Anaplastic thyroid cancer staging

According to the American Joint Committee on Cancer (AJCC)^[1] there are 4 stages of anaplastic thyroid cancer based on the clinical features and findings on imaging. Each stage is assigned a letter and a number that designate the tumor size, number of lymph node regions involved, and metastasis.

• No generally accepted staging system is available for anaplastic thyroid cancer. Because the prognosis for anaplastic thyroid cancer is so poor, all anaplastic thyroid cancer is classified as stage IV according to International Union Against Cancer (UICC) staging, regardless of tumor size, nodal status, and either absence or presence of distant metastasis.
• Undifferentiated (anaplastic) carcinomas are highly malignant cancers of the thyroid. They may be subclassified as small cell or large cell carcinomas. Both grow rapidly and extend to structures beyond the thyroid. Both small cell and large cell carcinomas present as hard, ill-defined masses, often with extension into the structures surrounding the thyroid. Small cell anaplastic thyroid carcinoma must be carefully distinguished from lymphoma. This tumor usually occurs in an older age group and is characterized by extensive local invasion and rapid progression. Five-year survival with this tumor is poor. Death is usually from uncontrolled local cancer in the neck, usually within months of diagnosis.^[2]

Primary tumor
Tumor size	Sub-stage	Finding	Image
T0		No evidence of primary tumor
T1		Tumor ≤2 cm in greatest dimension limited to the thyroid
T1	a	Tumor ≤1 cm, limited to the thyroid
T1	b	Tumor >1 cm but ≤2 cm in greatest dimension, limited to the thyroid
T2		Tumor >2 cm but ≤4 cm in greatest dimension, limited to the thyroid
T3		Tumor >4 cm limited to the thyroid, or gross extrathyroidal extension invading only strap muscles
T3	a	Tumor >4 cm limited to the thyroid
T3	b	Gross extrathyroidal extension invading only strap muscles (sternohyoid, sternothyroid, thyrohyoid, or omohyoid muscles) from a tumor of any size
T4		Tumor includes gross extrathyroidal extension beyond the strap muscles
T4	a	Tumor with gross extrathyroidal extension invading subcutaneous soft tissues, larynx, trachea, esophagus, or recurrent laryngeal nerve from a tumor of any size
T4	b	Tumor with gross extrathyroidal extension invading prevertebral fascia or encasing the carotid artery or mediastinal vessels from a tumor of any size.
TX		Primary tumor cannot be assessed
Regional lymph node involvement
Node involvement	Sub-stage	Finding	Image
N0		No lymph node involvement
N0	a	One or more cytologically or histologically confirmed benign lymph nodes
N0	b	No radiologic or clinical evidence of locoregional lymph node metastasis
N1		Metastasis to regional nodes
N1	a	Metastasis to level VI or VII (pretracheal, paratracheal, or prelaryngeal/Delphian, or upper mediastinal) lymph nodes. This can be unilateral or bilateral disease
N1	b	Metastasis to unilateral, bilateral, or contralateral cervical neck lymph nodes (levels I, II, III, IV, or V) or retropharyngeal lymph nodes
NX		Regional lymph node cannot be assessed
Distant metastasis
Presence of metastasis	Sub-stage	Finding	Image
M0		No distant metastasis
M1	IV	Distant metastasis