68 Ga-FAPI-04 PET/CT: A "New Kid on the Block" for Evaluation of Stewart-Treves Syndrome.

A 40-year-old woman, who underwent modified radical mastectomy and chemoradiotherapy for triple-negative left breast cancer, presented with increasing lymphedema of the proximal left upper limb of 2 years' duration. She underwent 18 F-FDG PET/CT (FDG) and 68 Ga-FAPI-04 PET/CT (FAPI) scans, which showed subcutaneous deposits that were more FAPI-avid than FDG. These were diagnosed as Stewart-Treves syndrome (STS), which is a rare, highly aggressive angiosarcoma arising in chronic lymphedema. Better performance of FAPI vis-à-vis FDG scan may aid in early diagnosis and timely intervention in this rare, albeit clinically important, pathology in breast cancer patients with chronic lymphedema.