Before the ACR published its point-based TI-RADS system in 2017, the most widely adopted thyroid nodule scoring method was the Kwak TIRADS classification, proposed by Kwak et al. in 2011. If your report references Kwak TIRADS or uses a numbering system that looks different from the ACR version, this guide explains how it works and how the two compare.
How Kwak TIRADS Works
The Kwak TIRADS classification evaluates thyroid nodules for the presence of five suspicious ultrasound features:
- Solid composition
- Hypoechogenicity or marked hypoechogenicity
- Irregular or microlobulated margins
- Microcalcifications
- Taller-than-wide shape
Rather than assigning weighted points to each feature as the ACR system does, Kwak TIRADS simply counts how many suspicious features are present and assigns a category based on the total count:
| Category | Features present | Estimated malignancy risk |
|---|---|---|
| Kwak 2 | 0 (benign pattern) | ~0% |
| Kwak 3 | 0 (not clearly benign) | 1.7% |
| Kwak 4a | 1 suspicious feature | 3.3% |
| Kwak 4b | 2 suspicious features | 9.2% |
| Kwak 4c | 3 or 4 suspicious features | 44.4% |
| Kwak 5 | 5 suspicious features | 72.4% |
The 4a/4b/4c subcategorisation was one of the system’s innovations, providing granularity within the moderately suspicious range that earlier classification attempts lacked.
Kwak TIRADS vs ACR TI-RADS
The key structural difference is counting versus weighting. In Kwak TIRADS, finding one suspicious feature scores the same regardless of which feature it is. In the ACR system, a taller-than-wide shape scores 3 points while a macrocalcification scores only 1. The ACR’s weighted approach reflects the differing predictive values of individual features.
| Kwak TIRADS (2011) | ACR TI-RADS (2017) | |
|---|---|---|
| Method | Count of suspicious features | Weighted point sum |
| Categories | 2, 3, 4a, 4b, 4c, 5 | TR1, TR2, TR3, TR4, TR5 |
| Granularity | All features weighted equally | Each feature has a specific point value |
| Biopsy thresholds | Category-based, varies by study | Standardised size thresholds per category |
| Prevalence today | Some Asian institutions | Standard in North America |
Both systems identify the same five suspicious features. The practical consequence is that the ACR system can distinguish between a nodule with one high-risk feature (like taller-than-wide at 3 points) and one with one lower-risk feature (like a macrocalcification at 1 point), while Kwak treats them identically.
When reviewing external reports or published studies, always confirm which TIRADS system was used. Kwak 4b is not equivalent to ACR TR4, and the biopsy thresholds differ between systems. Cross-referencing results across systems without noting the classification used introduces inconsistency.
If your report mentions Kwak TIRADS or uses categories like 4a, 4b, or 4c, your radiologist used the older system. The principles are similar to the ACR version, but the specific numbers are different. Ask your doctor which system they used and what the recommendation is for your nodule.
History of TIRADS Systems
The concept of a standardized thyroid nodule reporting system was first proposed by Horvath et al. in 2009, inspired by the successful BI-RADS system used for breast imaging. Kwak et al. published a simplified, practical version in 2011 that gained broad clinical adoption. The ACR formed a committee that produced the current ACR TI-RADS in 2017, and the European Thyroid Association published EU-TIRADS in the same year. The Korean Society of Radiology published K-TIRADS, and China introduced C-TIRADS in 2020.
Which System Should You Use?
Use whichever system your institution has adopted. In North America, ACR TI-RADS 2017 is standard. In Europe, EU-TIRADS dominates. In Korea, K-TIRADS applies. The TI-RADS calculator on this site implements the ACR 2017 system specifically.
Source
Kwak JY, Han KH, Yoon JH, et al. Thyroid Imaging Reporting and Data System for US Features of Nodules: A Step in Establishing Better Stratification of Cancer Risk. Radiology. 2011;260(3):892–899.
Frequently Asked Questions
What is Kwak TIRADS?
Kwak TIRADS is a thyroid nodule risk stratification system proposed in 2011 by Kwak et al. It classifies nodules from category 2 (benign) to 5 (highly suspicious) based on the number of suspicious ultrasound features present. It was one of the first standardized TIRADS systems and influenced the development of the ACR TI-RADS published in 2017.
Is Kwak TIRADS still used?
Some institutions continue to use the Kwak system, particularly in Asia. However, the ACR TI-RADS (2017) has largely replaced it in North America, while EU-TIRADS is standard across Europe. When comparing results across institutions, it is important to confirm which system was applied.
What is the difference between Kwak TIRADS and ACR TI-RADS?
Kwak TIRADS counts the number of suspicious features present and assigns a category based on how many are found. ACR TI-RADS assigns weighted points to specific features and sums them. ACR’s point-based approach is considered more granular because it accounts for the type of feature, not just how many are present.