Cytology reporting

Malignancy Risk by Category: — A Reference Table

Each cytology category carries a published malignancy risk range. The number tells you what that category means across the population studied — not what your result means for you specifically. Your oncologist applies these ranges to your full clinical picture.

Medically reviewed by Dr. C. Raghavendra Reddy, Medical Oncologist, MBBS (Gold Medal) · DNB · DM (Medical Oncology, Gold Medal) · Last reviewed September 2026

One table, three systems — Thyroid, salivary gland, and urine cytology all use numbered or named categories with published risk ranges — shown here side by side.
Category is not a diagnosis — A category assigns a population-level risk to a result type. It does not tell you whether you have cancer.
Figures are ranges, not certainties — Published risk of malignancy percentages vary by study and institution. They are population estimates, not individual predictions.
The organ matters as much as the number — Category I carries 5–10% malignancy risk in thyroid cytology and around 25% in salivary gland cytology. The same number means something different in each system.

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Cytology reporting systems attach a published malignancy risk range to each category. Ranges come from pooled studies where surgery confirmed or ruled out cancer. For thyroid (TBSRTC 2023), risk runs from 0–3% at Category II to 97–99% at Category VI. For salivary gland (Milan 2021) and urine (Paris 2022), the scales differ — this table covers all three.

What is the malignancy risk at each cytology category?

Tier	Thyroid — TBSRTC 2023	Salivary gland — Milan 2021	Urine — Paris 2022 (HGUC risk)
Non-diagnostic	Cat. I · ROM 5–10%	Cat. I · ROM ~25%	Non-diagnostic (no risk assigned)
Negative / Benign	Cat. II · ROM 0–3%	Cat. II · ROM ~10%	NHGUC · ~2–7%
Indeterminate / Atypia	Cat. III · ROM 10–30%	Cat. III · ROM ~20%	AUC · ~20–30%
Neoplasm — benign / low-grade	—	Cat. IVA · ROM ~5%	LGUN (low-grade neoplasm)
Neoplasm — uncertain malignant potential	Cat. IV · ROM 25–40%	Cat. IVB · ROM ~35%	—
Suspicious for malignancy	Cat. V · ROM 50–75%	Cat. V · ROM ~60%	SHGUC · ~50–80%
Malignant	Cat. VI · ROM 97–99%	Cat. VI · ROM ~90%	HGUC · ~80–90%
Sources	Cibas & Ali, TBSRTC 3rd ed. (2023)	Milan System 2nd ed. (2021)	Paris System Working Group, 2nd ed. (2022)

How reliable are these risk figures?

These figures are most reliable at the extremes — benign and malignant categories — and carry wider uncertainty in the middle tiers.

The ranges come from pooled studies where cytology categories were later confirmed by surgical pathology. Lower-risk categories are harder to verify because many patients with a benign result are never operated on, so the true false-negative rate remains uncertain. NCCN and ASCO note this limitation explicitly in their cytology guidance.

Higher categories are better supported by data. Suspicious and malignant results almost always lead to surgery, generating large, well-verified datasets. The Category V and VI ranges across all three systems are therefore among the most robust in oncology cytopathology.

Ranges also vary by institution. A centre that sees a higher proportion of high-risk patients will observe higher malignancy rates within the same categories. This is why the published figures are always expressed as ranges rather than single numbers.

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What do the terms on my cytology report mean?

ROM — Risk of malignancy: The proportion of cases in a given category that were found to be cancer on surgical follow-up. It is a population figure derived from studies, not a statement about your individual result.
AUS / FLUS: Atypia of undetermined significance / follicular lesion of undetermined significance — the Category III label in thyroid TBSRTC reporting. Cells are not clearly normal and not clearly malignant.
NHGUC: Negative for high-grade urothelial carcinoma — the lowest-risk category in the Paris System for urine cytology. It means high-grade cancer cells were not seen in the sample, not that all abnormality is excluded.
AUC: Atypical urothelial cells — the indeterminate category in the Paris System. Cells look abnormal but not enough to be called suspicious for high-grade cancer.
HGUC: High-grade urothelial carcinoma — the malignant-positive category in the Paris System, meaning high-grade cancer cells were identified in the urine sample.
FNA — Fine-needle aspiration: The sampling method used to collect cells from a nodule or mass. Most thyroid, salivary gland, and lymph node cytology reports come from FNA.

Did you know?

In the Milan System for salivary gland cytology, a non-diagnostic result carries an estimated malignancy risk of around 25% — higher than the indeterminate Category III in thyroid cytology (10–30%).

This reflects the underlying biology: salivary gland masses are more often malignant at baseline than thyroid nodules. The same category label carries a different meaning across organ sites, which is why the system name on your report matters as much as the category number.

Source: Milan System for Salivary Gland Cytopathology, 2nd edition (2021)

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Common questions

Frequently asked questions

What does my category number actually tell me?

It tells you how often cases in that category turned out to be cancer in the pooled studies that built the reporting system. It is a population-level estimate, not a verdict about your result. A Category III thyroid result with a 10–30% ROM means that between one in ten and three in ten cases in that category were found to be malignant at surgery — not that your chance of cancer is necessarily in that range. Your oncologist applies this figure alongside your scan findings, family history, and clinical examination before recommending next steps.

Why does the same category number mean different things across organs?

Because each system was built from studies of that specific organ. The baseline rate of malignancy differs — salivary gland masses are more often malignant than thyroid nodules at the time of testing — so the risk attached to each category reflects that starting point. Category I in TBSRTC (thyroid) carries a 5–10% ROM; Category I in the Milan System (salivary gland) carries around 25%. This is not an inconsistency. It is an accurate reflection of the underlying biology of each organ site.

Why does a non-diagnostic result still carry a malignancy risk?

A non-diagnostic result means the sample did not contain enough cells to classify confidently. It is a statement about sample quality, not about whether cancer is present. Because some of those inadequate cases later turned out to be malignant at surgery — due to poor sampling of a cancerous area — the category carries a residual risk. This is why your team will usually recommend repeat sampling rather than reassurance after a non-diagnostic result.

Do these published risk figures apply to Indian patients?

The published ranges come largely from North American, European, and East Asian datasets. Direct Indian data from studies of equivalent scale are limited, and some cancer types show different rates in Indian populations. ICMR has not yet published a parallel Indian dataset of comparable size. Your oncologist will apply the published figures as a guide while accounting for your individual clinical picture. We do not yet know how closely these ranges track across Indian referral populations, and that uncertainty is worth acknowledging rather than papering over.

Are there cytology systems not covered in this table?

Yes. The Bethesda System for cervical cytology uses text-based categories — NILM, ASC-US, LSIL, HSIL — and measures risk of CIN2 or higher pre-malignant change rather than outright malignancy, which is a different metric. The Papanicolaou Society of Cytopathology system for pancreaticobiliary samples uses Categories I–VI with its own published risk ranges. The International System for Serous Fluid Cytopathology covers pleural, pericardial, and ascitic fluid. Each of these is addressed separately in this series.

What system was used for my report, and where do I find it?

The report header or the final diagnostic line usually names the system. A thyroid FNA report may say 'Bethesda Category III' or 'TBSRTC Category III.' A salivary gland report may say 'Milan Category II.' A urine cytology report may say 'Paris System: NHGUC.' If the system is not named, the category number alone is ambiguous. Ask your reporting pathologist or treating oncologist to confirm which system was used before looking up a risk range.

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