What happened, in summary

A 61-year-old woman had been followed for 17 years after treatment for papillary thyroid cancer. Multiple tiny lung nodules had remained stable and were considered metastases from the thyroid cancer. During follow-up, however, one nodule in the right lower lung slowly enlarged to about 25 mm and was removed by wedge resection. The smaller miliary nodules matched her previous thyroid cancer, but the enlarging nodule had a different immunohistochemical pattern: it was positive for TTF-1 and napsin A and negative for thyroglobulin and PAX8. These findings supported a separate primary lung adenocarcinoma rather than another thyroid-cancer metastasis. Molecular testing of the lung adenocarcinoma identified a RET-GOLGA5 fusion that was not detected in the original thyroid tumor. No additional lung-cancer treatment is described after the resection. She was alive 19 months after surgery. The longstanding miliary lung nodules remained present and continued to be attributed to metastatic papillary thyroid cancer rather than spread from the lung adenocarcinoma.