Brain stem glioma in a child: False diagnosis of radiation necrosis with FDG PET

Abstract

Brainstem gliomas constitute 10-20% of all pediatric central nervous system tumors, and are diagnosed using computed tomography (CT) and magnetic resonance imaging (MRI) with surgical biopsy infrequently performed [1]. One clinical problem that is difficult to resolve in the treatment of brainstem gliomas is the clinical deterioration seen following completion of fractionated radiation therapy. While CT and MRI will delineate the anatomic lesion, neither can distinguish between radiation necrosis and progressive tumor, which provide similar signs and symptoms. Positron emission tomography (PET) with 18-fluoro-deoxyglucose (FDG) has been reported to be beneficial in resolving this dilemma [2][3][4][5]. Our experience with an 11-year-old girl is instructive in this regard.She presented with a 2-month history of progressive signs and symptoms relevant to a brain stem lesion. An MRI showed an expansile mass in the pons extending into the right cerebellar hemisphere with nodular enhancement (Fig. 1). The findings were consistent with a diffuse pontine glioma. Dexamethasone was started. Conformal radiation therapy to a total dose of 54 Gy was given with concurrent temozolomide. The steroids were slowly tapered and she recovered normal neurologic function, approximately 2 months after diagnosis. MRI scans were performed every 3 months, and showed stable to decreased size of the pontine mass. Ten months after diagnosis, the patient presented with decreased dexterity in her left hand; MRI showed enlargement of the pontine mass with increased nodular enhancement. The differential diagnosis was tumor progression vs. radiation necrosis, albeit the latter would be unlikely after this dose and so short an interval. An FDG PET scan was performed and showed no evidence of abnormal metabolic activity involving the pons (Fig. 2), suggesting radiation necrosis as the etiology for the patient's symptoms. She was followed without treatment, and one month later returned with severe left hemiparesis, headache, and difficulty swallowing. MRI revealed progressive tumor, and for the first time metastatic disease was seen (Fig. 3). Terminal care was arranged and the patient died 2 months later. DISCUSSIONPET was developed in the mid 1970s with the primary goal of providing quantitative measurements and anatomic localization of metabolic processes in vivo. PET scanning relies on the ability of radionuclides to emit positively charged particles or positrons, which, having traveled a short distance in tissue (millimeters), interact with electrons and emit energy in the form of two high energy (511 keV) gamma rays. A wide variety of ligands labeled with positron emitters have been developed to provide insights into many aspects of brain tumor biology. Among these metabolic radiotracers, 18-fluoro-deoxy glucose (FDG) has been the most frequently used for the clinical investigation of brain tumors. Similar to glucose, FDG enters the cell through several membrane transporters and is phosphorylated by hexokinase into a monophosphate derivative. In...