Anaplastic Thyroid Carcinoma Arising in the Metastatic Lesion of Papillary Thyroid Carcinoma: Comparative 18 F FDG and 68 Ga Ga-FAPI-04 PET-CT Imaging for Disease Staging & Exploratory Theranostics

An 86-year-old gentleman, case of treated classical papillary thyroid carcinoma (PTC) developed scar site recurrence, which revealed focal anaplastic thyroid carcinoma (ATC) arising in the background of PTC. 18 F-FDG PET-CT done for staging revealed metastatic disease in the bilateral lungs, left lung pleura, and C6 vertebra. The patient also underwent 68 Ga Ga-FAPI-04 PET-CT to evaluate for FAP expression, which demonstrated intense FAP expression at the above-mentioned metastatic sites. In this case, FAPI PET-CT also better delineated the disease extent as compared to FDG PET- CT. This case highlights the rare phenomenon of Anaplastic transformation of metastatic PTC and provides a clinical context relevant to ongoing investigations of FAP-targeted approaches in aggressive thyroid carcinomas.