Fused PET and MR make the grade in pediatric brain cancers

A connection between cerebral blood volume (CBV) and tumor metabolism in 3D PET and MR characterized a range of pediatric brain tumors. The two modalities were found to sync complementary functional and metabolic information in 94 percent of cases and could have an impact on risk stratification and prognosis with further study, according to research published June 25 in the Journal of Nuclear Medicine.

Katherine A. Zukotynski, MD, from the department of medical imaging at Sunnybrook Health Sciences Centre at University of Toronto in Canada, and colleagues evaluated F-18 FDG PET parameters, MR permeability and cerebral blood volume as a means of characterizing disease in a variety of pediatric brain tumors in this multicenter study of the Pediatric Brain Tumor Consortium.

“A comprehensive assessment of tumor anatomy and physiology often involves spatial correlation of imaging acquired on separate MR and PET scanners,” wrote the authors. “MR imaging with contrast has high soft tissue resolution, multiplanar capability, and lack of ionizing radiation, making it the modality of choice for determining tumor size, location, and characterization.”

Preliminary F-18 FDG PET research indicates a correlation between avid FDG uptake and a greater histologic grade and aggressiveness of cancer and discouraging outcomes. Widely considered a modality for diagnostic definition and extent of tumors, MR had limitations in imaging physiological detail until recent technological improvements pushed the boundaries of functional brain imaging. Modern MR can evaluate tumor vasculature, revealing new vessel development and the increased capillary leakage characterized by highly malignant tumors.

“More recently, advanced MR imaging techniques have been used to characterize tumors in functional and metabolic terms,” wrote the authors. “PET with F-18 FDG is used as a tool to demonstrate metabolically active disease and to complement anatomic imaging in these children.”

For this study, F-18 FDG PET and MR brain imaging was performed within a two-week period on 24 children across 10 institutions in a phase II study of bevacizumab and irinotecan. Of the childrens’ tumors, nine were low-grade glioma, seven were supratentorial high-grade astrocytoma, four were brain stem glioma, and two each were ependymoma and medulloblastoma. Tumors were imaged in stages of both pre- and post-treatment. A total of 12 children underwent pretreatment MR and F-18 FDG PET only, while three children underwent post-treatment MR and F-18 FDG PET alone. Nine patients had both pretreatment and post-treatment FDG PET and MR scanning.

A 1.5T scanner was used for MR imaging in all cases. Gadolinium contrast was administered for both diffusion and perfusion pediatric brain imaging.

Every case was assessed for coregistration quality in cortical surface, gray matter, tumor site and regional alignment. Regions of interest were manually selected and calculations were made for maximum tumor permeability, maximum CBV and FDG PET maximum tumor uptake ratio.

“Thirty-one of 33 cases (94 percent) had good to excellent registration of the 18F-FDG PET and MR permeability images including the 3D ROI [region of interest],” the researchers wrote. “Further, the correlation was good to excellent across a spectrum of brain tumor histologies irrespective of anatomic variations caused by the patients’ disease.”

Once coregistration was evaluated, MR permeability and PET parameters were compared. No significant association was found between MR permeability and F-18 FDG uptake and researchers attributed this to differing aspects of tumor physiology. However, there was a correlation between FDG uptake and CBV, a potential indicator of tumor grade. Additional studies will need to be conducted to verify the association with a covariate analysis of tumor histology using these parameters for assessment.