What happened, in summary

A man in his 60s had previously undergone surgery for a 4.8 cm invasive non-mucinous adenocarcinoma of the lung. The tumour contained acinar, papillary and micropapillary patterns and was staged as pT2bN1cM0, Stage IIB. He then received postoperative chemotherapy and entered follow-up.

Five years later, ultrasound detected a thyroid nodule and an enlarged lymph node in his neck. The initial clinical concern was a new primary thyroid cancer. Cytology raised a different possibility: recurrence of the earlier lung cancer. He underwent removal of the left thyroid lobe with regional lymph-node dissection. The tumour had invaded nearby strap muscle and involved 2 lymph nodes. Its appearance closely matched the original lung adenocarcinoma. The cells were positive for TTF-1 and Napsin A and negative for PAX8, supporting metastatic lung cancer rather than a thyroid primary. Some tumour cells close to normal thyroid follicles also stained for thyroglobulin, an unusual finding that could have caused diagnostic confusion.

A postoperative PET/CT showed no apparent residual cancer, so observation was initially chosen. Three months later, CT identified several small right-lung nodules suspicious for further metastases. Molecular testing of the thyroid metastasis found an EGFR exon 19 deletion. He started osimertinib approximately 3 months after surgery.

By 5 months after the thyroid operation, he was disease free. His journey involved a rare metastatic site and required pathology, immunohistochemistry and genomic testing to distinguish recurrent lung adenocarcinoma from a separate thyroid cancer and guide targeted treatment.