Molecular Imaging Tracers Help Determine Brain Metastases Treatment Strategies

Imaging with the molecular imaging tracer ¹⁸F-FDOPA (L-3,4-dihydroxy-6-[¹⁸F]fluoro-phenylalanine) can help differentiate radiation-induced lesions from new tumor growth in patients who have been treated with radiotherapy for brain metastases, according to new research.

Using this amino acid tracer, researchers discovered that physicians could effectively differentiate the two types of lesions 83% of the time. Progression-free survival could also be predicted through evaluating the ¹⁸F-FDOPA imaging findings.

Brain metastases occur in 20%–40% of all cancer patients, especially in those with melanoma, non-small-cell lung cancer, and breast cancer. They account for 170,000 new cases yearly in the United States alone, and prognosis is poor. Treatment for brain metastases usually includes a combination of surgery, radiation treatment, and chemotherapy.

“Histopathological changes in neural tissue treated with radiation could trigger clinical and imaging manifestations which are very similar to those caused by tumor growth,” said Karlo J. Lizarraga, MD, MS, from the department of neurology, Miller School of Medicine, University of Miami (FL, USA), and lead author of the study. “The challenge is then to differentiate whether these manifestations are due to radiation or to tumor progression or recurrence. Accurate and timely distinction between these two possibilities can significantly affect patient care, and outcome for treatment modalities are completely different for each case.”

In the retrospective study, published ahead of print October 28, 2013, and slated for the January 1, 2014, issue of the Journal of Nuclear Medicine, researchers analyzed images from 32 patients who had 83 earlier irradiated brain metastases and who underwent ¹⁸F-FDOPA positron emission tomography (PET). The studies were analyzed both semi-quantitatively and visually to determine whether lesions were caused by radiation injury or were recurrent or progressive brain metastases. Results were verified by histopathologic analysis or clinical follow-up. The prognostic ability of ¹⁸F-FDOPA in predicting progression-free survival and overall survival was also studied with the Kaplan-Meier and Cox regression techniques.

The best overall accuracy for differentiating between the two types of lesions was achieved using visual scoring, which had a sensitivity of 81.3%, a specificity of 84.3%, and an overall accuracy of 83.1%. The semi-quantitative analysis resulted in a sensitivity of 81.3%, a specificity of 72.5% and an overall accuracy of 75.9%.

Researchers also discovered that evaluations with ¹⁸F-FDOPA PET was highly prognostic of progression-free survival, as lesions with a negative PET result had a mean time to progression that was 4.6 times longer than lesions with positive ¹⁸F-FDOPA PET findings. Moreover, a trend toward predicting overall survival was also seen.

“¹⁸F-FDOPA PET imaging is currently available in few centers. The longer physical half-life of ¹⁸F-FDOPA, when compared to other amino acid tracers, gives it the advantage of potential automated production and transport to PET centers for widespread use,” noted Dr. Lizarraga.

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Miller School of Medicine, University of Miami