Carbon-11 methionine boosts PET for nerve sheath tumors

FDG-PET has shown a remarkable sensitivity in detecting malignant peripheral nerve sheath tumors, but it lacks specificity. The addition of carbon-11 methionine to FDG-PET studies could boost sensitivity to 95%, according to researchers from Massachusetts General Hospital in Boston.

Differentiating between benign neurofibromas and malignant peripheral nerve sheath tumors has important prognostic and therapeutic implications for patients with neurofibromatosis type 1 (NF1). NF1 can dwell in a patient's body for decades without becoming malignant. Most cases receive a clinical diagnosis, but in rare cases of malignant transformation disease, NF1 spreads rapidly and becomes harder to detect by CT or MRI. By using FDG-PET, physicians will be able to determine what has transformed and what hasn't, according to Dr. Miriam A. Bredella, an assistant in radiology at MGH who presented the study at the 2006 International Skeletal Society meeting in Vancouver.

Dr. Abass Alavi, a professor of radiology at the University of Pennsylvania, agrees that C-11 methionine has potential.

"Even though methionine's utility has not been fully established, there might be some NF1 tumors that can't be visualized with FDG where methionine could play an important role," Alavi said.

Compared with other malignancies, slow-growing tumors such as NF1, prostate, and thyroid lesions prompt a different FDG reaction. FDG uptake may not be visualized in the slower growing lesions and, therefore, can be interpreted in at least two ways, Alavi said. Either the FDG is not sensitive enough, or the tracer is indicating the tumor is not aggressive. C-11 methionine could help in these situations.

Bredella and colleagues performed 47 whole-body FDG-PET scans on 45 patients with NF1 and suspected malignant peripheral nerve sheath tumors based on clinical symptoms and previous imaging studies. Nine patients underwent additional C-11 methionine PET scanning.

The investigators detected 15 cases of malignant peripheral nerve sheath tumors at surgery, all of which had showed increased uptake on PET. They classified all malignant tumors correctly using FDG or C-11 methionine. Twenty-five patients had benign neurofibromas, which were confirmed at surgery or follow-up. Eight showed false-positive results on FDG-PET. Five patients underwent additional C-11 methionine scanning that showed no evidence of malignancy.

FDG-PET sensitivity and specificity values to detect malignant peripheral nerve sheath tumors were 100% and 72%, respectively. Adding C-11 methionine PET increased specificity to 95%.