Background Patients with metastatic cancer to the brain have a poor prognosis. In clinical practice, MRI is used to delineate, diagnose and plan treatment of brain metastases. However, MRI alone is limited in detecting micro-metastases, delineating lesions and discriminating progression from pseudo-progression. Combined PET/MRI utilises superior soft tissue images from MRI and metabolic data from PET to evaluate tumour structure and function. The amino acid PET tracer 18F-FACBC has shown promising results in discriminating high- and low-grade gliomas, but there are currently no reports on its use on brain metastases. This is the first study to evaluate the use of 18F-FACBC on brain metastases. Case presentation A middle-aged female patient with brain metastases was evaluated using hybrid PET/MRI with 18F-FACBC before and after stereotactic radiotherapy, and at suspicion of recurrence. Static/dynamic PET and contrast-enhanced T1 MRI data were acquired and analysed. This case report includes the analysis of four 18F-FACBC PET/MRI examinations, investigating their utility in evaluating functional and structural metastasis properties. Conclusion Analysis showed high tumour-to-background ratios in brain metastases compared to other amino acid PET tracers, including high uptake in a very small cerebellar metastasis, suggesting that 18F-FACBC PET can provide early detection of otherwise overlooked metastases. Further studies to determine a threshold for 18F-FACBC brain tumour boundaries and explore its utility in clinical practice should be performed.

PurposeThe study aims to evaluate whether combined 18F-FACBC PET/MRI could provide additional diagnostic information compared with MRI alone in brain metastases.Patients and MethodsEighteen patients with newly diagnosed or suspected recurrence of brain metastases received dynamic 18F-FACBC PET/MRI. Lesion detection was evaluated on PET and MRI scans in 2 groups depending on prior stereotactic radiosurgery (SRS group) or not (no-SRS group). SUVs, time-activity curves, and volumetric analyses of the lesions were performed.ResultsIn the no-SRS group, 29/29 brain lesions were defined as “MRI positive.” With PET, 19/29 lesions were detected and had high tumor-to-background ratios (TBRs) (Dmax MR, ≥7 mm; SUVmax, 1.2–8.4; TBR, 3.9–25.9), whereas 10/29 lesions were undetected (Dmax MR, ≤8 mm; SUVmax, 0.3–1.2; TBR, 1.0–2.7). In the SRS group, 4/6 lesions were defined as “MRI positive,” whereas 2/6 lesions were defined as “MRI negative” indicative of radiation necrosis. All 6 lesions were detected with PET (Dmax MR, ≥15 mm; SUVmax, 1.4–4.2; TBR, 3.6–12.6). PET volumes correlated and were comparable in size with contrast-enhanced MRI volumes but were only partially congruent (mean DSC, 0.66). All time-activity curves had an early peak, followed by a plateau or a decreasing slope.Conclusions18F-FACBC PET demonstrated uptake in brain metastases from cancer of different origins (lung, gastrointestinal tract, breast, thyroid, and malignant melanoma). However, 18F-FACBC PET/MRI did not improve detection of brain metastases compared with MRI but might detect tumor tissue beyond contrast enhancement on MRI. 18F-FACBC PET should be further evaluated in recurrent brain metastases.