Serial O-(2-[18F] fluoroethyl)-L-tyrosine PET for monitoring the effects of intracavitary radioimmunotherapy in patients with malignant glioma

Intracavitary radioimmunotherapy (RIT) offers an effective adjuvant therapeutic approach in patients with malignant gliomas. Since differentiation between recurrence and reactive changes following RIT has a critical impact on patient management, the aim of this study was to analyse the value of serial O-(2-[F-18]fluoroethyl)-L-tyrosine (FET) PET scans in monitoring the effects of this locoregional treatment. Methods: Following conventional therapy, 24 glioma patients (5 WHO III, 19 WHO IV) underwent one to five RIT cycles with either I-131-labelled (n=19) or Re-188-labelled (n=5) anti-tenascin antibodies. Patients were monitored with serial FET PET scans (2-12 scans). For semiquantitative evaluation, maximal tumoural uptake (TUmax) was evaluated and the ratio to background (BG) was calculated. Results of PET were correlated with histopathological findings (n=9) and long-term clinical follow-up for up to 87 months. Results: In seven tumour-free patients, PET revealed slightly increasing but homogeneous FET uptake surrounding the resection cavity with a peak up to 18 months following RIT (TUmax/BG 2.07 +/- 0.25) but stable or decreasing values during further follow-up (last follow-up: TUmax/BG 1.63 +/- 0.22). Seventeen patients developed regrowth of residual tumour/tumour recurrence showing additional nodular FET uptake (TUmax/BG 2.79 +/- 0.53). A threshold value of 2.4 (TUmax/BG) allowed best differentiation between recurrence and reactive changes (sensitivity 82%, specificity 100%). Conclusion: FET PET is a sensitive tool for monitoring the effects of locoregional RIT. Homogeneous, slightly increasing FET uptake around the tumour cavity with a peak up to 18 months after RIT, followed by stable or decreasing uptake, points to benign, therapy-related changes. In contrast, nodular uptake is a reliable indicator of recurrence.