Are Dual-Phase 18F-Fluorodeoxyglucose PET-mpMRI Diagnostic Performances to Distinguish Brain Tumour Radionecrosis/Recurrence after Cranial Radiotherapy Usable in Routine?

Abstract

Brain metastases or primary brain tumours had poor prognosis until the use of high dose radiotherapy. However, radionecrosis is a complex challenge in the post-radiotherapy management of these patients due to the difficulty of distinguishing this complication from local tumour recurrence. MRI alone has a variable specificity and sensibility, as does PET-CT imaging. We aimed to investigate the diagnostic performance of dual-phase ¹⁸F-FDG PET-mpMRI to distinguish cerebral radionecrosis from local tumour recurrence after cranial radiotherapy. A retrospective analysis was conducted between May 2021 and September 2022. Inclusion criteria encompassed patients with inconclusive MRI findings post-radiotherapy and history of cerebral radiotherapy for primary or metastatic brain lesions. Lesions are assessed qualitatively and semi-quantitatively. The gold standard to assess radionecrosis was histopathology or a composite criterion at three months. The study evaluated 24 lesions in 23 patients. Qualitative analysis yielded 85.7% sensitivity and 75% specificity. Semi-quantitative analysis, based on contralateral background noise, achieved 100% sensitivity and 50% specificity. Moreover, using contralateral frontal lobe background noise resulted in higher performances with 92% sensitivity and 63% specificity. Stratification by lesion type demonstrated 100% sensitivity and specificity rates for metastatic lesions. The diagnostic performance of dual-phase ¹⁸F-FDG PET-mpMRI shows promising results for metastatic lesions.

Keywords: PET-MRI; brain radionecrosis; cerebral tumour recurrence; cranial radiotherapy; dual-phase FDG.

Figure 1

A patient with a lung cancer metastatic cerebral lesion for which multiparametric MRI alone could not differentiate radionecrosis from local recurrence. Early acquisition (a) ¹⁸F-FDG PET-mpMRI showed a lesion in green ROI with SUVmax = 5.69 and contralateral ROI in yellow with SUVmax = 8.66. Late acquisition (b) showed lesion SUVmax = 5.04, contralateral SUVmax = 10.53. The Horky ratio is calculated at −0.27, which supports radionecrosis. This diagnosis has been confirmed with patient follow-up (imaging stability over 3 years).

Figure 2

A patient with non-small cell lung cancer with a single metastatic brain lesion was treated with surgical excision followed by radiotherapy on a tumour bed. Due to clinical symptoms (cerebellar syndrome), an ¹⁸F-FDG PET-mpMRI was performed. The late acquisition (b) showed significant FDG retention within the tumour compared to the early acquisition (a), with a retention index of 0.87, indicating a local recurrence. Therefore, this patient underwent surgery, and the histopathological results confirmed this hypothesis.

Figure 3

Patient treated for a glioblastoma (radiotherapy–temozolomide) with a large area of radiation necrosis (red circle, (a)). Through ¹⁸F-FDG PET-mpMRI co-registration, the patient was reclassified as having a recurrence within this area of radiation necrosis. Diffusion and perfusion sequences were not interpretable due to haemorrhagic artifacts ((b)—SWI sequence); however, thanks to PET co-registration, a hypermetabolism was identified (c).