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