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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.

TEMPI syndrome (TEMPI) compounds telangiectasias and polycythemia with elevated erythropoietin levels, monoclonal gammopathy, perirenal fluid collections, and intrapulmonary shunt. Although the pathophysiology of this syndrome remains unclarified, prior research has been established that it is a plasma cell neoplasm, often containing less than 10% bone marrow plasma cells. 18F-FDG PET serves as a valuable instrument for initial staging and treatment monitoring in multiple myeloma management. Thus, 18F-FDG PET can be legitimately applied for TEMPI assessment. Here, we present the first 18F-FDG PET images for the initial evaluation and treatment monitoring of TEMPI in a 51-year-old woman, who exhibited polycythemia (EPO:5,448 mIU/ml) without JAK2 mutation, telangiectasias, monoclonal IgG lambda gammopathy (13.9) g/L and 7% dysmorphic plasma cells (CD38 + CD138+), occasionally clustered, in favor of tumoral plasmacytomas. The first PET scan exhibited hypermetabolic diffuse bone marrow, potentially related to polycythemia, accompanied by non-lytic bone hypermetabolic lesions in the femoral and humeral diaphysis, and ametabolic peri-renal fluid collections, brown fat, and pleural talcoma. Post-treatment 18F-FDG PET (Daratumumab Bortezomib Thalidomide Dexamethasone) revealed a completely reduced signal of bone lesions, suggesting a complete response, which was substantiated both clinically and biologically, with the concurrent disappearance of telangiectasia and the monoclonal component, and the normalization of the EPO level. In future, additional data will be required to confirm the added value of 18F-FDG PET with TEMPI. Nevertheless, 18F-FDG PET can be a preferred tool for the extension workup and therapeutic evaluation of TEMPI syndrome.