Comparison of the effectiveness of MRI perfusion and fluorine-18 FDG PET-CT for differentiating radiation injury from viable brain tumor: a preliminary retrospective analysis with pathologic correlation in all patients

Abstract

Objectives

Differentiating radiation injury from viable tumor is important for optimizing patient care. Our aim was to directly compare the effectiveness of fluorine-18 fluorodeoxyglucose (FDG) positron emission tomography–computed tomography (PET-CT) and dynamic susceptibility-weighted contrast-enhanced (DSC) magnetic resonance (MR) perfusion in differentiating radiation effects from tumor growth in patients with increased enhancement following radiotherapy for primary or secondary brain tumors.

Materials and methods

We retrospectively identified 12 consecutive patients with primary and secondary brain tumors over a 1-year period that demonstrated indeterminate enhancing lesions after radiotherapy and that had undergone DSC MR perfusion, FDG PET-CT, and subsequent histopathologic diagnosis. The maximum standardized uptake value (SUV) of the lesion (SUV_{lesion max}), SUV_ratio (SUV_{lesion max}/SUV_{normal brain}), maximum relative cerebral blood volume, percentage of signal intensity recovery, and relative peak height were calculated from the positron emission tomography and MR perfusion studies. A prediction of tumor or radiation injury was made based on these variables while being blinded to the results of the surgical pathology.

Results

SUV_ratio had the highest predictive value (area under the curve=0.943) for tumor progression, although this was not statistically better than any MR perfusion metric (area under the curve=0.757–0.829).

Conclusions

This preliminary study suggests that FDG PET-CT and DSC MR perfusion may demonstrate similar effectiveness for distinguishing tumor growth from radiation injury. Assessment of the SUV_ratio may increase the sensitivity and specificity of FDG PET-CT for differentiating tumor and radiation injury. Further analysis is needed to help define which modality has greater predictive capabilities.

Introduction

Radiation therapy can help prolong survival for many patients with primary or metastatic brain tumors [1], [2]. To assess treatment response, the current standard of care involves following patients clinically and with serial MRI examinations in order to assess treatment response. The detection of new or increased enhancement at the site of previously irradiated tumor may represent radiation injury or tumor growth. Resolving the etiology of increasing enhancement is frequently very difficult because radiation injury and tumor growth often demonstrate a similar appearance with conventional imaging [3], [4].

Magnetic resonance (MR) perfusion and fluorodeoxyglucose (FDG) positron emission tomography–computed tomography (PET-CT) are advanced imaging techniques that are commonly utilized as problem-solving tools to help make the correct diagnosis in this setting. Both studies provide insightful data about the biology and physiology of abnormal tissue enhancement that cannot be perceived with conventional imaging alone. Even though radiation injury and tumor can both enhance on MR imaging, they are fundamentally different on a pathologic and mechanistic basis. Tumor growth promotes angiogenesis and microvascular proliferation [5], [6]. In contrast, radiotherapy decreases microvascular density and capillary perfusion because it typically induces endothelial cell damage and small-vessel injury [7], [8], [9]. In addition, histology-based research has demonstrated that the degree of capillary permeability is significantly different between the two entities [10], [11].

The ability of MR perfusion to quantify certain hemodynamic properties of enhancing brain lesions has shown promise for distinguishing malignant gliomas and metastases from radiation injury [12], [13], [14]. Several retrospective studies have also suggested the efficacy and usefulness of FDG PET-CT for differentiating tumor growth from radiation injury based on the quantification of their metabolic activities [15], [16], [17]. Although both MR perfusion and PET-CT are commonly performed, their comparative predictive value, sensitivity, specificity, and indications remain uncertain.

We hypothesized that dynamic susceptibility-weighted contrast-enhanced (DSC) MR perfusion is more effective than FDG PET-CT in distinguishing radiation injury from viable tumor. The high metabolic activity of background normal brain on FDG PET-CT often limits optimal assessment of brain lesions. In addition, there are several studies that suggest FDG PET-CT is not sensitive or specific enough to be used routinely for this task [18], [19], [20]. Therefore, the main objective of this preliminary study was to directly compare the effectiveness of MR perfusion and FDG PET-CT for discriminating between radiation injury and viable tumor with pathologic tissue diagnosis as the reference standard in all cases.