Prospective study of 11C\u2013methionine PET for distinguishing between recurrent brain metastases and radiation necrosis:\xa0limitations of diagnostic accuracy and long-term results of salvage treatment

Shoji Yomo,Kazuhiro Oguchi

doi:10.1186/s12885-017-3702-x

Abstract

BackgroundOn conventional diagnostic imaging, the features of radiation necrosis (RN) are similar to those of local recurrence (LR) of brain metastases (BM). 11C–methionine positron emission tomography (MET-PET) is reportedly useful for making a differential diagnosis between LR and RN. In this prospective study, we aimed to investigate the diagnostic performance of MET-PET and the long-term results of subsequent patient management.MethodsThe eligible subjects had enlarging contrast-enhanced lesions (>1 cm) on MR imaging after any form of radiotherapy for BM, suggesting LR or RN. However, it was difficult to differentiate LR from RN in these cases. From August 2013 to February 2017, MET-PET was performed for 37 lesions in 32 eligible patients. Tracer accumulation in the regions of interest was analysed as the standardised uptake value (SUV) and maximal lesion SUV/maximal normal tissue SUV ratios (LNR) were calculated. The cut-off value for LNR was provisionally set at 1.40. Salvage treatment strategies determined based on MET-PET diagnosis and treatment results were investigated. The diagnostic accuracy of MET-PET was evaluated by receiver operating characteristic (ROC) curve analysis.ResultsThe median interval from primary radiotherapy to MET-PET was 19 months and radiotherapy had been performed twice or more for 13 lesions. The MET-PET diagnoses were LR in 19 and RN in 18 lesions. The mean values and standard deviation of LNRs for each diagnostic category were 1.70 ± 0.30 and 1.09 ± 0.25, respectively. At the median follow-up time of 18 months, final diagnoses were confirmed histologically for 17 lesions and clinically for 20 lesions. ROC curve analysis indicated the optimal LNR cut-off value to be 1.40 (area under the curve: 0.84), and the sensitivity and specificity were 0.82 and 0.75, respectively. The median survival times of patient groups with LR and RN based on MET-PET diagnosis were 14.8 months and 35.1 months, respectively (P = 0.035, log-rank test).ConclusionsMET-PET showed apparently reliable diagnostic performance for distinguishing between LR and RN. The provisional LNR cut-off value of 1.4 in our institution was found to be appropriate. Limitations of diagnostic accuracy should be recognised in cases with LNR close to this cut-off value.

Highlights

On conventional diagnostic imaging, the features of radiation necrosis (RN) are similar to those of local recurrence (LR) of brain metastases (BM). 11C–methionine positron emission tomography (MET-PET) is reportedly useful for making a differential diagnosis between LR and RN
In the course of follow-up for BM treated with any type of radiotherapy, including conventional fractionated radiotherapy, stereotactic radiosurgery and particle therapy, lesions with continuous enlargement of Gd-enhanced areas documented on serial magnetic resonance (MR) scans and suspected to be local recurrence (LR) or radiation necrosis (RN), which are difficult to differentiate from each other, were studied using MET-PET
The median Karnofsky performance status score at the time of MET-PET was 90

Summary

Introduction

The features of radiation necrosis (RN) are similar to those of local recurrence (LR) of brain metastases (BM). 11C–methionine positron emission tomography (MET-PET) is reportedly useful for making a differential diagnosis between LR and RN. Local changes in the area of irradiation application at follow-up, are not uncommonly seen on Gd-enhanced and T2-weighted MR imaging [4, 5] The interpretation of such changes is often difficult and it may even be impossible to differentiate radiation-induced changes from local tumor recurrence [6], which poses a critical dilemma in decision-making for subsequent treatment. Amino acid tracers such as 11C- methionine (MET) are reportedly useful for positron emission tomography (PET), in the field of neuro-oncology, because of high amino acid uptake by tumor tissue with low uptake by normal brain tissue, resulting in an enhanced tumor-to-background contrast [7, 8]. Most previous studies investigated imaging changes within already treated BM by focusing on assessment of the diagnostic accuracy of the imaging modalities using receiver-operating characteristic (ROC) curve analysis [11, 12, 14]

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