Is classic papillary thyroid carcinoma definitely an "indolent cancer"?-a case of locally advanced classic occult papillary thyroid carcinoma: a case report.

Background: Occult papillary thyroid carcinoma (PTC) refers to the PTC accidentally found due to its occult lesions. Classic, as the most common subtype of PTC, is usually considered to have a low degree of malignancy and a favorable prognosis. Currently, the American Thyroid Association Management Guidelines adopted active surveillance (AS) as an alternative to immediate surgery in some low-risk PTC patients with less than 1 cm in diameter.

Case description: The patient was admitted to the clinic because of an incidentally detected right neck mass, and neck ultrasonography (US) and computed tomography (CT) showed an enlarged lymph node in right neck, which had an unclear border with the right internal jugular vein and the sternocleidomastoid muscle, whereas no abnormality was detected in thyroid glands; fine-needle aspiration biopsy (FNAB) of the enlarged lymph node showed metastatic carcinoma, and the possibility of follicular epithelial origin from the thyroid was highly considered; then the enlarged lymph node resection + right thyroid lobectomy + freezing + left thyroid lobectomy and isthmus resection + lymph nodes and adipose tissue dissection in II-VI regions in right neck + complete resection of the invaded right sternocleidomastoid muscle and right internal jugular vein were performed; postoperative pathology was classic PTC and lymph node metastasis with blood vessels and striated muscle tissues involved.

Conclusions: In this case, lateral cervical lymph node metastasis had already occurred without the preoperative detection of a primary thyroid focus, suggesting that the potential risk of classic PTC <1 cm cannot be ignored. Besides, how to accurately identify the malignant degree of low-risk PTC <1 cm preoperatively is worthy of further study.