Case Report: Contribution of [¹⁸F]FET PET in differential diagnosis between radionecrosis and progression in metastasis-reproducibility and superiority of dynamic acquisitions

We present the case of a 67-year-old woman with metastatic invasive ductal carcinoma of the left breast, in whom a follow-up magnetic resonance imaging, 3 months after encephalic radiotherapy, revealed a significant increase in the size of two brain metastases, potentially indicating progressive disease within the radiation field. Subsequent [¹⁸F] fluorodeoxyglucose ([¹⁸F]FDG) and [¹⁸F] fluoroethyl-L-tyrosine positron emission tomography ([¹⁸F]FET PET) scans were performed to distinguish radionecrosis from tumor progression. Despite a dynamic [¹⁸F]FET time-activity curve (TAC) against progression, the exceeding of the 1.9 cutoff by mean tumor to brain ratio (TBR) and interdisciplinary considerations led to the resection of one lesion. Histopathology revealed necrosis due to radiotherapy, without viable tumor proliferation. To verify radionecrosis, a second [¹⁸F]FET PET scan was conducted, showing consistent findings. In metastasis differentiation, the mean TBR cutoff of 1.9 and TAC analysis achieved a sensitivity of 95% and specificity of 91%. The discrepancy between the TAC and TBR emphasizes the need for consideration, and a time delay between radiotherapy and PET may impact TBR cutoffs. In addition, differences in radiosensitivity suggest a lower metastasis pre-test probability of progression, and it might be why a TAC analysis could be more effective in distinguishing true progression from treatment related changes in metastasis. This case demonstrates the accuracy of dynamic [¹⁸F]FET PET and suggests its utility for post-treatment metastasis evaluation, and further research on post-treatment delay could lead to improved performances of dynamic [¹⁸F]FET PET.