Nonspecific incidental brain lesions (NILs) are being detected more frequently because of an increasing number of screening or research MRI scans of the brain, and their natural course is uncertain.
Methods: In a prospective cohort study starting in 1999, we determined the outcomes of patients with incidental, nonenhancing, supratentorial, lobar, and small-volume (<10 mL) lesions, depending on the findings of MRI and PET with the (18)F-labeled amino acid fluoroethyl-l-tyrosine ((18)F-FET). Patients with seizures, focal neurologic deficits, signs of local or systemic infection or inflammation, known brain disease, or any kind of previous cerebral treatment were excluded. Finally, 21 patients were eligible. MRI was performed in 19 of these patients because of nonspecific symptoms (such as headaches, dizziness, or sudden deafness), whereas 2 patients were healthy volunteers in MRI studies. Clinical follow-up and MRI scans were obtained at 4- to 6-mo intervals, and follow-up ranged from 3 to 8.5 y. Mean lesion-to-brain (L/B) ratios of >or=1.6 on (18)F-FET PET were rated as positive.
Results: Four different outcome groups were identified. In group A, 5 NILs regressed or vanished completely. All of these lesions were circumscribed on MRI, and (18)F-FET uptake was negative, with an L/B ratio of 1.2+/-0.2 (mean +/- SD). In group B, 10 NILs were stable, without growth. All of these lesions were circumscribed on MRI, and (18)F-FET uptake was negative (L/B ratio: 1.0+/-0.1). In group C, 2 NILs grew slowly over years, and an astrocytoma of World Health Organization (WHO) grade II was diagnosed after resection in each case. The lesions were circumscribed on MRI, and (18)F-FET uptake was negative (L/B ratios: 0.7 and 1.0). In group D, 4 NILs showed sudden and rapid growth, with clinical deterioration, and a high-grade glioma of WHO grade III or IV was diagnosed after resection in all cases. The lesions were diffuse on MRI, and (18)F-FET uptake was significantly increased (L/B ratio: 2.0+/-0.4) (P<0.01 for group D vs. group A or group B).
Conclusion: For NILs, a circumscribed growth pattern on MRI and normal or low (18)F-FET uptake on PET are strong predictors for a benign course, with the eventual development of a low-grade glioma. In contrast, NILs with a diffuse growth pattern on MRI and increased (18)F-FET uptake indicate a high risk for the development of a high-grade glioma.