan System
Proposed by an international panel of experts (American Society of Cytopathology and International Academy of Cytology). Six categories:
| Category | Diagnosis | Risk of Malignancy | Usual Management |
|---|---|---|---|
| I | Non-diagnostic | 25% | Clinical/radiological correlation; repeat FNAC |
| II | Non-neoplastic | 10% | Clinical follow-up and radiological correlation |
| III | Atypia of Undetermined Significance (AUS) | 20% | Repeat FNAC or surgery |
| IVA | Benign Neoplasm | <5% | Conservative surgery or clinical follow-up |
| IVB | Salivary Gland Neoplasm of Uncertain Malignant Potential (SUMP) | 35% | Conservative surgery* |
| V | Suspicious for malignancy | 60% | Surgery* |
| VI | Malignant | >90% | Surgery* (extent depends on type and grade) |
Intraoperative frozen section may help determine extent of surgery
- Source: Bailey & Love's Short Practice of Surgery 28th Ed., Table 54.4; Cummings Otolaryngology Head & Neck Surgery
Category details:
- Non-neoplastic (II): Includes inflammatory, metaplastic, and reactive processes - requires clinicoradiologic correlation
- AUS (III): Reactive atypia or poorly sampled neoplasms - requires repeat FNA or surgery
- Benign Neoplasm (IVA): Clear-cut benign neoplasms (e.g. pleomorphic adenoma, Warthin tumour)
- SUMP (IVB): Neoplasm identified but specific entity cannot be determined and malignancy cannot be excluded
3. CERVICAL / ANAL CYTOLOGY - Bethesda Classification
| Category | Meaning |
|---|---|
| Negative for Intraepithelial Lesion or Malignancy (NILM) | Normal / reactive changes |
| ASC-US | Atypical Squamous Cells of Undetermined Significance |
| ASC-H | Atypical Squamous Cells - cannot exclude HSIL |
| LSIL | Low-Grade Squamous Intraepithelial Lesion (CIN 1, HPV effect) |
| HSIL | High-Grade Squamous Intraepithelial Lesion (CIN 2/3) |
| SCC | Squamous Cell Carcinoma |
| AGC | Atypical Glandular Cells |
| AIS | Adenocarcinoma in Situ |
| Adenocarcinoma | Malignant glandular lesion |
The same Bethesda classification is applied to anal cytology with parallel terminology.
- Source: Tietz Textbook of Laboratory Medicine 7th Ed.; Pfenninger and Fowler's Procedures for Primary Care
4. BREAST FNAC - UK/NHS Classification (C-coding)
The standard UK breast FNAC reporting uses a 5-tier C-code system:
| Code | Category | Description |
|---|---|---|
| C1 | Inadequate/Non-diagnostic | Insufficient material for diagnosis |
| C2 | Benign | Benign epithelial cells, no malignant features |
| C3 | Atypia - probably benign | Atypical cells but most likely benign (equivocal) |
| C4 | Suspicious - probably malignant | Atypical cells with high suspicion of malignancy |
| C5 | Malignant | Diagnostic of malignancy |
Note: Breast FNAC is increasingly being supplemented or replaced by core biopsy, which provides architectural assessment and allows ER/PR/HER2 receptor testing.
5. GENERAL / GENERIC FNAC Classification
For non-organ-specific or general purpose reporting, the following broad five-tier scheme is used in many centers:
| Category | Description |
|---|---|
| Unsatisfactory | Insufficient cells for diagnosis; <6 follicles of 10+ cells (thyroid equivalent) |
| Benign / Non-neoplastic | No evidence of malignancy; benign features |
| Indeterminate / Atypical | Equivocal; cannot classify as clearly benign or malignant |
| Suspicious of malignancy | Features highly suggestive but not diagnostic of malignancy |
| Malignant | Diagnostic of malignancy (specify type: carcinoma, lymphoma, sarcoma, etc.) |
Histological Cytological Images - Bethesda Thyroid Categories
Representative cytologic features by Bethesda category: (A) Cat. II - benign colloid nodule with bland follicular cells in macrofollicular pattern; (B) Cat. III - AUS with nuclear enlargement and occasional intranuclear grooves; (C) Cat. IV - follicular neoplasm with highly cellular aspirate in microfollicles; (D) Cat. V - suspicious for PTC with nuclear enlargement and pale chromatin; (E) Cat. VI - PTC with "Orphan Annie eye" nuclei and intranuclear pseudoinclusions.
- Source: Sabiston Textbook of Surgery
Summary Table - FNAC Classification Systems by Organ
| Organ / Site | Classification System | No. of Categories |
|---|---|---|
| Thyroid | Bethesda System (BSRTC) - International | 6 |
| Thyroid | BTA/RCPath Thy System - UK | 6 (Thy1-Thy5 + subcategories) |
| Thyroid | Italian TIR System | 6 |
| Thyroid | Japanese System | 5 |
| Thyroid | Australian System | 6 |
| Salivary Gland | Milan System | 6 (I-VI) |
| Breast | C-coding System (UK/NHS) | 5 (C1-C5) |
| Cervix / Anal | Bethesda Cervical Classification | Multiple (NILM, ASC-US, LSIL, HSIL, etc.) |
| General / Generic | Five-tier scheme | 5 |
Key Clinical Principle: Regardless of organ system, all FNAC classification systems share the same fundamental principle - stratifying lesions by risk of malignancy to guide clinical management (repeat FNA, observation, surgery). The non-diagnostic and indeterminate categories are the most clinically challenging and often require additional workup (repeat FNA, molecular testing, or core biopsy).
Sources: Bailey & Love's Short Practice of Surgery 28th Ed. | Sabiston Textbook of Surgery | Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 1 | Cummings Otolaryngology Head & Neck Surgery | Tietz Textbook of Laboratory Medicine 7th Ed.
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