Table 1. AJCC TNM seventh and eighth edition: definitions.

Seventh edition	Eighth edition
Tumor
T1a: tumor ≤1 cm limited to the thyroid	T1a: tumor ≤1 cm limited to the thyroid
T1b: tumor >1 cm but ≤2 cm limited to the thyroid	T1b: tumor >1 cm but ≤2 cm limited to the thyroid
T2: tumor >2 cm but ≤4 cm limited to the thyroid	T2: tumor >2 cm but ≤4 cm limited to the thyroid
T3: tumor >4 cm limited to the thyroid or minimal extrathyroid extension (for example, perithyroidal soft tissues or sternothyroid muscle) from a tumor of any size	T3a: tumor >4 cm limited to the thyroid T3b: gross extrathyroidal extension invading only strap muscles (sternohyoid, sternothyroid, thyrohyoid, omohyoid) from a tumor of any size
T4a: gross extrathyroidal extension invading subcutaneous soft tissues, larynx, trachea, esophagus, or recurrent laryngeal nerve from a tumor of any size	T4a: gross extrathyroidal extension invading subcutaneous soft tissues, larynx, trachea, esophagus, or recurrent laryngeal nerve from a tumor of any size
T4b: gross extrathyroidal extension invading prevertebral fascia or encasing the carotid artery or mediastinal vessels from a tumor of any size	T4b: gross extrathyroidal extension invading prevertebral fascia or encasing the carotid artery or mediastinal vessels from a tumor of any size
Node
Nx: regional lymph nodes cannot be assessed	Nx: regional lymph nodes cannot be assessed
N0: no evidence of locoregional lymph node metastasis	N0a: one or more cytologically or histologically confirmed benign lymph nodes N0b: no radiologic or clinical evidence of locoregional lymph node metastasis
N1a: ipsilateral or bilateral metastasis to level VI (pretracheal, paratracheal, or prelaryngeal/Delphian) lymph nodes	N1a: ipsilateral or bilateral metastasis to level VI or VII (pretracheal, paratracheal, or prelaryngeal/Delphian, or upper mediastinal) lymph nodes; this can be unilateral or bilateral disease
N1b: metastasis to unilateral, bilateral, or contralateral lateral neck lymph nodes (levels I, II, III, IV, or V) or retropharyngeal or superior mediastinal lymph nodes (level VII)	N1b: metastasis to unilateral, bilateral, or contralateral lateral neck lymph nodes (levels I, II, III, IV, or V) or retropharyngeal lymph nodes
Metastasis
M0: no distant metastasis	M0: no distant metastasis
M1: distant metastasis	M1: distant metastasis

AJCC, American Joint Cancer Committee; TNM, tumor, node, and metastasis.