Effect of ¹¹C-Methionine-Positron Emission Tomography on Gross Tumor Volume Delineation in Stereotactic Radiotherapy of Skull Base Meningiomas

Purpose: To evaluate the effect of trimodal image fusion using computed tomography (CT), magnetic resonance imaging (MRI) and ¹¹C-methionine positron emission tomography (MET-PET) for gross tumor volume delineation in fractionated stereotactic radiotherapy of skull base meningiomas. Patients and Methods: In 32 patients with skull base meningiomas, the gross tumor volume (GTV) was outlined on CT scans fused to contrast-enhanced MRI (GTV-MRI/CT). A second GTV, encompassing the MET-PET positive region only (GTV-PET), was generated. The additional information obtained by MET-PET concerning the GTV delineation was evaluated using the PET/CT/MRI co-registered images. The sizes of the overlapping regions of GTV-MRI/CT and GTV-PET were calculated and the amounts of additional volumes added by the complementing modality determined. Results: The addition of MET-PET was beneficial for GTV delineation in all but 3 patients. MET-PET detected small tumor portions with a mean volume of 1.6 ± 1.7 cm³ that were not identified by CT or MRI. The mean percentage of enlargement of the GTV using MET-PET as an additional imaging method was 9.4% ± 10.7%. Conclusions: Our data have demonstrated that integration of MET-PET in radiotherapy planning of skull base meningiomas can influence the GTV, possibly resulting in an increase, as well as in a decrease.