A 45-yr-old patient presents with a 4 x 5 cm mass in the neck. Histopathological examination showed metastasis of squamous cell origin. The patient was thoroughly examined using laryngoscope, whole body CT and PET CT. No primary is found and the diagnosis of Carcinoma of unknown primary (CUP) is made. TNM staging as per AJCC is:
Head and neck squamous cell carcinoma of unknown primary is defined as the identification of squamous cell carcinoma in one or more lymph nodes within the head and neck region that are not solely in the supraclavicular region, without an identifiable primary tumor.
Isolated supraclavicular lymphadenopathy, including those of squamous cell origin, almost always originate from cancer in the skin or a primary site beneath the clavicles (ie, lung, breast, colon, prostate, esophagus, cervix, urethral, and lymphoma primaries).
Head and neck cancers arise from a variety of sites within the head and neck region, which is divided into five basic areas:

Oral cavity
Pharynx
Larynx
Nasal cavity & paranasal sinuses &
Salivary glands

Cancers of the brain, thyroid, eye, ear, scalp, skin, teeth, bones (including skull base), and muscles, are not included with squamous cell cancers of the head and neck.
Most head and neck cancers begin in the mucosal surfaces of the head and neck region (upper aerodigestive tract) and are predominantly squamous cell carcinomas.
Cancers of the Head and Neck are classified using the tumor node metastases (TNM) staging system of the American
Joint Committee on Cancer (AJCC).
The T classifications indicating the extent of the primary tumor are site specific but there is considerable overlap in the cervical node (N) classifications.
Despite the variation in the T classification TX and T0 remain the same in all head and neck cancers.
TX — the primary tumor cannot be assessed.
T0 — no evidence of primary tumor.
T1N1M0
T1N2M0
T0N1M0
T0N2M0