Poorly differentiated adenocarcinoma of an unknown primary with a thyroid tumour and an aggressive course: thyroid or lung carcinoma?

Cancers of unknown primary (CUPs) are challenging for physicians to diagnose and treat. Metastases to the thyroid gland are rare, representing less than 1% of all thyroid malignancies. Here, we report a case of a 69-year-old Asian man who had both thyroid gland and lymph node enlargement in the neck and shoulders but no nodules/tumours in the lung field. The patient died 51 days after his first visit to our office, although pembrolizumab was administered on day 34 based on programmed cell death-ligand 1 (PD-L1) expression. Immunohistochemistry (IHC) with paired box 8 (PAX8) may be useful to diagnostically distinguish poorly differentiated lung adenocarcinomas from napsin A-positive thyroid carcinomas.