Assessment of Tumor Volumes in Skull Base Glomus Tumors Using Gluc-Lys[¹⁸F]-TOCA Positron Emission Tomography

Purpose: To assess a threshold for Gluc-Lys[¹⁸F]-TOCA positron emission tomography (PET) in target volume delineation of glomus tumors in the skull base and to compare with MRI-based target volume delineation. Methods and Materials: The threshold for volume segmentation in the PET images was determined by a phantom study. Nine patients with a total of 11 glomus tumors underwent PET either with Gluc-Lys[¹⁸F]-TOCA or with ⁶⁸Ga-DOTATOC (in 1 case). All patients were additionally scanned by MRI. Positron emission tomography and MR images were transferred to a treatment-planning system; MR images were analyzed for lesion volume by two observers, and PET images were analyzed by a semiautomated thresholding algorithm. Results: Our phantom study revealed that 32% of the maximum standardized uptake value is an appropriate threshold for tumor segmentation in PET-based target volume delineation of gross tumors. Target volume delineation by MRI was characterized by high interobserver variability. In contrast, interobserver variability was minimal if fused PET/MRI images were used. The gross tumor volumes (GTVs) determined by PET (GTV-PET) showed a statistically significant correlation with the GTVs determined by MRI (GTV-MRI) in primary tumors; in recurrent tumors higher differences were found. The mean GTV-MRI was significantly higher than mean GTV-PET. The increase added by MRI to the common volume was due to scar tissue with strong signal enhancement on MRI. Conclusions: In patients with glomus tumors, Gluc-Lys[¹⁸F]-TOCA PET helps to reduce interobserver variability if an appropriate threshold for tumor segmentation has been determined for institutional conditions. Especially in patients with recurrent tumors after surgery, Gluc-Lys[¹⁸F]-TOCA PET improves the accuracy of GTV delineation.