Abstract
High-precision radiotherapy (HPR) of recurrent high grade glioma (HGG) requires accurate spatial allocation of these infiltrative tumors. We investigated the impact of 18F-FET PET on tumor delineation and progression of recurrent HGG after HPR with carbon ions. T1 contrast enhanced MRI and 18F-FET-PET scans of 26 HGG patients were fused with radiotherapy planning volumes. PET-positive (PET+) tumor volumes using different isocontours (I%) were systematically investigated and compared with MRI-derived gross tumor volumes (GTV). Standardized uptake ratios (SUR) were further correlated with GTV and tumor progression patterns. In grade IV glioma, SUR > 2.92 significantly correlated with poor median overall survival (6.5 vs 13.1 months, p = 0.00016). We found no reliable SUR cut-off criteria for definition of PET+ volumes. Overall conformity between PET and MRI-based contours was low, with maximum conformities between 0.42–0.51 at I40%. The maximum sensitivity and specificity for PET+ volumes outside of GTV predicting tumor progression were 0.16 (I40%) and 0.52 (I50%), respectively. In 75% of cases, FLAIR hyperintense area covered over 80% of PET+ volumes. 18F-FET-PET derived SUR has a prognostic impact in grade IV glioma. The value of substantial mismatches between MRI-based GTV and PET+ volumes to improve tumor delineation in radiotherapy awaits further validation in randomized prospective trials.
Highlights
The prognosis of patients diagnosed with high grade glioma (HGG) remains poor, with median overall survival times of 10–15 months in grade IV glioma and 40–50 months in grade III glioma[1]
Our results indicate that 18F-FET positron emission tomography (PET) is well covered by CTVFLAIR with limited added value for tumor delineation, especially when considering the low specificity of PET positive tumor volume to predict regions at risk for tumor progression
Our results indicate that fixed standardized uptake ratio (SUR) thresholds are not suitable for delineation of 18F-FET PET in radiotherapy target definition generating variable conformity with the magnetic resonance imaging (MRI) based gross tumor volume (GTV)
Summary
The prognosis of patients diagnosed with HGG remains poor, with median overall survival times of 10–15 months in grade IV glioma (glioblastoma multiforme, GBM) and 40–50 months in grade III glioma[1]. High precision radiotherapy (HPR) holds the promise to escalate the dose in the tumor and improve local control while sparing normal tissue. Despite different methodological approaches employed most studies reported substantial mismatches between MRI based standard radiotherapy (RT) target definitions and 18F-FET active volumes. The extent of these mismatches has not been well studied and there is no consensus on a segmentation strategy that provides the optimum conformity or additional value of 18F-FET PET in RT target delineation. The presented study aims to quantify the mismatches and overlaps between T1 CE MRI and 18F-FET PET depending on the PET contouring technique It was investigated whether the mismatches between the two modalities can be explained by tumor volumes not visible in the treatment planning MRI (i.e. areas of tumor recurrence) and should be included in the target volume
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