Standardized Ultrasound Evaluation for Active Surveillance of Low-Risk Thyroid Microcarcinoma in Adults: 2024 Korean Society of Thyroid Radiology Consensus Statement

Abstract

Active surveillance (AS) has been widely adopted as an alternative to immediate surgery owing to the indolent nature and favorable outcomes of papillary thyroid microcarcinoma (PTMC). AS is generally recommended for tumors measuring ≤1 cm without aggressive cytological subtypes, risk of gross extrathyroidal extension (ETE), lymph node metastasis (LNM), or distant metastasis. AS requires careful patient selection based on various patient and tumor characteristics, and ultrasound (US) findings. Moreover, during AS, regular US is performed to monitor any signs of tumor progression, including tumor growth, new US features of potential gross ETE, and LNM. Therefore, appropriate imaging-based assessment plays a crucial role in determining whether AS or surgery should be pursued. However, detailed recommendations concerning US evaluation are currently insufficient, necessitating the formulation of this guideline. The Korean Society of Thyroid Radiology has developed a consensus statement for low-risk PTMC, covering US assessment methods when considering AS as a management option and conducting follow-up imaging tests during AS. This guideline aims to provide optimal scientific evidence and expert opinion consensus regarding a standardized US-based assessment protocol for low-risk PTMC.

Keywords: Active surveillance; Consensus; Papillary thyroid cancer; Practice guideline; Recommendation; Thyroid neoplasms; Watchful waiting.

Fig. 1. Classification of subcapsular tumors based on US imaging. A: Anterior versus posterior subcapsular location. The thyroid capsule (echogenic line) contacting the anterior strap muscles can be regarded as the anterior capsule (solid line). Other areas can be classified as the posterior capsule (dotted line). B: Posteromedial versus posterolateral thyroid capsule. The posterior thyroid capsule can be divided into halves, namely, the posteromedial and posterolateral capsule.

Fig. 2. Imaging-based appropriateness criteria for AS in thyroid cancer (≤1 cm). A: Ideal tumor characteristics include tumors confined to the thyroid gland with no contact with the thyroid capsule and with no metastasis. B: Appropriate tumor characteristics. Among anterior subcapsular tumors (near the yellow line), tumors showing capsular contact (a), disruption (b), and protrusion (c) are appropriate for AS. Paratracheal tumors (near the orange line) abutting the trachea with acute angle (d) are appropriate for AS. Among posteromedial subcapsular tumors (near the red line), tumors showing preserved thyroid parenchyma between the tumor and tracheoesophageal groove are appropriate (e). Posterolateral subcapsular tumors with capsular abutment (f, g) are appropriate for AS. C: Inappropriate tumor characteristics. Anterior subcapsular tumors (near the yellow line) demonstrating strap muscle replacement (a), paratracheal tumors (near the orange line) showing right- or wide-angle abutment to trachea (b) (including tumors showing obvious tracheal cartilage invasion; c), posteromedial subcapsular tumors (near the red line) with loss of normal thyroid parenchyma (d, e) or obvious protrusion (f), and posterolateral subcapsular tumors (near the blue line) with obvious protrusion (g, h), biopsy-proven metastasis (arrows) are inappropriate candidates for AS. AS = active surveillance

Fig. 3. Basic planes for nodule size measurement on ultrasound. A, B: Transverse (A) and longitudinal planes (B). The transverse plane for size measurement of nodules should be set in the slice that shows the maximum dimension (transverse diameter) on transverse scans, and the longitudinal plane should be determined parallel to the central longitudinal plane of the thyroid gland, not necessarily the true anatomical sagittal plane. C, D: Angulation of the probe to <45° can be used to measure the maximal tumor dimension on the transverse (transverse diameter) (C) or longitudinal plane (craniocaudal diameter) (D).

Fig. 4. Recommended reproducible size measurement methods for obliquely oriented thyroid nodules under active surveillance. A: A 48-year-old female patient with papillary thyroid microcarcinoma in the left thyroid lobe. B, C: Incorrect axis measurement method. (B) The transverse and anteroposterior diameters are measured in the plane of the thyroid gland, and correct caliper placement for size measurement (C) should be placed along the orientation of the nodule (i.e., axes of maximum dimension of the nodule in the transverse image and maximum dimension perpendicular to the first measurement). Inaccurate caliper placement at baseline or follow-up may potentially lead to inadvertent decision of progression and surgery conversion.