Amino acid PET tracer tells brain metastases from radiation injury

Delayed brain injury resulting from aggressive radiation treatment can be differentiated from recurrent brain tumors using F-18 FDOPA PET imaging, according to a study published Oct. 28 in the Journal of Nuclear Medicine.

Radiation injury is conventionally assessed with MR imaging, but the modality is not ideal for telling apart radiation-related injury and recurrent brain tumors, which can have similar physical symptoms. In the first study of its kind, Karlo J. Lizarraga, MD, from the department of neurology in the Miller School of Medicine at the University of Miami, and colleagues reviewed PET imaging studies to assess both the diagnostic and prognostic accuracy of amino acid-based F-18 FDOPA PET for detecting late or delayed radiation injury in the brain, which crops up in as many as 24 percent of cancer patients between three to 13 months following the end of radiation treatment.

“First, both types of lesions can have similar clinical presentations, such as seizures, focal neurologic deficits, and increased intracranial pressure,” wrote Lizarraga et al. Second, both types of lesions can have similar MR imaging appearances, such as perilesion edema, rimlike enhancement, and central hypointensity on T2-weighted imaging.”

To negate this and administer the appropriate treatment for either late radiation injury or brain metastases, researchers retrospectively reviewed 32 patients with a range of primary cancers treated with various radiation therapies, including whole-brain radiation therapy, stereotactic radiosurgery, or stereotactic radiotherapy. All subjects received follow-up metabolic PET imaging using F-18 FDOPA to identify brain metastases in contrast to radiation injury. PET scans were co-registered with MR images. Results showed that FDOPA PET picked up the increased metabolic activity of metastases better than other metabolic tracers like FDG.

Of the patients’ brain lesions, 32 out of 83, or 38.6 percent, were considered recurrent brain metastases. In sharp relief, 51 out of the 83, or 61.4 percent, were found to be related to late or delayed radiation injury. Sensitivity of F-18 FDOPA PET for distinguishing between the two was estimated at 81.3 percent, and specificity at 84.3 percent. Sensitivity was much higher for FDOPA than FDG, which was estimated at about 65 percent in previous studies.

“A visual score comparing tumor F-18 FDOPA uptake and striatum F-18 FDOPA uptake provided the highest sensitivity and specificity and was predictive of disease progression,” wrote the authors.

Researchers urged additional studies to include standardization of quantitative analysis to account for both clinical and technical variables in imaging, including those related to different metastases, treatment and follow-up timelines.