Synchronous metastatic differentiated high-grade thyroid carcinoma in lymph node from classic papillary thyroid carcinoma: a case report and literature review.

Differentiated high-grade thyroid carcinoma (DHGTC) is relatively uncommon. This case report described a 61-year-old male initially diagnosed with classic papillary thyroid carcinoma confined to the thyroid gland, accompanied by skip metastasis to a Level II lymph node. The metastatic lymph node exhibited histopathological progression to DHGTC. Following thyroidectomy for the primary tumor and neck dissection for the metastatic lymph node, the patient received adjuvant radiotherapy to the neck. Subsequently, pathological examination confirmed multiple pulmonary metastases (PMs) from DHGTC within six months of the initial diagnosis. Unfortunately, a post-therapeutic ¹³¹I scan revealed no uptake in the PMs. Genetic testing of the PMs revealed no mutations in BRAF, KRAS, or NRAS, and no RET rearrangement. The patient's condition was subsequently managed with targeted therapy and immunotherapy, which achieved disease stabilization.