False-positive MRI detection of recurrent or metastatic pediatric infratentorial tumors.

Gadolinium-DTPA enhanced MRI is the modality of choice when imaging central nervous system infratentorial tumors in the pediatric population. The detection of a new enhancing lesion following initiation of therapy is typically considered pathognomonic for recurrent or metastatic tumor. We report two pediatric patients with infratentorial central nervous system tumors who demonstrated new areas of enhancement with gadolinium-DTPA enhanced MRI following initiation of multimodality therapy. Both patients had surgical biopsy of the new lesions with histologic review failing to demonstrate viable tumor. These studies suggest caution in considering new enhancing lesions detected by MRI in a child with a brain tumor pathognomonic for new sites of active tumor.